3548 - Outcomes of Re-Irradiation with Pencil Beam Scanning Proton Therapy for Rectal Cancer Following Prior Pelvic Radiation: Toxicity, Recurrence, and Symptom Improvement

02:30pm - 03:45pm PT

Hall F

Screen: 5

POSTER

Presenter(s)

Austin Thompson, MD - University of Maryland Medical Center, Baltimore, MD

A. D. Thompson¹, A. Koroulakis^2,3, A. Patel⁴, J. K. Molitoris⁵, W. F. Regine Jr⁵, M. A. L. Vyfhuis⁵, S. P. Kanani⁶, D. Kunaprayoon⁵, and M. Kim⁶; ¹Department of Radiation Oncology, University of Maryland Medical Center, Baltimore, MD, ²Department of Radiation Oncology, Anne Arundel Medical Center, Annapolis, MD, ³Annapolis Radiology Associates Radiation Oncology, Annapolis, MD, ⁴University of Maryland Radiation Oncology at the Tate Cancer Center, Glen Burnie, MD, ⁵Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, ⁶Radiation Oncology Dept, University of Maryland, Baltimore, MD

Purpose/Objective(s): Re-irradiation (re-RT) for rectal cancer after prior pelvic radiation therapy (RT) poses challenges due to toxicity risks and uncertain outcomes. While prior studies show adequate cancer control and toxicity profiles, limited data exist on using pencil beam scanning proton therapy (PBSPT) and deep-tissue hyperthermia (DTT). This study evaluates PBSPT’s impact on toxicity, symptom relief, and cancer outcomes, while assessing the influence of DTT, surgery, and chemotherapy.

Materials/Methods: A retrospective review was conducted on 69 patients treated with PBSPT for rectal cancer after prior pelvic RT. ?2 and Mann-Whitney U tests assessed demographic and treatment differences. Kaplan-Meier analysis and Cox regression models evaluated toxicity (CTCAEv5), recurrence, and overall survival (OS).

Results: The median follow-up was 23 months (mos), with a median age of 70 years and 59 mos between RT treatments. 59.4% had rectal cancer during their first RT course. 61.7% experienced symptom relief after RT, with higher rates observed in those undergoing treatment with curative intent (p=0.014) rather than palliative intent. Late low-grade toxicity (grade 1-2) was associated with surgery (p = 0.033), recurrent rectal cancer (p = 0.005) and RT dose of their first treatment (p = 0.007). 33.3% of patients had late toxicity (23.2% >grade 3), with a median onset of 12 mos (95% CI: 11-24). Three grade 5 events occurred (rectal bleed, bowel perforation, and small bowel obstruction). Median time to local and distant recurrence were 17 mos (95% CI: 12-31) and 10 mos (95% CI: 7-15), respectively. BID fractionation had lower rates of distant metastasis (p=0.0444). Median survival was 30 mos with a 2y OS of 70.9%. Worse performance status (p=0.012) and acute high-grade toxicity (p=0.0494) were associated with an increased risk of mortality. Surgery was associated with lower mortality (HR: 0.326, CI: 0.135-0.789, p=0.003). No statistically significant effects from DTT (n=21), concurrent (n=57) or adjuvant chemotherapy (n=38) were observed.

Conclusion: PBSPT for re-RT in rectal cancer following prior pelvic RT achieves acceptable toxicity, significant symptom relief, and favorable cancer outcomes. Curative-intent RT was associated with improved symptom relief without increasing toxicity. BID fractionation was associated with lower rates of distant metastasis, while surgery was associated with increased late toxicity but improved survival. DTT and chemotherapy did not improve outcomes or increase toxicity, though this may be underpowered due to limited sample size. These findings support the use of PBSPT in this challenging population, with further research needed to refine patient selection and treatment strategies.