ESTRO 2021 Abstract Book

S1582

ESTRO 2021

been manually optimized for PTV coverage and maximum OAR doses. Additional mean dose constraints applied to eyes, lenses and pituitary. All 20 patients were retrospectively replanned using the original beam setup and the DVH estimation model. The resulting dose distributions were evaluated in terms of PTV coverage and mean dose to critical structures. Plan complexity was taken into account by comparing monitor units (MUs). Results The knowledge-based replanning resulted in improved dose distributions overall. Most of the critical structures received a lower mean dose (12 structures per patient on average; range 8-14). The remainder received a higher mean dose (2 structures per patient on average; range 0-5). The mean difference in mean dose after replanning was -21%. A large variation between patients was observed here (-4% to -35%). Sorting by structure, the pituitary displayed the largest shift in absolute mean dose (-6.8 Gy), followed by the left optic nerve (-6.1 Gy) and the optic chiasm (-6.0 Gy). Replanning only had a small effect on PTV coverage. The group mean V95% decreased from 99.1% to 98.7%. No systematic change was observed for the number of MU which remained around 225 MU/Gy. Conclusion The knowledge-based approach is an important step forward in treatment planning for brain tumors. It consistently achieved additional sparing in GBM patients with respect to the conventional method. There was no obvious trade-off in terms of target coverage or plan complexity. The results also indicate that DVH estimation helps to remove some of the inherent variability in manual optimization. Our department has added the brain model to its set of DVH estimation models for routine clinical use. Purpose or Objective The aim of this work is to compare the constraints for target dose conformity and dose fall-off around the target (dose gradient) from the UK SABR Consortium guide and ROSEL trial. Materials and Methods Two new quality metrics have been described by the UK SABR Consortium [Physics and Imaging in Radiation Oncology 11 (2019) 41–46]: 1) Prescription Dose Spillage (PDS), defined as the ratio of the prescription dose (PD) volume to the volume of PTV receiving at least the PD (PTV V100%); 2) Modified Gradient Index (MGI), defined as the ratio of the patient´s volume receiving at least 50% of the PD to the PTV V100%. Dosimetric quality of the lung SBRT computed in our department is evaluated using the R100% and R50% metrics of the ROSEL study [Radiat Oncol. 2009 Jan 12;4:1]. R100% and R50% are defined as PSD and MGI, respectively, but using the PTV volume as reference instead of PTV V100%. One hundred twenty six (126) lung SBRT treatment plans (5 to 8 fractions) were retrospectively selected for this study. All plans were calculated with the AAA algorithm (0.1 or 0.2 cm-calculation grid size) of the Eclipse TPS (v. 13.7) using a coplanar IMRT technique with 6 MV beams from a Varian Clinac 2100 CD (120 MLC). PTV ranged from 2.7 to 192.3 cm 3 . The PDS, MGI, R100% and R50% were calculated for all plans and resulting data was analyzed according to the constraints given by the UK SABR Consortium guideline and ROSEL study. Results were catalogued as ‟ no deviation”, ‟ minor deviation” and ‟ major deviation” according to the quality criteria established by both studies. Results Table 1 shows the passing rates for each metric according to the constraints of each study. Higher rates of major deviations were found using the UK SABR Consortium guideline (PDS and GMI), in comparison with the ROSEL study-based analysis (R50% and R100%). Specially, the target conformity worsened by 14% of cases when UK SABR Consortium guideline (PDS) was used instead the ROSEL study (R50%). Digital Poster: Treatment planning: applications PO-1857 Retrospective analysis of conformity and gradient indexes of lung SBRT plans J. Calvo-Ortega 1 , E. Puertas 1 , S. Moragues-Femenía 1 , J. Casals-Farran 1 1 hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain

Conclusion Adopting the UK SABR Consortium guideline implies a more demanding control quality of the SBRT plans respect to the ROSEL study, especially for target conformity.

PO-1858 Adrenal SBRT: The Effects of a Full and Empty Stomach on Radiation Dose to Organs at Risk O. Tanaka 1 , T. Taniguchi 1 , K. Ono 1 , S. Nakaya 1 , C. Makita 2 , M. Matsuo 2 , T. Kiryu 1 1 Asahi University Hospital, Department of Radiation Oncology, Gifu city, Japan; 2 Gifu University Hospital, Department of Radiation Oncology, Gifu city, Japan Purpose or Objective Adrenal recurrence after radical resection of the primary lesion is common. However, the adrenal glands are surrounded by radiosensitive organs such as the pancreas, kidneys, small intestine, and stomach. Treatment planning therefore requires many regulations due to the dose limit to the organ at risk. Radiotherapy can be adjusted for gastric capacity. We therefore performed an OAR analysis for SBRT to the left adrenal gland based on gastric state (empty or full stomach) at the time of irradiation. We examined whether it was possible to reduce the dose to OAR.