ESTRO 2021 Abstract Book

S1287

ESTRO 2021

Shuttle with CBCT control twice a week. The data of 911 sessions of radiation therapy were analyzed. Offsets in each direction (vertical, longitudinal, lateral) were determined by analyzing the systematic and random components of displacements according to van Herk method. Relations between the magnitude of interfractional displacements and the position of the Belly Board aperture relative to the bone structures of the small pelvis were also examined. Additionally, the correlation of body mass index and gender of patients with position reproducibility were analyzed too. Results In case of supine positioning the calculated margin values were: 0.64 cm in the vertical direction, 0.46 cm in the longitudinal and 0.41 cm in the lateral directions. For patients positioned on the Belly Board device the margin values were 1.07 cm in the vertical direction, 0.72 cm in the longitudinal and 0.74 cm in the lateral directions. The resulting margin values ensure that 95% of patients receive the prescribed dose in the absence of daily CBCT control. An excess of 1 cm PTV margin for patients prone-positioned on the Belly Board in all directions was observed in 86% of cases. For 14% of patients this trend was not observed, while the BMI index of each of them exceeded 25 For female patients, margins in the longitudinal and lateral directions are smaller than for male patients (0.66 and 0.62 compared to 0, 82 and 0.78 respectively), in the vertical direction the results are close. Conclusion Results for prone Belly Board positioned patients lead to either increasing the PTV margin to 1 cm in the vertical and 0.7 in the longitudinal and lateral directions respectively, while maintaining the current CBCT control schedule (twice a week), or to choosing the daily CBCT control while maintaining recommended by RTOG 0822 PTV margin 0.5 cm. However, as the PTV margin increases, the radiation exposure to normal tissues also increases. Protocol for patient immobilization using Belly Board device with daily CBCT was developed and used. PO-1563 A dose-escalation for early-regression based ART for rectal cancer: a planning feasibility study A. Cicchetti 1 , C. Fiorino 2 , P. Passoni 3 , P. Mangili 2 , N. Slim 3 , A. del Vecchio 2 , N.G. Di Muzio 3 , S. Broggi 2 1 IRCCS San Raffaele Scientific Institute, Medical Physics, Milan, Italy; 2 IRCCS San Raffaele Scientific Institute, Medical Physics, milan, Italy; 3 IRCCS San Raffaele Scientific Institute, Department of Radiation Oncology, milan, Italy Purpose or Objective Adaptive RT (ART), boosting the residual tumor after early regression, is used at our Institute since 2009 in the neoadjuvant treatment of rectal cancer pts. New protocol feasibility further escalating the tumor dose on pts with favourable pCR prediction (based on a previously validated early regression index, XXXX et al)) was here tested in a planning investigation. Materials and Methods Twenty pts previously treated with ART between 09/15 and 12/18 were considered to simulate the new protocol. They underwent two Oxaliplatin cycles (days -14 and 0), while a continuous fluoropyrimidines infusion was delivered from day -14 to the end of RT. The original RT schedule consisted of 41.4Gy (2.3Gyx18 fractions) to mesorectum and regional pelvic lymph- nodes. After a mid-course MRI scan (at the 9th fraction), a residual GTVboost was delineated. This structure was defined as the whole rectum portion included in the tumour's craniocaudal limits still visible on the RM images. Margins of 5mm were added to obtain the corresponding PTVboost, which was treated on the last 6 fractions with a simultaneous integrated boost of 3.1 Gy/fraction. The new ART scheme (SuperART) consisted of a further dose escalation during the same last 6 fractions (4Gy/fr) to the effective residual tumor plus a margin of 5 mm (PTVsuperboost). SuperART plans were compared against ART plans. Particularly, DVHs (converted to EQD2, a/b=5) were computed for anorectum, anal canal, bladder and small bowel. Moreover, EUD-based NTCP from published models of toxicity was defined for every patient and organ. Clinical endpoints were late rectal bleeding (QUANTEC, Michalski IJROBP 2010), fecal incontinence (Peeters IJROBP 2006), urinary incontinence (XXX et al) and bowel acute obstruction/perforation (Burman IJROBP 1991), respectively. Results Table1 shows average EUDs on the study group, intra-patients EUD variations and NTCP increased risk for the studied organs. The structure mostly affected is the anorectum, with 70% of pts with a 5% increased risk of grade 2 bleeding. However, its maximum absolute NTCP was 11.5% with mean values equal to 3.4 % and 8.9 % with ART and SuperART, respectively. Most of SuperART treatments for the other organs did not affect NTCP more than 3%. Figure1 depicts the average ART and SuperART DVHs: considering the importance of the ERT cranial-caudal extension on the planning performances, we divided patients under and over the average value (4cm).