ESTRO 2021 Abstract Book

S1325

ESTRO 2021

Figure 1 omparison of CCT for He70.4% and He78.8% The gamma pass rate for the C-Shape IMRT plan was 99.1% for tolerance criteria of 3%/3mm and 97.2% for 3%/2mm Conclusion Topping up Helium from 70.4% to 78.8% on the Elekta Unity MR Linac had minimal impact on overall dosimetry although max differences were on the order of 1% for limited gantry angles We gratefully acknowledge support and assistance by Jason Arts and Armia George, Elekta AB PO-1603 One more image A. Walewska 1 , P. Kukolowicz 1 1 Maria Sklodowska-Curie National Research Institute of Oncology, Medical Physics Department, Warsaw, Poland Purpose or Objective Due to the localization, size of the Planning Target Volume (PTV) and the need to use boluses, obtaining the correct position of Soft Tissue Sarcoma (STS) patients during radiotherapy is difficult to achieve. Patient’s repositioning is often needed several times before start irradiation. The purpose of this study was to check the effectiveness of repositioning using in vivo dosimetry. Materials and Methods Eleven STS patients were treated with IMRT and VMAT (Eclipse v.13.6, Varian). 25 Gy was delivered in 5 fractions with 6MV/15MV photon beams. In-vivo measurements with gafchromic films EBT3/EBT-XD (Ashland Inc.) were done for each patient and fraction. For each patient, the dose was measured under the bolus in 4 points. One, located on the intersection of the central axis and the skin (IP) and three points lying at least 5 cm away from IP (cranial, caudal and right of left). The results of the measurements were compared with the dose calculated with Treatment Planning System (TPS). The relative mean standard deviation (mSD) (normalized to dose calculated with TPS) was calculated for each patient and measurement point as well. To highlight the difficulty of the patient’s positioning, the total number of verification kV/MV (pairs of images) and CBCT acquired during the entire treatment was recorded for each patient. All treatment plans were checked with pre-treatment Portal Dosimetry. Results The uncertainty of a single in vivo measurement was 1,1% (1SD). Gamma analysis (3%/3mm) results of pre- treatment verification were >95% for all plans. The mean number of verification images per patient per fraction was: 1,1 (CBCT) and 3,3 (pair kV/kV or kV/MV); minimum: 1/1, maximum: 2/12 CBCT and orthogonal pair of images, respectively. The mean difference between measured and calculated dose for all patients and all measurement points was 2,7%. The maximum value of the relative mean standard deviation was 3,8%. Conclusion For any patient, the mean value (for all measurements) of the difference between in vivo dose measurements and TPS calculation did not exceed 4,3%. The difference between in vivo dose measurements and TPS calculation and the low value of the relative mean standard deviation for all patients provide a good quality of irradiation STS patients. It was possible thanks to a high repeatability patient’s treatment position obtained after the required repositioning based on verification imaging.

PO-1604 Risk evaluation for the radiotherapy treatment of a pregnant patient P. Weber 1 , G. Guibert 1 , T. Risse 1 , B. Yanes 1 , B. De Bari 1 1 RHNe, Radio-Oncology, la Chaux-de-Fonds, Switzerland

Purpose or Objective The head and neck postoperative radiotherapy treatment of a pregnant patient – scheduled between the pregnancy weeks 25 to 28, and indeed with an upside down fœtus position – needed a risk evaluation, and thus a decision to treat the patient. Materials and Methods A head and neck radiotherapy treatment, prescribed with 30x2Gy, has been planned on the Philips Pinnacle TPS with 2 VMAT arcs at 6MV for a pregnant patient. The simulation CT scanning region went from the head down to the liver, in order to calculate the scattered dose as close to the fœtus as possible, but avoiding