Popliteal fossa metastasis from rectal carcinoma in a young adult: A rare case report

Abstract

Signet-ring cell carcinoma (SRCC) of the rectum is a rare and aggressive subtype of colorectal cancer, typically metastasizing to regional lymph nodes, peritoneum, and liver. Metastasis to the popliteal fossa is exceedingly rare. We report a 27-year-old male with biopsy-proven rectal SRCC who developed a painful soft tissue mass in the left popliteal fossa within 1 year despite ongoing chemoradiotherapy. MRI demonstrated a heterogeneous mass with central necrosis, peripheral enhancement, and diffusion restriction. ¹⁸F-FDG PET/CT revealed peripheral FDG avidity (SUVmax 2.8). Ultrasound-guided core biopsy confirmed metastatic adenocarcinoma of colorectal origin. This case highlights an extremely rare metastatic pattern of rectal SRCC and emphasizes the importance of a multimodality imaging approach, including ultrasound, MRI, and PET/CT, in evaluating atypical soft tissue lesions in oncology patients.

Introduction

Colorectal cancer (CRC) remains a significant global public health concern. It is the third most commonly diagnosed malignancy in males, following prostate and lung cancer, and ranks also third in females, after breast and lung cancers [1]. Furthermore, despite notable advancements in therapeutic strategies, CRC continues to account for approximately 10% of all cancer-related mortality worldwide [1]. In Pakistan, CRC is the second most common cancer, accounting for 4.8% of all diagnosis [1]. Signet ring colon carcinoma (SRCCs) comprise 1.0% of all cases of colon cancer, and 15.3% of all SRCC cases. When compared to nonvariant adenocarcinomas, SRCCs have an earlier mean age of onset of 3.5 years, compared to 6.6 years for gastric SRCCs [2].

While the liver and lung are primary targets for distant metastasis from colon cancer, the metastasis to other distant sites is rarely found. Metastasis from colon cancer to skeletal muscles is rarest manifestation. The possible mechanism of metastatic spread of adenocarcinoma of colon to the skeletal muscles could be by lymphatics, hematogenous route, and direct extension of primary disease and by manipulation during surgery [3].

Given the rarity and diagnostic challenge of such metastases, awareness and careful radiological evaluation are essential in patients with a history of aggressive colorectal malignancies such as SRCC.

In this report, we describe a rare case of biopsy-proven rectal SRCC metastasizing to the popliteal fossa, with a focus on radiological findings, differential diagnosis, and potential pathways of metastatic spread.

Case presentation

Our case presents a 27-year-old male with no known comorbidities presented in walk-in clinic of Shaukat Khanum Memorial Hospital with complaints of rectal bleeding, recurrent episodes of anal pain, constipation, and a prolapsed anal mass persisting over the preceding 6 months. He had previously received treatment for presumed hemorrhoids at multiple other surgical facilities without resolution.

Biopsy was performed from growth at anal verge, which revealed poorly differentiated mucinous adenocarcinoma with signet ring cell features. Immunohistochemical staining showed CDX2 positivity, supporting a colorectal origin.

An MRI of the pelvis demonstrated a locally aggressive anorectal malignancy with diffuse infiltrative changes, positive circumferential resection margin (CRM), and multiple meso-rectal lymph nodes (Fig. 1). Based on radiologic and pathologic findings, the tumor was staged as T3N2Mx as tumor invades through the muscularis propria into the sub serosa(T3), metastasis in < 4 regional lymph nodes with no distant metastasis on CT scan of the chest and abdomen.

Fig. 1.

(A) Sagittal T2W image shows (arrow) irregular circumferential mass like wall thickening seen involving anal canal and rectum. (B) Axial T2W image shows diffuse infiltrative changes resulting in positive CRM (c) Axial T2W image shows Multiple meso-rectal nodes.

The patient had a diverting colostomy to relieve his obstructive symptoms. A gastrointestinal multidisciplinary team (GI-MDT) assessed his case and made plans for radiation, neoadjuvant chemoradiotherapy, and a colonoscopy. Following that, he endured 3 cycles of capecitabine in addition to radiation therapy (50.4 Gy in 28 portions). However, post-treatment MRI showed interval increase in tumor size (Fig. 2). An examination under anesthesia showed a firm, stenotic tumor that affected the levator muscles and the entire anal canal. The tumor was unresectable since the top portion could not be reached.

Fig. 2.

Post radio-chemotherapy assessment scan showed progressive disease with progressive thickening of rectal wall tumor, which now extends to the anal canal as well as bulging exophytic lesion through the anal verge (A & B). Interval development of left inguinal necrotic lymph node (C).

He started experiencing pain and limited movement in his left lower limb about 4 months after finishing 7 cycles of chemotherapy. Upon physical examination, a hard mass was found above the left knee's posteromedial aspect. Although ultrasonography is commonly used as the first-line investigation for popliteal fossa lesions, given the patient’s known history of aggressive rectal carcinoma and the firm, painful nature of the swelling, advanced cross-sectional imaging was prioritized. Plain radiography did not reveal osseous involvement, following which MRI was performed for detailed soft tissue characterization.

MRI of the lower limb showed a neoplastic mass along the posterior aspect of the left femur, demonstrating diffusion restriction and contrast enhancement (Fig. 3). Histopathology of a biopsy from the knee lesion confirmed metastatic adenocarcinoma, with CK20 +, CDX2 +, and CK7 − immunophenotype, consistent with a colorectal carcinoma (Fig. 4). For evaluation of other possible metastatic deposits, a whole-body PET-CT scan was performed which revealed a heterogeneous FDG-avid lesion with SUV of 2.8 in the left popliteal fossa, with additional metastatic deposits observed near the hip joint and pelvis.

Fig. 3.

MRI of the left distal thigh demonstrating a heterogeneous soft tissue mass along the posterior aspect of the femur. (White arrows) (A) axial T2-weighted image, (B) sagittal T1-weighted post-contrast image showing peripheral enhancement with central necrosis, and (C) axial diffusion-weighted imaging demonstrating diffusion restriction in the peripheral viable tumor.

Fig. 4.

Histopathology of core biopsy sample from left distal femur mass exhibits malignant neoplasm composed of infiltrative tumor cell showing (A, B) intracytoplasmic mucin vacuoles and (C) eccentric nuclei (signet ring cells).

The patient was determined to be a candidate for systemic therapy when the case was re-examined in a sarcoma MDT. He then underwent 2 portions of palliative radiation to his left knee joint.

A follow-up CT scan revealed progressive disease, including tumor involvement at the colostomy site, requiring conversion to a transverse colostomy. The patient was sent to palliative care due to the disease's advancement and deteriorating performance (Fig. 5).

Fig. 5.

Whole-body PET-CT scan showing a heterogeneous FDG-avid lesion (SUV 2.8) in the left popliteal fossa soft tissue (black arrows).

6 months later, he passed away from his disease.

Discussion

Epidemiologically, Signet ring cell carcinoma (SRCC) is rare: about 10% of gastric cancer (GC) and 1% of colorectal cancer (CRC)[5]. SRCCs comprise 0.7% of all cases of rectal cancer, and 4.3% of all SRCC cases. When compared to nonvariant adenocarcinomas, SRCCs have an earlier mean age of onset of 6.6 years like gastric SRCCs, compared to 3.5 years for colon SRCCs [2]. Rectal SRCC patients have the worst overall survival relative to comparative nonvariant cancers of all the SRCCs, with 5- and 10-year survival at 24.9% vs 59.8% and 19.8% vs 50.7%, respectively [2].

Primary signet ring cell carcinoma of colorectum is diagnosed when the following criteria are satisfied. Firstly, tumor must be primary, histological material is adequate and signet ring cell present more than 50% of cancer [4]. Signet ring cell carcinoma of colorectum has an aggressive clinical course and poorer prognosis. There is high incidence of peritoneal metastases and relatively low incidence of hepatic metastases, a characteristic feature distinguishing colorectum signet ring cell carcinoma from non signet colorectum carcinoma [5]. All the 3 criteria were satisfied in our case.

A study conducted by Hugen et al. [6] demonstrates that SRCC patients exhibit a divergent pattern of metastases, with involvement of rare metastatic sites such as the heart, bone, pancreas, and skin. Therefore, attention should be paid to uncommon findings on imaging in SRCC patients during clinical follow-up, as these may reflect metastatic disease. The higher rate of distant metastases in rectal cancer can be explained by the venous drainage of the rectum bypassing the liver and draining directly into the inferior vena cava.

Our case also presents with unusual metastasis of signet cell type adenocarcinoma of the rectum which, despite ongoing chemoradiotherapy, exhibited disease progression by developing a painful lump in the popliteal region. The possible differential diagnosis included a soft tissue primary lesion, with a less likely possibility of metastatic deposit. An MRI was performed to differentiate between benign and malignant lesions. MRI demonstrated malignant features, and subsequent biopsy confirmed the diagnosis of metastatic deposit rather than soft tissue primary disease. The interval development of a necrotic left inguinal lymph node raises the possibility that the popliteal metastasis represents either a skip metastasis or part of a complex retrograde lymphatic spread. Hematogenous dissemination with subsequent soft tissue implantation also remains a plausible mechanism, particularly in aggressive histological subtypes such as SRCC.

Ultrasonography serves as a valuable diagnostic tool, particularly in the differentiation of solid versus cystic lesions. It is often employed as the first-line investigation when evaluating soft tissue abnormalities, especially in cases where no primary malignant lesion has yet been identified, as it aids in distinguishing between benign and malignant soft tissue lesions.

However, in the present case, the patient had a known metastatic rectal growth, and the X-ray findings were highly suspicious for malignant involvement in the soft tissues. This prompted the need for a more advanced imaging modality. Accordingly, a Magnetic Resonance Imaging (MRI) study was performed, given its superior soft tissue contrast resolution and multiplanar capability, which surpasses ultrasonography in accurately characterizing the extent, nature, and malignant potential of soft tissue lesions.

PET-CT plays a key role in staging and identifying atypical metastatic sites. Both ¹⁸F-FDG PET/CT and dedicated PET are highly sensitive for diagnosing malignant soft tissue lesions, with PET/CT showing greater specificity (85% vs 71%) and accuracy (89% vs 85%). A SUV threshold of 2.4 may help distinguish malignant from benign lesions, supporting the use of whole-body ¹⁸F-FDG PET/CT for initial evaluation of suspicious musculoskeletal tumors. In the present case, the popliteal lesion demonstrated peripheral FDG uptake with a measured SUVmax of 2.8, exceeding the commonly cited threshold of 2.4 for malignant soft tissue lesions, while the central necrotic component showed no FDG uptake.

Although mucinous and signet-ring cell carcinomas may show variable FDG avidity, the metabolically active rim corresponded closely with the enhancing and diffusion-restricting components on MRI, supporting true viable tumor tissue.

Conclusion

Popliteal fossa metastasis from rectal SRCC is exceptionally rare and diagnostically challenging. Radiologists should maintain suspicion for metastatic disease in patients with known colorectal malignancy presenting with atypical soft-tissue lesions. A multimodal imaging approach with tissue histopathology confirmation is essential for accurate diagnosis and management.

Learning Points

• 1.SRCC of the rectum may metastasize to highly unusual extra-regional sites.
• 2.Solid popliteal fossa lesions in oncology patients should not be assumed benign.
• 3.PET-CT and ultrasound-guided biopsy are critical in identifying atypical metastases.

Authors’ contributions

A.S conceived and designed the case report. A.S and F.I, S.A were responsible for data collection and acquisition of data. A.S performed the literature review and wrote the manuscript. A.S, A.N, S.K., and K.S reviewed and critically revised the manuscript. All authors have approved the final manuscript.

Patient consent

Written informed consent for the publication of this case report was obtained from the patient’s father. All patient data were de-identified to maintain confidentiality.