ESTRO 2023 - Abstract Book

S1588

Digital Posters

ESTRO 2023

Conclusion Substantially reduced planning margins are feasible for the investigated treatment region when using PGI for treatment verification and intervention in clinical routine - resulting in a considerable dosimetric benefit. PGI acts as an important enabler for closed OAPT workflows not only due to the improved treatment accuracy as shown here, but also by providing an independent QA measure on patient level for the online-adapted plan.

PO-1849 PSQA of Stereotactic Partial Breast Irradiations with GammaPod: initial experience at Udine Hospital

E. Moretti 1 , P. Scalchi 1 , D. Marfisi 1 , V. Gagliardi 1 , M. Guernieri 1 , M. Trovo' 2 , C. Reverberi 2 , A. Prisco 2

1 ASU FC, Medical Physics, UDINE, Italy; 2 ASU FC, Radiation Oncology, UDINE, Italy

Purpose or Objective GammaPodTM (GP), manufactured by Xcision, is the world's first radiotherapy technology designed for delivering Stereotactic Partial Breast Irradiation (S-PBI). The GP combines a rotating irradiation unit of Co-60 sources and dynamic table motion. The patient’s prone breast is immobilized by a vacuum assisted breast cup. In December 2021, our institution became the first site in Europe to go live with GP. We began with 4-fractions adjuvant PBI and then moved toward single-fraction patterns: preoperative, postoperative S-PBI (RX 30, 33 and 18.4 Gy, respectively), tumor cavity boost (RX 7.9 Gy) before whole-breast irradiation. In this work, we present our Patient-Specific QA (PSQA) strategy of GP-based S-PBI. Materials and Methods The GP-workflow entails daily morning checks, breast cup fitting, CT-imaging, segmentation, optimization, treatment. Steps following CT-run are carried out as soon as possible, since the patient lies on the CT-couch to minimize movements. Accordingly, there is no room for conventional paradigm of pre-treatment PSQA. On treatment day, for each GP-plan, we perform an independent check of the delivery time by means an in-house way based on target volume and dose. After completion of the S-PBI, a dual off-line PSQA session is carried out. A microchamber (CC01, IBA) is located in a high-dose region with low-dose gradient (less than ±1%), in a water-filled breast cup matching the patient. Further, in a PMMA breast-phantom, relative dose distributions are acquired with the 2D-detector SRS MapCHECK® (SNC) in axial plane. The SRS-MapCHECK maps had been previously validated with radiochromic dosimetry (EBT3/EBT-XD® depending on the dose level). Results To date we treated 88 patients. Point-doses on average agreed to TPS-values within -1.5% (1.3% SD), ranging from -5.4% to -1.6%. This trend may be due to uncertainties in the collimator openings or approximations in the conversion dose to water dose to breast tissue assumed by TPS. We observed major discrepancies with posterior targets. Concerning the planar maps, the passing gamma rates (GPR), obtained with 3%/1 mm-gamma criteria and 10% RX threshold, varied from 91% to 100% with a mean GPR of 96.3±4.9%. Preoperative S-PBI (range of treated volumes: 1.9-7.7 cc) yielded understandably optimal results: average GPR of 99.6% (1.1% SD) and range of 94.8-100%. Postoperative plans (13.1-107.5 cc) presented slightly worst performance: mean GPR of 96.1±4.5%, range: 83.4-100%. Conclusion The analysis of PSQC data collected during our initial clinical operation highlighted a general good accuracy of GP S-PBI and guided us to modulate the patient’s eligibility criteria according to target extent and position. However, the GP-workflow, mainly in the case of single fraction, emphasizes the need for an online, robust and independent plan verification (fast Monte Carlo and, much more challenging, a GPR-prediction tool based on plan-complexity and control points of the couch travel). A research project on these topics is being developing.

PO-1850 Examining the FLASH effect by performing radiobiological Monte Carlo simulations with TOPAS-nBio

L. Derksen 1 , V. Flatten 1,2 , R. Engenhart-Cabillic 2,3 , K. Zink 1,2,3 , K. Baumann 1,2,4

1 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany; 2 University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany; 3 Marburg Ion-Beam Therapy Center, MIT, Marburg, Germany; 4 Marburg Ion-Beam Therapy Center , MIT, Marburg, Germany