ESTRO 2022 - Abstract Book

S443

Abstract book

ESTRO 2022

1 Campus Bio-Medico University, Radiation Oncology, Rome, Italy

Purpose or Objective In the current study we analysed the dosimetric data on patients enrolled in a prospective phase II study on volume de- escalation in neoadjuvant chemoradiotherapy (CRT) of locally advanced rectal cancer (LARC) compared to an historical matched-control patient group. Materials and Methods This study included 103 patients with LARC (T2 low-lying/T3, N0-N1) and compared a prospective group with an historical group. Fifty-two patients were enrolled in the prospective study. De-escalated treatment volumes, due to the exclusion of elective nodal irradiation, were delineated. The CTV included the primary tumor and mesorectum with vascular supply containing the perirectal and presacral nodes. The PTV was defined by the CTV with a margin of 1 cm in all directions. Radiation therapy was delivered with a total dose of 50.4 (28fx/1.8Gy) using 3-dimensional conformal techniques. The control group (51 patients) received standard treatment. In both, concomitant fluoropyrimidine-based chemotherapy was associated. We analysed dose parameters of the main organs at risk (OARs) for the two groups (reduced vs standard volumes): small bowel (V5, V10, V15, V20, V30, V45), bladder (Dmax, V5, V10, V15, V35, V50) and femoral heads (V52). Treatment interruptions ( ≥ 5 days) due to adverse events were the primary clinical endpoint of this analysis. The dosimetric variables were compared using the independent sample t-test. A 2-sided p value of 0.05 was considered significant. Results The mean volume of de-escalated PTV was 495.5cc ± 169.8cc (range, 231.0cc– 920.3cc) compared with standard PTV (mean 1055.37cc ± 204.44cc; range, 620.6cc – 1696.9cc). The V5, V10, V15, V20, V30, V45 of small bowel were significantly lower in the reduced volumes group (mean 170.9cc, 131.8cc, 54.1cc, 35.2cc, 25.5cc, 13.4cc respectively), in comparison with the standard group (mean 531.3cc, 429.4cc, 302.0cc, 231.8cc, 139.3cc, 60.2cc respectively; p<0.001). The Dmax, V5, V10, V15, V35, V50 of the bladder were also significantly lower in the experimental group (mean 65.7Gy, 94.1%, 90.4%, 65.3%, 13.1%, 2.5% respectively) than in the control group (mean 55.9Gy, 100.6%, 99.2%, 94.1%, 56.7%, 6.6% respectively; p<0.033). Moreover, the left and right femoral heads V52Gy was lower in the first group (p=0.042). Treatment interruptions due to gastrointestinal toxicity in the two groups was 11.5% vs 41.1%, respectively (p=0.001). Conclusion This is the first study to show significant dose reduction for OARs given by the reduction of target volume in LARC compared to the historical group, while improving clinical outcomes as shown in the final report of the study protocol. This is probably related to the biological effect associated with the low number of interruption days and to the early stage of the selected population. A. Castelluccia 1 , D. Marchesano 1 , G. Grimaldi 2 , I. Annessi 1 , F. Bianciardi 1 , C. Borrazzo 1 , A. Di Palma 1 , R. El Gawhary 1 , M. Masi 1 , M. Rago 1 , M. Valentino 1 , L. Verna 1 , P. Gentile 1,2 1 San Pietro Fatebenefratelli Hospital, Radiation Oncology, Rome, Italy; 2 UPMC San Pietro FBF, Radiation Oncology, Rome, Italy Purpose or Objective Short-course irradiation reduces the risk of local recurrence and showed overall survival improvement with a lower rate of early toxicity when compared to chemoradiation. Previous studies demonstrated that radiotherapy dose of neoadjuvant treatments is a significant predictor of achieving a local response. The purpose of this study is to measure the effects of stereotactic MR-guided adaptive radiotherapy (SMART) for rectal cancer patients in terms of early toxicity, radiological and pathological response . Materials and Methods Patients diagnosed with local advanced rectal cancer with positive lymph node staging, resectable (cT3 with > 5 mm extramural invasion and uninvolved MRF) or unresectable unfit for chemotherapy, underwent SBRT on rectal lesion and mesorectum with patological nodes using hybrid MR-Linac (MRIdian ViewRay). Treatment prescription at 80% isodose for the rectal lesion and mesorectum with pathological nodes was 40Gy (8Gy/fr) and 25 Gy (5Gy/fr),respectively, delivered on 5 days (3fr/week). The gross target volume (GTV) was identified on a true fast imaging (TRUFI) MR scan acquired during simulation and prior to each fraction to adapt the treatment plan of the day. New plans were calculated and delivered every fractions because of rectal and bowel motion. An intrafraction motion management strategy was applied, consisting of a gating approach based on the real-time acquisition of a sagittal cine MRI during the whole delivery time (temporal resolution: 8 frames/s). Response assessment by MRI was performed 3 weeks after SMART, than patients fit for surgery underwent total mesorectal excision. Adverse effects of RT were scored according to the NCI Common Toxicity Criteria (CTC) scale, version 5.0. Results Twenty patients underwent rectal SMART. No G3-5 toxicity was recorded. A moderate or good radiological response (mrTRG ≤ 3) was observed in all of this patients. Twelve patients were elegible for total mesorectal excision. Mean interval between the completion of SMART and surgery was 4 weeks. Pathological downstaging occurred in all patients, 2 patients achieved complete response (pCR). pCR occured with a prolonged time to surgery (>7 weeks). Patients baseline characteristics, pathological stage and response are listed in Table 1. PD-0495 Stereotactic MR-guided Adaptive RT for rectal cancer: toxicity, radiological & pathological response