3800 - A Retrospective Analysis of Simultaneous Integrated Boost (SIB) to Close or Positive Parametrial Margins in Adjuvant Pelvic Radiotherapy for Endometrial Cancer
Presenter(s)
Z. Y. Yakub1, P. Patel2, K. L. Schnarr1, J. Sienna1, and E. Donovan1; 1Juravinski Cancer Centre, McMaster University, Hamilton, ON, Canada, 2McMaster University, Hamilton, ON, Canada
Purpose/Objective(s): Patients with close or positive parametrial margins following surgical resection for endometrial cancer are considered at increased risk of locoregional recurrence. Radiation oncologists employ clinical equipoise to manage margin status, with limited existing evidence that guides dose escalation. Advances in radiation planning and delivery have enabled the use of simultaneous-integrated boost (SIB) to escalate dose and treat residual microscopic disease in parametrial tissue. While this is a common practice, there are no guidelines that inform the dose fractionation of SIB, and there is limited data regarding toxicity and efficacy. We report institutional outcomes of adjuvant pelvic radiotherapy incorporating SIB for close or positive parametrial margins.
Materials/Methods: We conducted a retrospective review of patients treated between 2015–2025 who received adjuvant pelvic radiotherapy with SIB following definitive surgery for endometrial cancer. SIB was defined as >220 cGy per fraction to the parametrial margin. Close margins were defined as <5 mm. Outcomes included SIB dose, chemotherapy utilization, =grade 3 acute and late toxicity (CTCAE), local control, distant relapse, and overall survival (OS).
Results: There were 27 patients treated with an SIB for close (n=14, 51.9%), positive (n=12, 44.4%), or negative (n=1, 3.7%) margins. Total SIB doses were 50 Gy (n=9, 33.3%), 52 Gy (n=3, 11.1%), 54 Gy (n=8, 29.6%), and 55 Gy (n=7, 25.9%). Vaginal brachytherapy (VBT) doses were 5.5 Gy (n=13, 48.1%), 7 Gy (n=3, 11.1%), and 11 Gy (n=6, 22.2%); five patients (18.5%) did not receive VBT. Seventeen (63%) received adjuvant chemotherapy.
Median follow-up was 25 months (range, 5-73). Four of 27 patients (14.8%) developed grade 3 toxicity, all due to vaginal stenosis interfering with examination. No treatment-related =grade 3 bowel or bladder toxicities were observed. Three patients (11.1%) developed distant relapse. Local control was 100%. Overall survival was 88.9% at last follow-up.Conclusion: Adjuvant pelvic radiotherapy with SIB to close or positive parametrial margins was associated with excellent local control and low rates of severe non-vaginal toxicity when retrospectively analyzed. While control rates are excellent, further prospective studies may consider whether boosting patients is necessary at all. Future studies comparing individuals treated with or without SIB are needed to clarify its incremental benefit, inform dosing and evidence-based practice, and better characterize long-term toxicity.