ESTRO 2021 Abstract Book

S1347

ESTRO 2021

Materials and Methods CT data set of 10 low/intermediate-risk Medulloblastoma / PNET patients (age 13.1 ±6.5 years) who previously received the VMAT based CSI treatment (23.4 Gy in 13 fractions) in Novalis was planned in the newly commissioned Halcyon ring gantry linear accelerator in Eclipse v15.6 TPS. Multiple isocenters (2 or 3) were placed by equally dividing the (brain+spine) PTV length, with 1st Iso at 1st, 2nd Iso at -3rd and 3rd Iso at 5th period. For Novalis, and Halcyon 2 and 3 arcs were used per isocentre. For Brain, PTV 360֯ was used while at spine iso the arc span was 260֯-180֯-100֯ for both the plans. For a fair comparison, all plans were normalized to 97% dose to cover 97% of the target volume. With equal PTV coverage, OAR doses, MU, Volume receiving 20% and 50% doses were compared. Statistical comparison is done by Student t-test with significance level Average MU for HA and NT plans were 1795±148 and 1524 ±213 and the difference was statistically significant (P=0.048). With equal coverage of PTV, 29 OAR’s were compared (HA-NT) for maximum and (or) mean dose as. Central organs viz. Pharynx, Larynx, Heart, Bladder, Bowel, Rectum shows a mean dose difference between 0%-4.1%. lateralised organs viz. bilateral eye, lens (max dose), parotid, submandibular gland, femur head, stomach, liver, shows a dose variation between 1.1 to 7.9%. Esophagus, Rt and Lt kidney max dose difference (HA-NT) was -13.9%± 6.3%, -12.4%±5.3%, and -8.5%±5.3%; first two were statistically significant p<0.04. The difference of mean volume receiving 50% and 20% dose for HA-NT were - 650±157 cc and - 361.3±215; and both were statistically significant p=0.03. Conclusion Conclusion: Ring gantry (HA) based CSI plans are dosimetrically comparable with NT standard LA plans in terms of PTV dose coverage; for most of the OAR’s the doses are comparable and slightly favored the HA plans, esophagus and Lt Kidney shows a statistically significant difference. Although HA plans require a larger MU; it reduces the lower isodoses like 50% and 20%. Figure-1 shows the visual difference in 50% isodose lines. This is because of the difference in MLC speed between both LA, HA = 6 cm/sec and NT= 3 cm/sec. Higher MLC Speed produces a larger modulation demands larger MU but reduces the dose spade. Low-risk Medulloblastoma/PNET patients have long survival. Halcyon and Novalis plans were comparable in terms of PTV/OAR dose parameters. Halcyon produces a significantly lower low dose spillage – which may be helpful for patients in terms of quality of life and induction of radiation-induced second cancer. Figure-1: Volume receiving 50% of the prescription dose - A visual change is observable between Halcyon and Novalis dose distribution, with Novalis showing high low dose spillage than Halcyon linear accelerator. p<0.05. Results

PO-1627 Dosimetric characterization of 3D-printed electron compensator: an analysis of printing parameters F. Biltekin 1 , G. Yazici 1 , G. Ozyigit 1 1 Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey Purpose or Objective To evaluate the effects of printing parameters on dosimetric characterization of 3D-printed electron compensator used in high-energy electron beam therapy. Materials and Methods In the present study, electron compensator with a dimension of 5 × 5 × 1 cm was modeled in SketchUp Pro 2017 3D modeling program and the output of the modelled electron compensators were printed in MakerBot Replicator Z18 3D-printer using polylactic acid (PLA) filament. As printing parameters 5 different infill percentages (0%, 20%, 40%, 60%, 80%, and 95%), 5 different infill patterns (linear, hexagonal, diamond fill, sunglasses fill, and donut fill) and 2 different printing directions (horizontal and vertical) were selected and electron compensators were printed for all defined conditions. Then, two-dimensional (2D) dose measurements were performed in Varian Clinac DHX linear accelerator using 6 MeV and 12 MeV electron energies. During the measurement, PTW Seven29 2D-Array ionization system was used and cross calibration was performed with PTW Roos electron chamber for both electron energies. To evaluate the dosimetric characterization of 3D-printed electron compensator, 2D dose maps obtained with 3D-printed electron compensator at the depths of 1.5 cm for 6 MeV and 3 cm for 12 MeV electron energies were compared with the dose maps obtained with commercially available compensator using 2D gamma analysis method in PTW