CT-guided iodine-125 brachytherapy for sacral metastasis with neural canal invasion: a case report of clinical outcome and dosimetry

Abstract

Sacral metastasis with neural canal invasion may cause severe pain and progressive neurological dysfunction, while local treatment is challenging because of the proximity of the sacral nerve roots, lumbosacral plexus, rectum, and bladder. We report a 65-year-old man with lung adenocarcinoma and a progressive sacral metastasis causing radiating pain, sensory impairment, lower-extremity weakness, and difficulty walking despite systemic treatment. CT-guided permanent iodine-125 seed implantation was performed with a prescription dose of 140 Gy. A total of 84 seeds, each with an activity of 0.5 mCi, were implanted. An exploratory postoperative CT-based analysis of the anatomically estimated neural region yielded a calculated mean dose of 316 Gy and a maximum voxel dose of 907 Gy; however, interpretation of these values was limited by the highly heterogeneous dose distribution, contouring uncertainty, and the use of a point or voxel-based maximum dose. At 2 weeks after implantation, the Visual Analogue Scale score decreased to 3, and at 1 month it decreased to 1. Lower-extremity muscle strength improved to grade 4, and the patient regained independent ambulation. During the available 12-month follow-up, imaging showed a marked metabolic response and no radiographic evidence of local progression. No new or worsening clinically apparent neurological deficit attributable to the procedure was identified. This case suggests that CT-guided iodine-125 seed implantation may be a feasible local treatment option for selected patients with sacral metastasis involving neural structures. However, a single case with 12 months of follow-up cannot establish long-term neural tolerance or support modification of existing neural dose constraints.

1. Introduction

Lung adenocarcinoma is the most common histological subtype of lung cancer and is highly prone to bone metastasis (1, 2). When sacral metastases invade the sacral canal, they often cause severe neuropathic symptoms including intense pain, motor and sensory dysfunction in the lower limbs, and even incontinence, significantly compromising the patient’s quality of life (3, 4). The management of such cases often poses significant therapeutic challenges. Due to the unique anatomical constraints, while surgical resection serves as a crucial approach for relieving neural compression and achieving radical tumor removal, it is associated with substantial trauma and carries high operative risks, severely limiting its applicability (5). The procedure entails specific primary complications, including neural injury, massive hemorrhage, and disruption of pelvic stability. Local radiotherapy is a key modality for symptom palliation and tumor control. However, given the radiosensitivity of the neural canal invasion, sciatic nerve, and adjacent critical organs (rectum and bladder), conventional external beam radiotherapy imposes stringent dose constraints on neural structures (typically 45–50 Gy) due to concerns about radiation-induced neural injury (6). This constraint significantly limits the escalation of the radiotherapy prescription dose, potentially resulting in insufficient dose coverage to the tumor target volume. Consequently, it compromises the local control of tumors invading neural structures, particularly for radioresistant tumors. Systemic therapies (chemotherapy, targeted therapy) often provide only slow and incomplete relief for locally advanced compressive lesions (7). In recent years, low-dose-rate (LDR) brachytherapy, particularly Iodine-125 (I-125) radioactive seed implantation, has demonstrated unique advantages in the comprehensive management of tumors. I-125 seeds continuously emit low-energy gamma rays, creating a high-dose radiation zone within the tumor tissue. Concurrently, due to the rapid dose fall-off with distance, the rapid dose fall-off may help limit irradiation of adjacent normal tissues, although highly localized high-dose regions may occur near implanted seeds (8). The dose-rate characteristics and spatiotemporal dose distribution of this modality fundamentally differ from those of conventional external beam radiotherapy. The dose-rate characteristics and spatial dose distribution of permanent iodine-125 implantation differ substantially from those of conventionally fractionated external-beam radiotherapy. However, clinical evidence regarding dosimetry and neurological outcomes when iodine-125 seeds are implanted near sacral neural structures remains limited. This report describes CT-guided iodine-125 seed implantation in a patient with lung adenocarcinoma and a symptomatic sacral metastasis invading the neural canal. The purpose of this report is to present the technical procedure, exploratory postoperative dosimetry, and short-term clinical and neurological outcomes. The findings are intended to generate hypotheses rather than establish neural tolerance thresholds. In this case, the implanted seeds were positioned immediately adjacent to the sacral nerves. This report describes the use of CT-guided iodine-125 seed implantation in a patient with lung adenocarcinoma and a symptomatic sacral metastasis directly invading the neural canal. The implanted seeds were located close to the involved sacral nerve roots, resulting in high calculated physical doses within the contoured neural region. The patient experienced pain relief, improvement in neurological function, and local disease control during the available 12-month follow-up, without new or worsening clinically apparent neurological deficits attributable to the treatment. The purpose of this report is to present the technical procedure, postoperative dosimetric findings, and short-term clinical outcome. This observation should be regarded as hypothesis-generating and does not establish long-term neural tolerance or justify modification of existing dose constraints.

2. Case report

2.1. Case presentation

Patient: Male, 65 years old, smoking history of 40 pack-years (quit 5 years ago). In January 2023, he presented with a 2-month history of paroxysmal irritating dry cough and exertional dyspnea, without hemoptysis or fever. Physical examination: diminished breath sounds in the right upper lobe, otherwise unremarkable. Imaging (chest PET/CT) revealed multiple space-occupying lesions in the right lung, accompanied by multiple pulmonary metastatic foci. Whole-body PET/CT imaging further confirmed an osteolytic destructive lesion in the sacrum, suggestive of metastasis. CT-guided percutaneous needle biopsies of the lung lesion and the sacral lesion were performed. Pathology showed poorly differentiated adenocarcinoma (Figure 1). Immunohistochemistry (IHC): NapsinA (+), TTF-1 (+), C-erbB2 (-), P53 (+, mutant pattern expression), Ki-67 (approximately 10%+). Final diagnosis: Adenocarcinoma of the right lung, cT4N2M1 (Stage IV), with sacral metastasis.

Figure 1.

HE staining findings are indicative of poorly differentiated adenocarcinoma. (A) Glandular architecture appears abnormal, characterized by glandular confluence and infiltrative borders, distinct from classical acinar patterns. The complex glandular subtype exhibits more disorganized glandular structures; (B)Tumor cells demonstrate significant nuclear atypia, featuring pleomorphism with marked variation in nuclear size and contour.

2.2. Treatment course

Following the diagnosis, the patient received systemic chemoradiotherapy beginning in February 2023, consisting of radiotherapy (30 Gy in 10 fractions) combined with pemetrexed plus carboplatin. After two cycles, the overall response was assessed as stable disease. However, the sacral lesion progressed clinically during treatment, with persistent and worsening pain in the left gluteal region radiating to the left lower extremity, accompanied by numbness and progressive motor weakness. On examination, the patient had severe sacral tenderness, sensory impairment in the L5-S1 distribution, reduced left ankle dorsiflexion and plantarflexion strength, and difficulty walking independently. MRI and CT confirmed enlargement of the sacral metastasis with marked compression of the sacral plexus and sciatic nerve roots (Figure 2). Next-generation sequencing of the sacral biopsy specimen revealed an EGFR exon 21 L858R mutation, and chemotherapy was subsequently replaced by icotinib. Despite 4 weeks of targeted therapy combined with analgesic treatment, the patient’s pain and neurological dysfunction did not improve.

Figure 2.

MRI and CT images of the sacral metastasis before iodine-125 seed implantation. (A–D) Coronal MR and CT images; (E–H) Axial MR and CT images; (I–L) Sagittal MR and CT images. Arrows indicate the sacral metastatic lesion with invasion of the sacral canal and compression of the sacral plexus/sciatic nerve.

2.3. Preoperative planning and CT-guided iodine-125 seed implantation

Because the sacral lesion represented symptomatic oligoprogression with progressive pain and neurological dysfunction, local salvage treatment was considered necessary. Surgical resection was deemed unsuitable because of the complex anatomy of the sacral region and the high risk of injury to adjacent critical neurovascular structures. External beam radiotherapy (EBRT), although a standard palliative option, was considered suboptimal in this setting because dose escalation was constrained by the tolerance of nearby organs at risk, particularly the rectum and bladder, while the large tumor burden and delayed analgesic onset further limited its expected benefit. After multidisciplinary team discussion, CT-guided iodine-125 seed implantation was selected as the optimal local treatment strategy.

Thin-slice CT imaging data of the patient’s sacral region with a slice thickness of 1.25 mm were imported into a three-dimensional treatment planning system (TPS) for preoperative planning. The tumor target volume was delineated collaboratively by interventional radiologists and medical physicists, while the rectum and bladder were contoured as organs at risk. Based on TPS optimization, the prescribed dose was set at 140 Gy. The finalized plan required implantation of 84 iodine-125 seeds, each with an activity of 0.5 mCi, to achieve conformal dose coverage, with more than 90% of the target volume receiving the prescription dose while maintaining doses to the rectum and bladder below safety thresholds (Supplementary Figure 1).

With the patient in a prone position, local anesthesia was administered, and the implantation procedure was performed under 5-mm CT guidance (Figure 3). Using an 18-gauge coaxial trocar and a seed implantation device, the iodine-125 seeds were implanted into the sacral lesion with a three-dimensional oblique puncture technique according to the preoperative TPS plan.Intraoperative CT was used to guide needle placement and seed deposition throughout the procedure. No intraoperative or immediate postoperative complications, such as hemorrhage or infection, were observed.

Figure 3.

CT-guided iodine-125 seed implantation. (A–D) Axial CT images; (E–H) Coronal CT images; (I–L) Sagittal CT images.

2.4. Postoperative verification and exploratory dosimetric analysis of the anatomically estimated neural region

After implantation, postoperative CT images were imported into the TPS for dosimetric verification (Supplementary Figure 2). The postoperative analysis confirmed that the actual dose distribution was consistent with the preoperative plan. Post-implant dosimetry was performed to evaluate target coverage and the calculated doses to the rectum, bladder, and anatomically estimated neural region. Post-implant CT-based analysis showed that the minimum distance between the implanted seeds and the expected anatomical location of the nearest sacral nerve root was approximately 3 mm. Because MRI–CT fusion was not performed and the neural structures were not directly visualized, the following dose values were derived from an anatomically estimated neural region and should be interpreted as exploratory estimates rather than precise measurements of the dose to the sacral plexus. The calculated mean dose and maximum voxel dose within this estimated region were 316 Gy and 907 Gy, respectively. The maximum dose represented a highly localized voxel-based value close to an implanted seed and may not reflect the dose received by a clinically meaningful neural volume.

Because MRI–CT image fusion was not performed, the individual sacral nerve roots and sacral plexus could not be directly and completely visualized on the postoperative CT images. For the exploratory neural dosimetric analysis, an anatomically estimated neural region was retrospectively delineated on the postoperative CT images according to the expected course of the sacral nerve roots within the sacral canal and neural foramina and their relationship to the adjacent osseous landmarks. The contour was reviewed by an interventional radiologist and a radiation oncologist. Therefore, this contour represented an anatomically estimated neural region rather than a directly visualized or MRI-confirmed sacral plexus contour. All treatment plans were generated using the domestically developed Feitian TPS, specifically its Feitian Brachy v3.00.00 module for brachytherapy optimization.

2.5. Results and follow-up

At 2 weeks postoperatively, the patient reported significant pain relief, with the Visual Analogue Scale (VAS) score decreasing to 3. By 1 month postoperatively, the pain had essentially resolved (VAS 1), allowing discontinuation of analgesic medications. Lower extremity numbness diminished, muscle strength improved to grade 4, and the patient was able to ambulate independently without crutches.The patient underwent regular follow-up assessments. Treatment efficacy was evaluated using PET/CT at 3, 6, and 12 months postoperatively. Imaging findings revealed stable seed distribution. A distinct photopenic defect was evident in the implanted core region, with the maximum standardized uptake value (SUVmax) significantly decreased to background levels, suggesting a marked metabolic response (Figure 4). The lesion volume remained stable, decreasing from 7.84 cm × 7.24 cm × 4.11 cm to 5.47 cm × 4.18 cm × 3.40 cm. No radiographic evidence of local progression was observed during the 12-month follow-up (Figure 5). At the last follow-up (12 months postoperatively), the sacral lesion remained well-controlled locally. Neuropathic pain had not recurred. Muscle strength and sensory function in the lower extremities were stable. The patient could independently perform activities of daily living. Overall quality of life had significantly improved compared to the preoperative state.

Figure 4.

PET/CT images obtained at the 12-month follow-up after iodine-125 seed implantation. Follow-up PET/CT showed a marked reduction in FDG uptake within the treated lesion to approximately background level, suggesting a marked metabolic response. (A, D, G) Coronal fused images; (B, E, H) Sagittal fused images; (C, F, I) Axial fused images.

Figure 5.

CT follow-up results. (A–D) Coronal CT images; (E–H) Axial CT images; (I–L) Sagittal CT images.

3. Discussion

This case report presents a clinical treatment approach for sciatic nerve compression caused by sacral metastasis from advanced lung adenocarcinoma. Despite the patient receiving systemic chemoradiotherapy and targeted therapy, progression of the local lesion led to progressive compressive neuropathy, creating a therapeutic dilemma (9, 10). Within this challenging context, the CT-guided percutaneous iodine-125 radioactive seed implantation technique we employed achieved satisfactory analgesia and functional recovery. This discussion aims to delve into the therapeutic decision-making logic for this case and explore the potential clinical role of iodine-125 seed implantation in such complex scenarios. Limitations of Conventional Therapies and the Therapeutic Dilemma in this Case: Although surgical decompression represents the most direct theoretical approach for relieving nerve compression, its application in the sacral region is severely limited. The complex anatomy of the sacrum, adjacent to critical structures including the sacral plexus, sciatic nerve, iliac vessels, and rectum, poses significant challenges. Radical resection not only entails substantial trauma but also carries an substantial risk of permanent nerve damage (resulting in bowel and bladder incontinence, lower limb dysfunction), intraoperative massive hemorrhage (particularly from injury to the presacral venous plexus), and pelvic instability (11, 12). For this patient with advanced cancer, the devastating impact on quality of life imposed by such an extensive surgical procedure rendered it an unacceptable option. Although the patient had received standard external beam radiotherapy (EBRT) and targeted therapy, the lesion continued to progress. This was attributable to the dosage constraints of EBRT: to protect critical organs such as the spinal cord, small bowel, bladder, and rectum, the radiation dose deliverable to the sacral region has an upper limit. This dose may be insufficient to achieve complete tumor eradication in bulky lesions (13). Second, tumor heterogeneity and drug resistance: Advanced tumors exhibit spatial and temporal heterogeneity. The sacral metastasis likely harbored a subclone of tumor cells resistant to the targeted therapy, leading to its progression as an “outlier” despite reasonable systemic disease control elsewhere (14). This underscores the urgent need for a treatment modality capable of overcoming resistance and providing higher rates of local control. Within the therapeutic dilemma of this case, iodine-125 seed implantation proved instrumental in resolving the impasse. Its success hinges on several core mechanisms and advantages: First, iodine-125 seeds continuously emit low-energy gamma rays, delivering cumulative radiation damage to tumor cells over several months. This continuous low-dose-rate irradiation effectively eliminates hypoxic cells resistant to conventional external beam radiotherapy and disrupts tumor cell repopulation. Consequently, it achieves superior radiobiological effectiveness, overcoming the dose limitations inherent in external radiotherapy (15, 16). Second, utilizing CT-guided three-dimensional treatment planning system (TPS), we precisely implanted the seeds within the irregularly shaped sacral lesion. This achieved a highly conformal dose distribution, effectively concentrating high-dose radiation precisely within the irregular sacral lesion (17). Consequently, it maximally spared adjacent critical structures including the rectum, bladder, and uninvolved neural tissues—a level of organ preservation that surgical resection and external beam radiotherapy (EBRT) cannot match. Third, the fundamental therapeutic principle of this technique lies in inducing tumor necrosis, absorption, and fibrosis through potent radiation effects, leading to a reduction in lesion volume (18). In this patient, iodine-125 seed implantation was followed by pain relief, improvement in motor function, and radiographic local disease control. However, because the patient had also received previous external-beam radiotherapy, systemic chemotherapy, icotinib, and analgesic treatment, the observed clinical improvement cannot be attributed solely to iodine-125 seed implantation.

The calculated physical doses within the anatomically estimated neural region were substantially higher than dose values commonly discussed for conventionally fractionated external-beam radiotherapy. However, direct numerical comparison between these modalities is inappropriate. Permanent iodine-125 implantation delivers continuous low-dose-rate irradiation with radioactive decay and a highly heterogeneous spatial dose distribution. In addition, the biological effect depends on multiple uncertain parameters, including dose rate, repair kinetics, irradiation duration, tissue-specific α/β ratio, and the volume of neural tissue exposed. Therefore, the high calculated physical doses in this case should not be interpreted as biologically equivalent to the same numerical doses delivered using external-beam radiotherapy.

The interpretation of the reported neural doses is also limited by dosimetric and anatomical uncertainties. The maximum dose represented a single-voxel located close to an implanted seed and may not reflect the dose received by a clinically meaningful neural volume. Furthermore, the small size and limited visibility of the sacral nerve roots and plexus on CT may introduce contouring uncertainty. Volume-based parameters would provide a more clinically meaningful interpretation; however, these parameters were not available in the retrospective treatment record. Radiation-induced peripheral neuropathy may occur after a prolonged latent period and may be progressive. Consequently, the absence of new clinically apparent neurological deficits during 12 months of follow-up does not establish long-term neural safety. The lack of standardized prospective neurological assessments, electrophysiological testing, and longer follow-up further limits the ability to exclude subclinical or delayed neural injury.

The clinical improvement observed in this patient should also be interpreted in the context of concurrent and previous treatments. The patient received external-beam radiotherapy, systemic chemotherapy, icotinib, and analgesic therapy. Although symptoms had continued to progress before implantation and improved shortly thereafter, the contribution of concomitant systemic treatment cannot be completely excluded.

This report has several limitations. First, it describes a single patient and cannot establish a dose–toxicity relationship. Second, follow-up was limited to 12 months. Third, contouring of the neural structures and interpretation of point maximum doses were subject to uncertainty. Fourth, standardized longitudinal neurological examinations and electrophysiological assessments were not available. Finally, radiobiological equivalence between permanent low-dose-rate brachytherapy and external-beam radiotherapy could not be reliably determined. Therefore, the findings should be considered hypothesis-generating rather than evidence for redefining neural dose constraints.

4. Conclusion

CT-guided iodine-125 seed implantation was technically feasible and was associated with pain relief, improvement in neurological function, and local disease control during the available 12-month follow-up in this patient with sacral metastasis and neural canal invasion. No new or worsening clinically apparent neurological deficits attributable to the treatment were observed during this period.However, the reported neural dose values were derived from an anatomically estimated neural region on postoperative CT rather than from directly visualized or MRI-confirmed neural contours. In addition, the biological effects of permanent low-dose-rate iodine-125 irradiation cannot be directly inferred from physical dose values or compared numerically with conventional external-beam radiotherapy dose constraints. Therefore, this single case should be regarded as a hypothesis-generating clinical observation and cannot establish long-term neural safety, define a dose–toxicity relationship, or justify modification of existing neural dose constraints. Further studies with standardized neurological assessments, volume-based dosimetric analysis, MRI-based neural delineation, larger patient cohorts, and longer follow-up are required.

Funding Statement

The authors declared that financial support was received for this work and its publication. This work was supported by the 960th Hospital of PLA.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Ethics statement

The studies involving humans were approved by Institutional Ethics Committee of the 960th Hospital (Approval No.: KYLL-2025-124). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

Author contributions

QS: Conceptualization, Data curation, Writing – original draft, Writing – review & editing. XP: Methodology, Writing – original draft. SL: Project administration, Writing – original draft. ZS: Project administration, Writing – review & editing. BT: Data curation, Writing – original draft. HY: Resources, Writing – original draft. CZ: Resources, Writing – original draft. YH: Investigation, Writing – original draft. XM: Formal analysis, Project administration, Writing – original draft, Writing – review & editing. ML: Conceptualization, Investigation, Software, Writing – original draft, Writing – review & editing.

Conflict of interest

The authors declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The authors declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fonc.2026.1773723/full#supplementary-material