3046 - Short Course Radiotherapy with Simultaneous Integrated Boost in the Treatment of Rectal Cancer: Clinical and Pathological Response Rates

M. K. Melton¹, N. T. Pfister², M. Akce³, and R. Jacob¹; ¹University of Alabama at Birmingham, Birmingham, AL, ²Department of Radiation Oncology, Winship Cancer Institute of Emory University, Atlanta, GA, ³Winship Cancer Institute, Atlanta, GA

Purpose/Objective(s): To report early clinical and pathological response rates of dose-escalated short course radiotherapy (SCRT) in rectal cancer patients receiving total neoadjuvant therapy (TNT). Materials/

Methods: This prospective cohort study included 22 patients treated via a TNT approach for locally advanced rectal cancer. Prior to treatment, patients were clinically staged by MRI and case management was agreed upon in a multi-disciplinary setting. All patients were cT3/4 or node positive. They received SCRT followed by 6-9 cycles of systemic chemotherapy (XELOX/FOLFOX) and were evaluated for response within 4 weeks from end of chemotherapy. The planned radiation was a standard dose of 25 Gy in 5 fractions to the primary tumor and at-risk lymphatics (PTV_Low). The dose-escalation was achieved via SIB delivering 30 Gy to the primary tumor (PTV_Mid) and 40 Gy (PTV_High) to inoperable lateral pelvic lymph nodes (LN), when present, over 5 fractions. A 5-10 mm margin was added to the rectal tumor and lateral pelvic LN to create the SIB targets. All patients were planned for intensity modulated radiation therapy via a technology company treatment planning system. Prior to treatment patients underwent kV and Cone beam CT imaging for image guidance. Shifts were not performed solely to accommodate the boost target volume if it was felt to compromise the coverage of at-risk lymphatics. Normal tissue constraints were prioritized over boost coverage.
Results: Eleven of the 22 (50%) patients achieved a complete clinical response (cCR). Six (55%) of the 11 patients who achieved a cCR were offered active surveillance (AS), of which two chose resection, one failed 4.9 months from chemotherapy and received a margin negative (RO) salvage low anterior resection (LAR), and three remain on AS. In total, 8/11 patients with cCR proceeded to surgery, of which 3/8 (38%) achieved a pCR. Of the eleven patients who did not achieve a cCR, seven have gone to surgery, and one (14%) achieved a pCR. All 15 patients who underwent resection achieved RO resections, and sphincter preservation was achieved in 19/20 (95%) surgical candidates; only one patient required an abdominoperineal resection. Three of 3 patients who received SIB for lateral LN achieved a complete nodal response. No grade =3 radiation toxicities were observed. Six of 11 patients (55%) who had a cCR either demonstrated pCR on surgery, or continue to be on AS.
Conclusion: Dose-escalated SCRT is a safe alternative approach for carefully selected patients with rectal cancer receiving total neoadjuvant therapy, with high rates of cCR. Over 50% of patients who achieve cCR are likely to not require surgical intervention, though longer follow up is needed to confirm this. Further escalation of RT dose in a five-fraction regimen is likely necessary to improve cCR rates.

Abstract 3046 – Table 1: Response Rates

22 Patients	Initial Surgery	Salvage Surgery	pCR	Active Surveillance
11 cCR	7	1	3/8 (38%)	3
11 partial response	7		1 /7(14%)	0
Total:	14	1	4/15 (27%)