Short Course Radiotherapy (SCRT) with Simultaneous Integrated Boost (SIB) in the Treatment of Rectal Cancer: Feasibility and Early Toxicities

Abstract

To report the feasibility and early toxicities of dose-escalated SCRT in rectal cancer patients receiving total neoadjuvant therapy (TNT). This retrospective cohort study included 13 patients treated via a TNT approach (guided by RAPIDO trial) for locally advanced rectal cancer. 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 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 Varian EclipseTM (Palo Alto, CA) treatment planning system. Prior to treatment patients underwent kV and cone beam CT (CBCT) 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. Early toxicities were assessed for 30 days following treatment and graded using the Common Terminology Criteria for Adverse Events version 5.0. Offline image review was performed to determine the frequency at which the gross tumor volume (GTV) was captured within the 5 mm planning target volume (PTV_Mid) margin on daily CBCT scans. If a misalignment (tumor crossing outside of PTV_High) was discovered, the direction of misalignment (lateral/anterior/posterior) and location (superior/inferior) in relation to the superior border of the pubic symphysis was documented. An additional 5 mm margin from the PTV_Mid was created to determine if this would have captured the target. Twelve of the 13 patients were ≥ 30 days from treatment completion and were included in the toxicity analysis. The most common grade 1-2 toxicities were proctitis and diarrhea, which occurred in 42% and 25% of patients, respectively. Other toxicities were new onset rectal bleeding (17%), rectal pain (17%) and radiation dermatitis (8%). No grade 3+ toxicities were observed. Given five fractions for each of the 13 patients, a total of 65 CBCTs were reviewed and compared to the planning CT obtained at simulation. A total of 14/65 (22%) misalignments were discovered across six patients. All occurred above the superior edge of the pubic symphysis. Four misalignments were multi-directional. Nine were anterior, 6 were lateral, and 1 was posterior. Twelve of the 14 (86%) would have been covered within the additional 5 mm expansion volume. In this cohort of patients who received dose-escalated SCRT, the composite rate of acute toxicity was expectedly low. Feasibility of treatment delivery was established but could be improved upon with an addition 5 mm expansion, specifically in locations cranial to the superior edge of the pubic symphysis.