ESTRO 2022 - Abstract Book

S1358

Abstract book

ESTRO 2022

The chamber’s perturbation correction factors for all investigated field sizes are magnetic field dependent, where the observed differences are stronger in small field sizes. In reference field size, these factors P i , including the water-to-air stopping power ratios, can be considered constant beyond the build-up region. In small field sizes, the gradient perturbation shows depth-dependence and its changes in magnetic field also contribute strongest to the small field correction factors of the investigated ionization chambers in the presence of a magnetic field. The ∆ z-shift was also shown to be reduced in a magnetic field for both large and small field sizes. Conclusion The detailed simulations performed in this work has facilitated deep understandings of the behavior of ionization chambers in a magnetic field under small field condition. The influence of all detector’s components on its magnetic field dependent dose response has been quantified, where the contribution of the gradient perturbation caused by the extended air cavity is shown to be the largest. The correction factors derived from this study can be used to identify the limit of applicability of these chambers.

PO-1575 CyberKnife versus interstitial brachytherapy for partial breast irradiation: dosimetrical assessment

A. Herein 1 , G. Stelczer 1 , C. Pesznyák 1 , N. Mészáros 1 , Z. Takácsi-Nagy 1 , C. Polgár 1 , T. Major 1

1 National Institute of Oncology, Centre of Radiotherapy, Budapest, Hungary

Purpose or Objective To dosimetrically compare stereotactic radiotherapy with CyberKnife (CK) and multicatheter insterstitial brachytherapy (MIBT) for accelerated partial breast irradiation, focusing on the dose to organs at risk (OARs). Materials and Methods Treatment plans of thirty-one MIBT treated patients were selected, and CK plans were created on the same CT images. The OARs were the same for both plans of every patient: ipsilateral non-target and contralateral breast, ipsilateral and contralateral lung, skin, ribs and heart for left sided cases. The CTV was created from the outlined lumpectomy cavity with a total margin (surgical + radiation) of 20 mm in six main directions. The PTV was equal to CTV for MIBT, but in CK plans it was generated from CTV with the addition of a 2 mm isotropic margin for real-time marker tracking. The fractionation was identical (4 x 6.25 Gy). Dose-volume parameters were calculated for both techniques and compared using a Wilcoxon matched pair test. Results Regarding dose coverage, both techniques performed well, the D90 parameter was similar, but the V100 parameter was lower for MIBT than CK, V100 91.6% vs. 98.9%, p<0.001, respectively. Regarding the V100 of non-target breast the CK performed slightly better than the MIBT (V100: 1.1% vs. 1.6%), but for V90, V50 and V25 the MIBT resulted in less dose. The average dose of ipsilateral lung was lower for MIBT than for CK, 4.9% vs. 6.2%, p<0.001, respectively. For the heart, only the D 2cm 3 parameter was significantly lower for MIBT (17.3% vs. 20.4%, p=0.0311 for MIBT and CK, respectively). For all of the examined parameters of skin and ribs, the MIBT performed better. The dose to contralateral breast and lung was very low for both techniques, the MIBT performed better for contralateral lung (D 1cm 3 3.8% vs. 6.1%, p<0.001 for MIBT and CK, respectively), but no significant differences were found for the contralateral breast. Conclusion The target volume can be properly irradiated by both techniques with high conformity. MIBT provides more advantageous plans than CK regarding the OARs, except for the dosimetry of heart and contralateral breast and for dose conformity, but all of the OAR parameters for CK are also below the dose limits. 1 Pontificia Universidad Católica de Chile, Instituto de Física, Santiago, Chile; 2 Fundación Arturo López Pérez, Radioterapia, Santiago, Chile Purpose or Objective Peripheral dose is an issue of growing interest in today’s radiotherapy, as it contributes to the generation of deterministic late-effects on organs near the field edge as well as secondary cancer. The impact of out-of-field doses is particularly relevant for IMRT treatments, which are typically characterized by large irradiated volumes to low doses. Although this dose is not negligible, it is usually ignored, and then the cancer risk associated with the treatment. Knowing the dose to any organ within the patient would allow generating more personalized therapies. This work presents a new model that calculates peripheral photon dose (PPD) in 3D outside the 5% isodose. The model is easy to use, fast, and only requires two treatment parameters. Materials and Methods Considering the physical sources of PPD and a Monte Carlo (MC) simulation of a reference treatment (eight coplanar isocentric 10x10 cm 2 square fields equally distributed at mid-abdomen on the ICRP110 phantom), an analytical expression for PPD, as a function of the point´s position within the patient, was proposed. The model was fitted to the MC data using MATLAB (for doses outside the 5% isodose). Then, it was tested with a) experimental (TLD) data from a VMAT prostate PO-1576 Simple model for a 3D assessment of peripheral dose in coplanar isocentric photon radiotherapy I. López-Martínez 1 , I. Espinoza 1 , B. Sánchez-Nieto 1 , J. Rodríguez-Mongua 2