Simultaneous Integrated Boost (SIB) Versus Sequential Boost in Anal Cancer Patients: A Single-Center Experience.

Purpose: Concurrent chemoradiation is the standard of care for the treatment of anal cancer. Radiation can be delivered by sequential or simultaneous integrated boost (SIB) approach. The present study was conducted to compare the treatment outcomes and toxicity profile of patients with anal cancer treated with sequential boost and SIB approach.

Methods: A single-institution retrospective analysis of patients with squamous cell carcinoma of the anal canal treated between 2019 and 2022 with radical chemoradiation was performed. The sequential boost schedule consisted of 45 Gy in 25 fractions (1.8 Gy daily) to the gross tumor, nodes, and elective nodal volume, followed by a 9 Gy in five fractions boost to the gross disease. Patients receiving SIB were treated as per RTOG 0529 protocol. In both the groups, patients were treated with volumetric modulated arc therapy (VMAT). The two groups were compared in terms of overall survival (OS), colostomy-free survival (CFS), relapse-free survival (RFS), and acute toxicity profile. p-values < 0.05 were considered statistically significant.

Results: The patient and disease characteristics in both treatment arms were comparable. The only difference was a significantly longer overall treatment time of ≥ 50 days in the sequential arm (77.8% vs 43.8%, p = 0.04). The median follow-up was 18 months. The 2-year CFS was 80% in sequential vs 87.5% at 2 years for the SIB arm, 2-year OS 83.3% vs 58.6%, and 2-year RFS was 38.9% vs 41.7%, respectively. A total of 14 (77.8%) in sequential and 8 (50%) in the SIB arm had disease relapse. On univariate analysis, the involved pelvic lymph node significantly affected OS (HR 10.45, p = 0.03) while inguinal lymph node involvement adversely affected RFS (HR 6.16, p = 0.02). The most common acute toxicity was radiation-induced dermatitis, 15 (83.4%; 5 grade II, 10 grade III) in sequential vs 7 (43.8%; 3 each grade II and III) in the SIB group followed by hematological (61.1% vs 68.75%). However, the incidence of overall acute toxicities was significantly less in the SIB arm (p = 0.006).

Conclusion: Our study showed that concurrent chemoradiation with the SIB-VMAT approach is well tolerated in patients of anal carcinoma and resulted in lesser treatment interruptions and comparable outcomes as compared to the sequential approach. Our results warrant further evaluation in a prospective study.