ESTRO 2020 Abstract Book

S822 ESTRO 2020

time making SBRT of multiple targets clinically efficient and feasible. Moreover, the use of PET emission as “biological fiducial” to track multiple targets independently is poised to improve both treatment efficiency and accuracy. Further studies will be directed to determine the delivery time and treatment delivery accuracy for single isocenter SBRT of independently moving targets. PO-1521 Endorectal balloon air filling for extreme hypofractionated prostate SBRT S. Vieira 1 , J. Stroom 1 , J. Kociolek 1 , A. Soares 1 , C. Greco 1 1 Fundação Champalimaud, Radiotherapy, Lisboa, Portugal Purpose or Objective To assess the influence of varying volumes of endorectal balloon air filling on dosimetric plan quality in extreme hypofractionation SBRT (5x9Gy), for localized prostate cancer. Material and Methods CT and MR image sets were acquired for 10 prostate cancer patients with an endorectal rectal balloon. Balloon inflation ranged between 0cc and 250cc (fig1). The following structures were contoured on CT-MR fused images for each balloon filling: CTV, PTV (2 mm margin from CTV), urethral wall (UW), bladder wall (BW), rectal wall (RW), neurovascular bundles (NVBs) and urogenital diaphragm (UGD). A 4 arc VMAT plan was generated (Eclipse TM ) for each balloon filling. Plan priorities were determined for the 150cc scan of each patient using RapidPlan TM and subsequently used for all other scans to eliminate bias from planners. Average dose parameters were determined for each rectal balloon volume. Following clinical practice, PTV coverage was defined as the volume receiving 45Gy. Organs at risk were evaluated at D1cc (< 36Gy for RW and BW and <40.5Gy for BW). Evaluation of the NVBs and UGD was at maximum dose (D 0.1cc < 40.5 Gy and 42.75Gy respectively). Figure 1. CT images of a prostate patient set-up with six rectal balloon fillings: (a) 0cc, (b) 50cc, (c)100cc, (d) 150cc, (e) 200cc, and 250cc. Beacon transponders are also inserted in the urethra for online tumor tracking.

the CTV volumes for each balloon filling was checked to be <2%. A balloon volume up to 150cc was well-tolerated by all patients. A balloon volume of 200 cc produced some discomfort in 2 of the patients. On average dose tolerances were met for all organs at risk (fig2). However, two patient plans exceeded UW dose tolerances for the lowest balloon fillings. RW tolerance dose was exceeded in three patient plans for the highest balloon filling. Individual plan optimization would improve these results, but also increase the subjectivity of the study. No significant effect was observed (<3%) for BW, RW, and UW with respect to balloon filling (fig 2). Regarding RW, the highest priority was needed for the D1cc leading the optimization to the limit. For UGD and NVBs the Dmax appears to decrease with balloon volume. However, the increasing plan difficulty for large balloon volumes mostly manifests itself on the PTV coverage, which decreased 8% from 0cc to 250cc (fig.2a).

Conclusion An endorectal balloon filling of 150cc is well tolerated. Increased rectal balloon filling reduces PTV coverage if organ at risk doses are to be maintained. For our prostate hypofractionation we have chosen 150cc as the best compromise, accepting 5% decrease in dose coverage and prioritizing anatomy fixation and OAR sparing. PO-1522 3D vs 4D dose calculations for moving target near air cavity for lung VMAT SBRT planning Y. AlmutairI 1 , L. Leon Vintro 2 , B. McClean 3 1 University College Dublin & Saint Lukes Radiation Oncology Network, Physics, Dublin, Ireland ; 2 University College Dublin, School of Physics, Dublin, Ireland ; 3 Saint Lukes Radiation Oncology Network, Department of Physics, Dublin, Ireland Purpose or Objective An air cavity near a moving target is a non-trivial source of uncertainty in lung SBRT. A common method is to calculate a lung SBRT treatment plan using the average intensity projection (AIP) of the 4DCT scans. In this study, the extent of air cavity caused by emphysema in a poor pulmonary lung function, in addition to the dose deficiency due to set up errors was investigated. This effect is often ignored when the ITV concept is utilized.

Results In total we acquired 45 scans for the 10 patients (the smallest set with 4 scans for 250cc). The reproducibility of