ESTRO 38 Abstract book

S584 ESTRO 38

seed placement accuracy, treated as a normal distribution. In the three needle directions, the mean displacement (± standard deviation) was: 6.7 mm (±7.8 mm) in the shallow-deep direction, 0.75 mm (±4.4 mm) in the template left-right direction, and 0.10 (±4.0 mm) in the skin-chest wall direction. The skin and chest-wall were imposed as boundaries for seed placement in the simulation. Resultant CTV coverage was assessed using the V 100% . .

Conclusion Simulations of an idealized target geometry reveal an interplay between CTV/PTV size and post-implant dosimetry when seed delivery error is accounted for. This motivates the necessity of considering CTV size and the extent of PTV cropping when assessing PBSI treatment plans. PO-1050 A gynecological multichannel applicator including a real-time treatment verification system A. Romanyukha 1 , M. Carrara 2 , T. Giandini 2 , M. Petasecca 1 , T. Al-Salmani 1 , D. Cutajar 1 , A. Cerrotta 3 , B. Pappalardi 3 , F. Piccolo 3 , E. Pignoli 2 , A. Rosenfeld 1 1 University of Wollongong, Centre for Medical Radiation Physics CMRP, Wollongong, Australia ; 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Medical Physics, Milan, Italy ; 3 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 2, Milan, Italy Purpose or Objective To develop, calibrate, and test an innovative prototype of a vaginal multichannel applicator with an embedded system for real-time verification of source dwell position and time in HDR 192 Ir brachytherapy, to ensure that planned treatments are delivered in accordance to the original treatment plans. Material and Methods The applicator prototype is modeled based on a multichannel vaginal applicator (MVA) that is 30 mm in diameter and contains seven peripheral and one central catheter(s). Three p-type epi diodes are placed around the upper region of the MVA and were selected due to their efficiency in the measurement of high-energy photons associated with the 192 Ir source. The readout system is an AFE and FPGA system capable of 100 μs online readout for up to four diodes at a time. The ad hoc detectors and electronics have been all developed and optimized according to the specific task. In this study, a prototype of the MVA system was characterized, and an efficient method for real-time source localization and dwell time measurement was established. Once the method was defined and tested, the accuracy of the MVA system was then quantified in

Results In general, PTV cropping has a larger effect on dosimetry for small volume CTVs. In this simulation, large decreases in V 100% coverage are observed for the smallest PTV (0 mm skin-chest wall margin) on the three smallest CTVs, particularly in comparison to the largest CTV. For these CTV/PTV combinations, the mean V 100% was 46%, as opposed to 89% for the largest CTV. For all CTVs, a cropping to a 7 mm skin-chest wall margin likely does not substantially affect coverage compared to the 10 mm un- cropped PTV. Overall, the results show that the volume of the CTV is important when considering resultant V 100% coverage when seed delivery error is taken into account, particularly when patient anatomy requires severe cropping to the skin and chest wall.