Treatment of Rectal Mucosal Melanoma: Role of Radiotherapy
Surgery remains the primary treatment for rectal mucosal melanoma, with radiotherapy reserved for adjuvant treatment after incomplete resection, palliation of unresectable disease, or as part of multimodal therapy for recurrent disease. 1
Primary Treatment Approach
Wide local excision is the preferred surgical approach over radical abdominoperineal resection, as studies demonstrate comparable survival outcomes with significantly reduced morbidity and better quality of life. 1 Historically, radical resections were performed but resulted in high rates of post-operative complications without survival benefit. 1
Important Distinction from Rectal Adenocarcinoma
The provided guidelines 2, 3, 4, 5 address rectal adenocarcinoma, not mucosal melanoma. These are fundamentally different diseases with distinct biology and treatment paradigms:
- Rectal adenocarcinoma benefits from preoperative radiotherapy (25-50.4 Gy) to reduce local recurrence 2, 4
- Mucosal melanoma behaves as a systemic disease from onset, making local control strategies less impactful on survival 1
Radiotherapy Indications for Rectal Mucosal Melanoma
Adjuvant Setting
- Post-operative radiotherapy should be administered after incomplete surgical resection or when adverse pathologic features are present 1
- Hypofractionated regimens are preferred for melanoma due to radiobiological properties 6, 1
Palliative Setting
- Radiotherapy provides transient palliative benefit for anorectal melanoma when surgery is not feasible 7
- Consider for symptomatic local control in patients with unresectable disease or those refusing surgery 6
Recurrent Disease
- For pelvic recurrence, surgical resection followed by hypofractionated radiotherapy is a reasonable approach 1
- One case series demonstrated successful management of recurrent disease with surgery followed by radiation therapy and systemic immunotherapy 1
Radiotherapy Technique Considerations
Large fraction sizes (≥400 cGy per fraction) achieve higher complete remission rates compared to conventional fractionation for mucosal melanomas. 7 In head and neck mucosal melanomas, 6 of 7 patients treated with ≥400 cGy fractions achieved complete remission versus only 5 of 18 with smaller fractions. 7
Practical Fractionation Options
- Hypofractionated regimens are biologically more effective for melanoma 6, 1
- Low-dose radiotherapy (5.6 Gy × 4 fractions) has shown promise when combined with immunotherapy in metastatic melanoma 8
Critical Caveats
Anorectal melanoma is frequently systemic at presentation, making local control measures less impactful on overall survival. 1 The complete remission rate with radiotherapy alone is approximately 72% locally, but 7 of 18 patients achieving complete remission subsequently relapsed. 7
Radiotherapy alone as primary treatment carries poor salvage rates - only 1 of 7 patients who failed primary radiotherapy was successfully salvaged with subsequent surgery. 7 This contrasts with the better outcomes when radiotherapy is used adjuvantly after surgery.
Integration with Systemic Therapy
Multi-agent systemic therapy should be incorporated as anorectal melanoma behaves as a systemic disease. 1 Emerging evidence suggests combining radiotherapy with immunotherapy (checkpoint inhibitors or adoptive T-cell therapy) may enhance clinical benefit. 8
The overall treatment goal must prioritize quality of life and tumor control while minimizing treatment-related morbidities, given the poor prognosis of this disease. 1