Weekday-dependent long-term outcomes in gastrointestinal cancer surgery: a German population-based retrospective cohort study

Abstract

Background:

For most solid cancers, surgery represents the mainstay of curative treatment. Several studies investigating the effects of the weekday of surgery (WOS) on patient outcomes have yielded conflicting results. Barmer, the second-largest health insurance company in Germany, serves roughly 10% of the German population. The authors have used the Barmer database to evaluate how the weekday on which the surgery is performed influences long-term oncologic outcomes.

Methods:

For this retrospective cohort study, the Barmer database was used to investigate the effect of the WOS (Monday–Friday) on outcomes following oncological resections of the colorectum (n=49 003), liver (n=1302), stomach (n=5027), esophagus (n=1126), and pancreas (n=6097). In total, 62 555 cases from 2008 to 2018 were included in the analysis. The endpoints were overall survival (OS), postoperative complications, and the necessity for therapeutic interventions or reoperations. The authors further examined whether the annual caseload or certification as a cancer center influenced the weekday effect.

Results:

The authors observed a significantly impaired OS for patients receiving gastric or colorectal resections on a Monday. Colorectal surgery performed on Mondays was associated with more postoperative complications and a higher probability of reoperations. The annual caseload or a certification as a colorectal cancer center had no bearing on the observed weekday effect. There is evidence that hospitals schedule older patients with more comorbidities earlier in the week, possibly explaining these findings.

Conclusion:

This is the first study investigating the influence of the WOS on long-term survival in Germany. Our findings indicate that, in the German healthcare system, patients undergoing colorectal cancer surgery on Mondays have more postoperative complications and, therefore, require significantly more reoperations, ultimately lowering the OS. This surprising finding appears to reflect an attempt to schedule patients with higher postoperative risk earlier in the week as well as semi-elective patients admitted on weekends scheduled for surgery on the next Monday.

Introduction

Highlights

• This comprehensive study examines the weekday impact on long-term cancer outcomes using reimbursement data of ~10% of the German population.

• The analysis includes over 62 000 oncological resection cases from 2008 to 2018.

• Patients undergoing gastric and colorectal resections on Mondays experience impaired overall survival.

• Monday surgeries for colorectal cancer are linked to increased postoperative complication and reoperation rates.

• No correlation was found between annual caseload or cancer center certification and the observed weekday effect.

• Scheduling trends indicate older patients with more comorbidities are booked earlier in the week, potentially explaining outcomes.

For most solid cancers, surgery represents the mainstay of curative treatment¹. In recent decades, the centralization of complex procedures as well as advances in surgical techniques and perioperative medicine have led to a significant decline in mortality following high-risk cancer resections².

However, despite these substantial improvements, the healthcare setting remains a dynamic environment subject to multiple factors that could potentially influence patient outcomes. One such factor, which has been overlooked until recently, is the day of the week on which surgical procedures are carried out. Due to variances in operational schedules, case volumes, and stress and fatigue levels among medical practitioners and patients, it is reasonable to theorize that these different characteristics of each weekday might significantly influence the outcomes of surgical procedures.

Emerging evidence supports this theory, indicating higher mortality rates in patients who undergo elective surgery toward the end of the week^3–5. This observation hints at a potential systematic phenomenon tied to operational practices within healthcare settings. However, this ‘weekday effect’ hypothesis is not unanimously supported as other studies found no significant impact of the day of the week on the outcomes of elective oncological resections^6–8. This discrepancy in findings could be attributed to variations in healthcare systems between countries, differences in study design, or disparities in the clinical conditions being studied.

The contradictory results are most likely due to differences in healthcare systems between countries⁹. The German healthcare system, due to its universal coverage and accessibility, provides a distinctive context to investigate the existence and impact of the ‘weekday effect’. A few studies have investigated this concept in Germany, yielding mixed results. For example, higher mortality rates were observed following elective cancer surgeries of the colon and pancreas when procedures were performed early in the week¹⁰. Conversely, no such weekday effect was observed for D2-gastrectomy for gastric cancer¹¹.The question remains whether the weekday effect holds relevance in the German healthcare system, and if so, how it impacts patient survival post elective cancer surgery. Identifying weekday-dependent factors influencing patient outcomes could be key to further reducing mortality following tumor resections.

Barmer, the second-largest health insurance company in Germany, serves ~10% of the population of Germany. The cohort provided by Barmer, representative of Western European demographics, has been the subject of numerous scientific investigations^12–15.

The primary objective of this study is to investigate the influence of the weekday of surgery (WOS) on patient outcomes following elective oncological procedures on the colorectum, liver, stomach, esophagus, and pancreas using the Barmer database. These gastrointestinal malignancies were specifically selected as they are predominantly treated by visceral surgery, which is typically organized as joint departments in Germany. Given the similar organizational contexts across these procedures, it is plausible that any identified weekday effect may be applicable to all the aforementioned cancer types.

Methods

Study design and Barmer cohort

This retrospective cohort study utilized the Scientific Data Warehouse of the Barmer health insurance company, in which anonymized administrative data is recorded for scientific research purposes. Due to the anonymized nature of the data, an ethics approval was not required and the European General Data Protection Regulation does not apply to this study.

During the study period (2007–2018), Barmer insured between 8 and 9.2 million people (~10–12% of the German population). Cases were selected based on a defined set of International Statistical Classification of Diseases and Related Health Problems, 10^th revision [international classification of diseases (ICD)-10] and Operationen- und Prozedurenschlüssel (Operation and Procedure Classification System, OPS) codes (Supplementary Table S1, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). For this study, we focused primarily on the major gastrointestinal malignancies of the colorectum, liver, stomach, esophagus, and pancreas. In contrast to breast and prostate cancers, these are not subject to pronounced gender dependency and, apart from lung cancer, are among the most common causes of cancer-related death. The surgical therapy of these individual tumor types includes very different techniques and associated different intraoperative and postoperative complications, such as anastomotic leakage. However, the organizational context of oncological resections of these gastrointestinal cancer entities is similar as they are treated in joint visceral surgery departments in Germany. As a weekday effect must be a result of organizational rather than biological circumstances, we aimed to include biologically distinct but organizationally similarly treated tumor entities.

Surgeries performed Monday through Friday between 2007 and 2018 were analyzed, and all cases with uninterrupted insurance status and surveillance periods of at least 2 years preceding and 1-year following the oncologic resections were considered. As the Barmer dataset does not contain information regarding the emergency or elective nature of a case, it was not possible to stratify according to emergency status. Previous cancer diagnoses and/or oncologic treatments were exclusionary. The primary endpoints were overall survival (OS), postoperative complications, and the necessity for therapeutic interventions or reoperations. In addition, we examined whether the annual caseload or cancer center certification had an influence on weekday effects. This work has been reported in line with the strengthening the reporting of cohort, cross-sectional and case–control studies in surgery (STROCSS) criteria^16,17, Supplemental Digital Content 1, http://links.lww.com/JS9/A748.

Statistical analyses

The log-rank test was used to compare survival probabilities after oncologic resections, with data stratified by tumor type and WOS. A Cox proportional hazards model was used to calculate hazard ratios and CI and to test the effects of caseload or cancer center certification on WOS-related survival. The Wilcoxon rank-sum test was used to compare the average ages of patients among WOS groups. The Bonferroni correction was used to adjust for multiple testing in the survival analyses. To investigate differences in preoperative morbidity and their impacts on postoperative survival among the WOS, we used the Charlson comorbidity index (CCI) and the age-adjusted CCI (ACCI)^18,19 using the outpatient diagnoses (ICD codes) of the two calendar quarters preceding the quarter in which the operation occurred. The CCI, a method of categorizing comorbidities of patients based on the ICD diagnoses, is designed to predict the 1-year and 10-year mortality for a patient with a range of comorbid conditions. The ACCI is a modification of the CCI that also accounts for the age of the patient, providing a more holistic evaluation of the patient’s health status. The age-related resolution of ACCI data (10-year steps) appeared inadequate for this study; therefore, we employed a combination of age adjustment and stratification by CCI. A logistic regression model was used to study the WOS-related frequency of postoperative complications; odds ratios (OR) and CI were calculated.

As the colorectal cancer cohort is by far the largest cohort in this study and has the strongest weekday effect, we limited further analyses based on center size, certification status, or comorbidities on this cohort.

All statistical evaluations were performed using the software R (Version 4.2.1, R-Foundation). Graphs and figures were created with GraphPad Prism (Version 9.5.1, GraphPad Software). The significance threshold was set at P<0.05.

Results

Barmer cohort characteristics

Case numbers for colorectal (n=49 003), hepatic (n=1302), gastric (n=5027), esophageal (n=1126), and pancreatic (n=6097) resections (Table 1) were consistent with the size of the Barmer cohort (~10% of the German population). Colorectal cancer patients were the oldest (71.1±11.5 years), and patients receiving colorectal resections on Mondays were significantly older than those receiving surgery on other days (71.7±11.6 vs. 71.0±11.5 years, P<0.001; Table 1, Supplementary Fig. S1A, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). The age distribution in gastric cancer patients was similar (71.2±11.0 vs. 69.5±11.8 years, P<0.001), while the other cohorts showed no significant weekday-related age differences.

Table 1.

Barmer cohort characteristics and case numbers by weekday (♂=male, ♀=female, t=total, SD, c.p.d.=cases per day).

			Age (♂)			Age (♀)			Age (t)
Organ	WOS	N (♂) (%)	Median	SD	N (♀) (%)	Median	SD	N (t)	Median	SD	c.p.d. (%)
Colorectum	Monday	4310 (44.4)	70.2	11.0	5395 (55.6)	72.8	12.0	9705	71.7	11.6	19.8
	Tuesday	4241 (45.6)	69.6	10.9	5057 (54.4)	71.7	11.9	9298	70.8	11.5	19.0
	Wednesday	4455 (45.4)	69.6	10.8	5368 (54.6)	71.7	12.2	9823	70.7	11.6	20.0
	Thursday	4661 (45.6)	70.0	10.6	5556 (54.4)	72.0	11.9	10217	71.1	11.3	20.8
	Friday	4430 (44.5)	69.9	10.7	5530 (55.5)	72.6	11.8	9960	71.4	11.4	20.3
	Monday–Friday	22097 (45.1)	69.8	10.8	26906 (54.9)	72.2	11.9	49003	71.1	11.5
Liver	Monday	154 (53.7)	68.6	10.2	133 (46.3)	65.6	13.1	287	67.2	11.7	22.0
	Tuesday	171 (61.1)	68.7	10.5	109 (38.9)	66.8	12.8	280	68.0	11.5	21.5
	Wednesday	160 (59.9)	67.7	10.6	107 (40.1)	67.1	10.4	267	67.5	10.5	20.5
	Thursday	139 (54.5)	69.2	10.7	116 (45.5)	66.3	10.5	255	67.9	10.7	19.6
	Friday	114 (53.5)	69.4	8.5	99 (46.5	63.9	12.7	213	66.8	11.0	16.4
	Monday–Friday	738 (56.7)	68.7	10.1	564 (43.3)	66.9	11.9	1302	67.5	11.1
Stomach	Monday	553 (51.5)	70.6	10.6	521 (48.5)	72.0	11.4	1074	71.2	11.0	21.4
	Tuesday	497 (49.8)	69.2	10.5	501 (50.2)	69.7	12.4	998	69.5	11.5	19.9
	Wednesday	486 (49.3)	68.0	11.7	499 (50.7)	70.2	12.6	985	69.1	12.2	19.6
	Thursday	532 (50.5)	69.0	11.0	522 (49.5)	70.3	12.4	1054	69.6	11.7	21.0
	Friday	466 (50.9)	69.4	11.2	450 (49.1)	69.9	12.9	916	69.7	12.0	18.2
	Monday–Friday	2534 (50.4)	69.2	11.0	2493 (49.6)	70.4	12.3	5027	69.8	11.7
Esophagus	Monday	210 (80.2)	65.0	9.4	52 (19.8)	67.8	9.6	262	65.6	9.5	23.3
	Tuesday	160 (70.8)	63.9	9.2	66 (29.2)	66.2	9.2	226	64.6	9.2	20.1
	Wednesday	193 (76.3)	64.8	9.0	60 (23.7)	65.4	8.6	253	65.0	8.9	22.5
	Thursday	166 (71.6)	63.9	9.4	66 (28.4)	66.4	9.9	232	64.6	9.6	20.6
	Friday	108 (70.6)	64.3	9.5	45 (29.4)	66.0	12.1	153	64.8	10.3	13.6
	Monday–Friday	837 (74.3)	64.4	9.3	289 (25.7)	66.4	9.9	1126	64.9	9.5
Pancreas	Monday	640 (44.4)	67.6	9.8	801 (55.6)	68.2	10.5	1441	67.9	10.2	23.6
	Tuesday	573 (45.1)	67.7	9.7	698 (54.9)	68.1	10.5	1271	67.9	10.2	20.8
	Wednesday	499 (42.8)	67.7	9.7	668 (57.2)	68.3	10.5	1167	68.0	10.2	19.1
	Thursday	476 (41.0)	67.9	9.7	686 (59.0)	69.0	9.6	1162	68.6	9.6	19.1
	Friday	475 (45.0)	67.6	10.2	581 (55.0)	68.8	10.0	1056	68.3	10.1	17.3
	Monday–Friday	2663 (43.7)	67.7	9.8	3434 (56.3)	68.5	10.2	6097	68.1	10.1
	∑	28869 (46.1)			33686 (53.9)		53.9	62555

WOS, weekday of surgery.

The most and fewest cancer surgeries were performed on Thursdays and Tuesdays, respectively (12 920 vs. 12 073; Table 1 and Supplementary Fig. S1B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). More colorectal cancer resections were performed in the second half of the week (Supplementary Fig. S1B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749), while the number of liver resections decreased over the course of the week. Except for colorectal cancer cases, resections were performed least frequently on Fridays (Table 1 and Supplementary Fig. S1B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749).

The genders were equally distributed over the week in all analyzed tumor entities (data not shown).

Overall survival in relation to the weekday of cancer surgery

The outcomes after colorectal, hepatic, pancreatic, esophageal, and gastric cancer surgeries were in the expected ranges, with the lowest survival in the pancreatic surgery patients (5-year OS ~26%) and the best survival in the colorectal surgery group (5-year OS ~59%) (Supplementary Fig. S2, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). The OS after liver (Figs 1B, 2B), esophageal (Figs 1D, 2D), and pancreatic (Figs 1E, 2E) surgeries did not significantly differ between WOS. Gastric cancer patients operated on Mondays had significantly shorter OS compared with Tuesdays, Thursdays, and Fridays (Figs 1C, 2C). Gastric cancer cases trended toward better survival for surgeries performed on Wednesdays compared to Mondays, but this effect failed to reach statistical significance. For colorectal surgery, patients operated on Mondays had a definite survival disadvantage (Figs 1A, 2A).

Figure 1.

Kaplan–Meier estimates of overall survival for each weekday (Monday–Friday) following surgery for colorectal (A), liver (B), gastric (C), esophageal (D), and pancreatic (E) resections.

Figure 2.

Hazard ratios (OS) for each weekday (Monday to Friday) after surgery of colorectal, liver, gastric, esophageal, and pancreatic cancers (n.s., not significant; *P<0.05; **P<0.01; ***P<0.001).

Influence of comorbidity and age on the weekday effect in colorectal surgery

Comorbidities have a profound impact on the short-term and long-term postoperative outcome after oncologic resections of different solid cancers^20–22. To assess the severity of comorbidities and their impact on mortality, various scoring systems such as the widely used CCI and the Elixhauser comorbidity index were developed^18,23,24. The ACCI incorporates the patients’ age as a correction variable of the final score and has been shown to be a better survival predictor than the original CCI in some studies^25,26. We calculated the mean CCI and ACCI scores for all tumor entities depending on the WOS (Supplementary Tables S2A and S2B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749).

As the colorectal cancer group showed the strongest weekday effect and patients undergoing colorectal surgery on Mondays were significantly older, we examined the influence of age distribution and accompanying comorbidities on OS after colorectal cancer surgery. There were no significant differences in the CCI or ACCI between the WOS (Supplementary Tables S2A and S2B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). To verify whether age distribution could explain the weekday effect, we used an age-adjusted and CCI-stratified survival analysis (Supplementary Fig. S3, Supplemental Digital Content 1, http://links.lww.com/JS9/A749), which revealed a highly significant influence of the patient’s age on OS. Interestingly, age adjustment alone attenuated the weekday effect only slightly (Supplementary Fig. S3B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). The age-adjusted Cox model stratified for patients with no comorbidities (CCI score=0; CCI range: 0–18) showed a significant survival disadvantage for Monday compared to the other days of the week except Friday (Supplementary Fig. S3C, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). Further stratified analyses determined that the Monday effect was not detectable in patients with CCI scores of 1 or 2; however, like the group without comorbidities, patients with CCI greater than 2 had significantly lower OS after surgery on Mondays.

We next analyzed whether the observed weekday effects in colorectal and gastric cancer cohorts were dependent of the patient’s age. To this end, the cohorts were divided by the median age (71.1 years in colorectal cancer; 69.8 years in gastric cancer) and analyzed separately (Supplementary Fig. S4, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). While an impaired outcome for patients operated on Mondays could be seen both in younger (P=0.003) and older (P<0.0001) colorectal cancer patients, the effect could not be seen in either gastric cancer cohort (P=0.37 and P=0.095, respectively). In summary, the combination of age and comorbidities could not fully explain the observed weekday effect in the colorectal surgery group.

Postoperative complications in relation to the weekday of colorectal cancer surgery

As postoperative complications negatively influence not only overall but also long-term cancer-specific survival^27,28, we hypothesized that the lower OS of patients operated on Mondays may be related to higher postoperative complication rates. We therefore defined a set of typical complication ICD codes (Supplemental Table S3, Supplemental Digital Content 1, http://links.lww.com/JS9/A749) and evaluated their frequencies by WOS (Supplementary Table S4, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). As colorectal surgery accounted for the largest part of our cohort (> 78%) and showed the strongest weekday effect, we focused on this patient population. Patients operated on Mondays were significantly more likely to be diagnosed with any complication during their initial hospitalization (Supplementary Fig. S5A, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). Interestingly, complications emerging after hospital discharge did not correlate with the WOS (Supplementary Fig. S5B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749).

Increased rates of reoperations after oncological surgeries performed on Mondays

Complications requiring reoperation or therapeutic intervention within 30 days of the index operation (e.g. relaparotomy/thoracotomy or endoscopic implantation of a vacuum sponge) were most frequent after esophageal resection (13.8 and 11.5%, respectively; Supplementary Table S5, Supplemental Digital Content 1, http://links.lww.com/JS9/A749) and least frequent after colorectal surgery (9.2 and 0.8%, respectively; Supplementary Table S2, Supplemental Digital Content 1, http://links.lww.com/JS9/A749).

For all oncological procedures combined, there was a strong trend toward higher numbers of reoperations in patients operated on Mondays (Fig. 3A). A similar, weaker trend was seen for therapeutic interventions such as postoperative drainage of abscesses or endoscopic placement of endoluminal vacuum sponges (Supplementary Fig. S6, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). However, since colorectal procedures outnumbered all other procedures, they contributed most to the effects observed for all oncological procedures combined.

Figure 3.

Odds ratios for the occurrence of any type of complication requiring reoperation for each weekday (Monday to Friday) during 30 days after the index operation for all cancer entities (A) and for individual cancer resections (B–F) (n.s., not significant; *P<0.05; **P<0.01; ***P<0.001).

Individual analyses revealed significantly fewer reoperations after colorectal resections performed on Tuesdays (OR 0.85 (0.77–0.94), P=0.001), Wednesdays (OR 0.88 (0.80–0.97), P=0 .008), or Thursdays (OR 0.86 (0.78–0.95), P=0.002, all compared to Mondays). Operations on Fridays also trended toward lower reoperation rates, but this effect failed to reach statistical significance (OR 0.93 (0.84–1.02), P=0.119; Figure 3B and Supplementary Fig. S6B, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). Gastric cancer patients required significantly fewer reoperations after resections performed on Thursdays (OR 0.63 (0.46–0.85), P=0.003) and there was a significantly lower risk of reoperations after liver resections on Wednesdays (OR 0.58 (0,33–0.97), P=0.04). We observed no effect of the WOS on postoperative therapeutic (nonsurgical) intervention rates for colorectal, gastric, and liver cancer surgeries (Supplementary Fig. S6B, D, Supplemental Digital Content 1, http://links.lww.com/JS9/A749). The WOS had no significant impact on therapeutic intervention or reoperation rates after pancreatic or esophageal surgeries (Fig. 3C, E, and Supplementary Fig. S6C, E, Supplemental Digital Content 1, http://links.lww.com/JS9/A749).

Prognostic impact of the WOS in large-volume versus small-volume and certified versus noncertified centers

Cancer surgery carried out in high-volume centers have been shown to have better outcomes²⁹. We therefore investigated whether the annual caseloads of surgery centers influenced WOS-dependent OS. Centers with annual caseloads exceeding 40 colonic and 30 rectal resections were defined as large. Unsurprisingly, large centers had significantly better 5- and 10-year OS rates than small centers (Fig. 4A). However, the weekday effect was observed in both small and large surgery centers (Fig. 4C–E).

Figure 4.

Survival analysis with hazard ratios for patients after colorectal cancer resections according to annual caseload or certification as a colon cancer center: Kaplan–Meier curves showing overall survival of colorectal cancer patients over a 10-year period after surgery, according to colorectal resection caseloads (A) and cancer center certification status (B). Hazard ratios after colorectal cancer surgery for each WOS (Monday to Friday), by resection caseload (all observed centers (C), high-volume (D), and low-volume centers(E)) and certification status as a colon cancer center (with (F) and without (G) certification). *P<0.05.

Cancer treatment including major resections in cancer centers certified by the German Cancer Society (Deutsche Krebsgesellschaft) has better short-term and long-term results than treatment in noncertified institutions^30,31. Cancer centers must meet quality standards such as the commitment to national cancer treatment guidelines, discussion of all cases in interdisciplinary tumor boards, minimal numbers of certain operations and interventions, maximum number of revision surgeries, or postoperative complications to earn and keep certification (for further details, please refer to www.krebsgesellschaft.de)³². In addition to annual caseload requirements (≥ 30 colonic and ≥20 rectal resections), certified colorectal cancer centers must provide an interdisciplinary team with on-call board-certified visceral surgeons and endoscopists throughout the weekends, thus possibly providing better surveillance and care for patients experiencing complications on weekends. We opted to compare WOS effects in certified and noncertified centers in the colorectal cancer cohort because it had the strongest weekday effect and the most available certification status data. Of the 1190 centers, 296 (24.8%) were certified. As in our caseload-based analysis, we found that OS was significantly better in certified centers (Fig. 4B) but that the weekday effects were comparable in certified and noncertified centers (Fig. 4F, G).

Discussion

The main goal of this study was to investigate whether there is a weekday-dependency of long-term patient outcome after oncological resections of gastrointestinal malignancies in Germany. To date, the weekday effect in Germany has mainly been studied for emergency procedures and organ transplants^33–38.

The results of this retrospective cohort study indicate a significantly reduced OS for patients receiving gastric or colorectal resections on Mondays, whereas the outcomes of hepatic, esophageal, and pancreatic surgeries were independent of the WOS.

While our findings align with one of the few other German studies investigating this effect¹⁰, they contrast with several studies from other countries reporting higher postoperative mortalities and worse long-term survival after elective cancer surgeries performed in the second half of the week^39–41. This is most frequently attributed to reduced medical care on the weekend, as the first 48 h after surgery are considered a particularly vulnerable period and critical to postoperative outcomes^5,42,43. This hypothesis may also explain poorer outcomes after early-week surgeries for procedures involving intestinal anastomoses. Unlike primary anastomotic leakage resulting from technical issues (e.g. stapler failure or insufficient sutures), secondary anastomotic leakage typically occurs between postoperative days 5 and 9^44,45, which includes the weekend for patients operated on Mondays. Reduced staffing on weekends may lead to the delayed detection or inadequate treatment of anastomotic leakage and thus a failure to rescue in such cases. Consistent with this hypothesis, WOS does not affect the outcomes of hepatic resections, which often do not require intestinal anastomoses. However, contrary to this hypothesis, the outcomes of esophageal and pancreatic resections, in which anastomotic leakage is a frequent complication, were also independent of WOS. There was no increase in mortality after emergency colorectal cancer surgery or emergency stroke treatment later in the week and throughout the weekends in Germany^35,37. These findings indicate adequate emergency care for patients throughout the weekend in Germany. The results are also consistent with the observed Monday effect being independent of annual caseload or certification in the colorectal cohort, as the weekday effect would be smaller in larger or certified centers if this was a result of insufficient manpower on weekends. Overall, impaired medical care on the weekends does not seem to be the main reason for the observed Monday effect in the colorectal and gastric cancer cohort.

An alternative explanation for our findings is patient selection. Patients with colorectal or gastric cancer receiving oncologic resections on Mondays were significantly older, strongly suggesting an underlying patient selection bias for these tumor types. There was no significant age difference between the WOS for the other cancer entities, suggesting a more even distribution of patients over the course of the week and possibly therefore not producing a weekday effect. Surprisingly, we found no significant differences in comorbidity among the days of the week, and the results of our age-adjusted and comorbidity-stratified survival analysis could not fully explain the weekday effect in colorectal resections. Possible explanations for this are that the outpatient preoperative CCI did not capture all essential comorbidities or that factors influencing the risks associated with surgery or anesthesia could not be detected by the CCI (e.g. anticoagulation due to prior pulmonary embolism), thus limiting the significance of this analysis. Differences in age distributions also failed to fully explain the WOS-related survival disadvantage.

Another German group recently reported increased mortality after colon and pancreatic head surgeries performed early in the week, using data from the Study, Documentation, and Quality Center (StuDoQ, Studien-, Dokumentations- und Qualitätszentrum) of the German Society for General and Visceral Surgery¹⁰. In this cohort, patients operated on Mondays for colon cancer were significantly older and had higher T and Union internationale contre le cancer stages, while rectal cancer patients operated on Mondays had needed significantly more preoperative nursing care and suffered more frequently from coronary artery disease. Due to the limitations of the administrative Barmer cohort, we combined colon and rectal cancer cases for analysis; therefore, our study population is not entirely comparable to the StuDoQ cohort. However, factors such as cases with higher TNM (Tumor/Node/Metastasis) stages being operated on earlier in the week (suggesting that surgeons schedule more difficult cases earlier in the week) are likely to apply in our study cohort as well.

Importantly, we observed a weekday effect in colorectal cancer and, to a lesser extent, the gastric cancer cohort, while other cancer types seemed unaffected. One possible explanation for this discrepancy might lie in the typically better prognosis associated with colorectal cancer compared to liver, esophageal, pancreatic, and gastric cancers. This relatively positive prognosis might make outcomes following colorectal surgery more susceptible to subtle influences, like those associated with variations in weekday surgical practices. In contrast, the prognosis for liver, esophageal, pancreatic, and gastric cancers is often less favorable, largely due to the aggressive nature of these malignancies and the late stage at which they are frequently diagnosed. For these types of cancers, the primary factors affecting patient outcomes may be more directly related to the disease characteristics and the complex surgical interventions required. These factors could potentially overshadow the influence of the weekday effect. For example, the stage and aggressiveness of the disease, the complexity of the surgical procedure, and the postoperative care requirements may all play more substantial roles in determining the outcome compared to the WOS. However, understanding these differences is a complex issue, requiring further investigation.

Elderly patients, especially those over 80 years of age, have an increased risk for postoperative complications and prolonged hospital stay after major oncologic resections^46,47. Postoperative complications, such as anastomotic leakage after colorectal resections, are known to worsen overall and long-term cancer-specific survival^27,28. Consistent with this, the patients in our cohort receiving oncologic resections of the colorectum on Mondays had significantly more postoperative complications and also higher reoperation rates during their initial hospitalization than those operated on Tuesdays, Wednesdays, and Thursdays. Colorectal resections on Fridays trended toward the same results but failed to reach statistical significance.

The Barmer database does not provide any information about the urgency of a surgical procedure. A distinction between elective and emergency cancer resections was therefore not possible in this study. Elective oncologic resections are usually planned several days to few weeks in advance. There is no evidence that less well-planned cases are then scheduled for Mondays in the German healthcare system. However, there is a possibility that semi-elective cases (i.e. patients with large colorectal tumors suggesting impending bowel obstruction or gastric tumors with subacute bleeding) are admitted to the hospital at the weekend, treated conservatively first and then, with better staffing, scheduled for early surgery on the next Monday playing a potential role in our observed Monday effect. As such semi-elective oncological resections most frequently occur in colorectal and gastric cancer, this may serve as a possible explanation for the weekday effect in the colorectal and gastric cancer cohorts. This hypothesis also aligns with the German StuDoQ cohort, which observed higher TNM stages for colon cancer patients receiving surgery on Monday¹⁰.

Taken together, neither impaired medical care at weekends nor uneven age distribution during the week can fully explain the Monday effect after colorectal and gastric resections. Since the Barmer cohort is representative of the overall population of Germany, it has been used for numerous scientific studies^13,48,49. However, as it represents only one single database, it may despite its size and representativeness limit the general applicability of the results to the general population to some degree. As Barmer is a health insurance company, the principal limitation of this study is that, its database contains only information directly relevant for reimbursement and excludes relevant data such as TNM/union internationale contre le cancer stages, exact procedures performed (e.g. anastomotic technique), and intraoperative data such as blood loss, duration of the procedure, or surgeon experience. This precludes relevant further analyses such as the distribution of tumor stages over the week or stratified analyses according to tumor stage. Furthermore, the Barmer database cannot provide any information about ethnicity or socio-economic status and thus prevents further in-depth subgroup analyses.

In conclusion, our findings indicate that German patients having received colorectal cancer surgery on Mondays had more postoperative complications (e.g. anastomotic leakage) and, therefore, required significantly more reoperations, ultimately resulting in worse long-term and OS. We have also observed a Monday effect after gastric resections, whereas the outcome after oncological liver, pancreas, and esophagus resections does not seem to depend on the WOS. The most plausible explanation for these observations is a combination of selection bias toward operating older patients with higher tumor stages , thus potentially requiring more complex procedures on Mondays (reflecting an organizational attempt by treating physicians to minimize the potential negative effects of reduced staffing on weekends) and conservatively treated, semi-elective patients admitted on weekends and operated on the following Monday. We were not able to substantiate the hypothesis that patients with more severe comorbidity are scheduled earlier in the week as the CCI does not significantly vary between weekdays. Further studies including in-depth analyses of potential confounders are needed to verify this hypothesis. In order to substantiate the influence of the weekday of an operation on the short-term and long-term outcome after major oncological resections, prospective studies are required, recording not only detailed data of the comorbidities and the exact extent of the tumor, but also of the intraoperative course, including the surgical techniques. As organizational reasons are most likely to be the underlying reason of the weekday effect, such studies must be conducted as multicenter trials in order to be representative of a healthcare system. Knowledge of the weekday effect on patient survival after oncological resections and the underlying weekday-dependent negatively influencing factors may help to further reduce mortality after major tumor resections, leading to more favorable patient outcomes with fewer postoperative complications, shorter hospital stays, and reduced healthcare costs.

Ethical approval

Only anonymized data was used for this study, therefore no ethics approval was required (as per German law).

Consent

Only anonymized data was used, therefore no patient consent was required (as per German law).

Sources of funding

Lei Zhu was supported by the China Scholarship Council (CSC, no. 201908080072). Hector Stiftung II supports Sebastian Schölch.

Author contribution

C.F.M.: conceptualization, methodology, visualization, investigation, writing – original draft, writing – review and editing; C.S.: conceptualization, methodology, investigation, writing – original, draft, writing – review and editing; L.Z.: conceptualization, formal analysis, methodology, validation, visualization, writing – review and editing; M.M.N.: data curation, formal analysis, methodology, investigation, software; H.L.H.: data curation, formal analysis, methodology, investigation, software, validation, writing – review and editing; U.M.: resources, supervision, conceptualization, validation; C.R.: conceptualization, writing – review & editing; S.S.: supervision, conceptualization, formal analysis, validation, resources, supervision, writing – original draft, writing – review and editing.

Conflicts of interest disclosure

The authors declare that they have no financial conflict of interest with regard to the content of this report.

Research registration unique identifying number (UIN)

1. Name of the registry: Researchregistry.com

2. Unique Identifying number or registration ID: researchregistry8859.

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Guarantor

Ursula Marschall, Helmut L’hoest, and Sebastian Schölch.

Data availability statement

The original data extracted from the Barmer Data Warehouse can be shared upon request to the corresponding author.

Provenance and peer review

Original data can be shared upon request to the corresponding author.

Assistance with the study

None.

Presentation

None.