ESTRO 2023 - Abstract Book

S1809

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ESTRO 2023

radiotherapy. CTCAE v5.0 was used to evaluate acute GU and GI toxicity and late rectal morbidity was classified by the RTOG toxicity scoring system. Results Mean age at diagnosis was 74 years (range 53-86). The mean prostate volume was 46.02 cc (range, 20-102 cc). In 81 patients, hydrogel spacer was successfully injected, in 20 patients SpaceOARTM and in 69 SpaceOAR VueTM. After insertion, 55 patients received treatment with SBRT and 26 patients hypofractionated EBRT. The most frequent reason for not performing the procedure was failure to hydrodissect. The hydrogel spacer created a mean separation between the prostate and rectum of 11.45 mm (5.20 – 17.70 mm), with a mean width of 33.94 mm (22.00-56.10 mm) and length of 39.94 mm (13.50 mm – 62.82 mm). Rectal sparing was calculated for SBRT with a mean rectal V32, V28 and V18 of 2.81%, 4.89% and 14.21%, respectively and for hypofractionated EBRT with a mean rectal V40, V48, V52 V56 and V60 of 18.20%, 12.14%, 9.53%, 6.88% and 3.75%, respectively. Acute rectal toxicity was G1 in 2.5% and G2 in 1.2% and GU toxicity was G1 in 38.2% and G2 in 13% of patients. Late rectal toxicity was G1 in 3.7%, toxicity ≥ G2 was not reported. Conclusion Use of hydrogel spacers is safe in patients with intermediate-risk prostate cancer undergoing external beam radiotherapy and can reduce rectal doses with low-risk of rectal morbidity. Longer follow-up to better assess late toxicity and prospective data are needed to evaluate impact of rectal spacers in patient outcomes. Purpose or Objective PerFRACTION is a patient treatment quality assurance software that evaluates the daily treatment delivery of external beam radiotherapy. PerFRACTION calculates the delivered dose on the planning CT or a CBCT, using the treatment logs . The calculated dose is compared with the TPS dose distribution using different gamma index criteria. Users can set tolerances, based on the passing rate, to define failing fractions. This work aims to analyze PerFRACTION results for plans from five treatment sites delivered in a Halcyon (Varian Medical Systems) linac. Materials and Methods The daily PerFRACTION calculated data during five months were retrospectively analyzed. Treatment plans were calculated in Eclipse with AAA (v.15.6) algorithm, and delivered on a Halcyon using 6 MV FFF photon beams. Data were extracted from the PerFRACTION database with a SQL script. Only fractions calculated using a CBCT were considered. Lung, breast, prostate, rectum and gynecologic plans were studied with 352, 911, 550, 223 and 165 fractions, respectively. The passing rate of the PTV was evaluated with three global gamma index criteria: 3 %/3 mm, 5 %/3 mm and 5 %/5 mm with a 20 % threshold. The percentage of fractions with passing rate below 90 % was determined, considering them as failing fractions. The passing rate for all treatment fractions was studied to identify possible temporal trends, evaluating separately lung treatment replannings. Results In lung treatment plans, the percentages of failing fractions were 68.2 %, 18.5 % and 9.4 % for criteria 3 %/3 mm, 5 %/3 mm and 5 %/5 mm, respectively. The average passing rate of each criteria was 80 %, 94.6 % and 96.7 %. A 54 % of lung treatment plans required a replanning. Figure 1a shows the passing rate trend for replanned plans. Failing fractions were related to anatomical changes in the PTV and the lung. The average passing rate was 95 %, 88 % and 99 % at the end of the first, second and third weeks, respectively. Non-replanned plans showed a constant trend (Figure 1b). PO-2038 Analysis of daily treatment fractions using PerFRACTION on a Halcyon linac S. Pie-Padro 1 , P. Navarro-Palomas 1 , D. Sánchez-Artuñedo 1 1 Hospital Universitari Vall d'Hebron, Servei de Física i Protecció Radiològica, Barcelona, Spain