ESTRO 2022 - Abstract Book

S494

Abstract book

ESTRO 2022

Conclusion Oncentra’s AGA model agrees well with measurements in LO, IU and needles. CR is a useful method to determine the contorted source path within challenging applicator types when autoradiographs cannot accurately be used. Subsequent LO commissioning is done using CT only which is efficient and less time consuming. Annual quality assurance of the applicators are also done using CT images

PD-0563 QA primary program for a skin surface brachytherapy plan

C. Arrichiello 1 , F. Buonanno 2 , G. Ametrano 1 , F. Gherardi 1 , E. Iannacone 1 , P. Muto 1

1 Istituto Nazionale Tumori - IRCCS - Fondazione G. Pascale, Radiotherapy Unit, Naples, Italy; 2 Università degli Studi di Napoli Federico II, Post Graduate School in Medical Physics, Department of Advanced Biomedical Sciences, Naples, Italy Purpose or Objective Among the radiation techniques for cutaneous malignancies treatment, the choice of brachytherapy is rapidly rising. Last year, in our department, more than 20 patients were enrolled for skin brachytherapy target. This approach is especially pointed at large targets or irregularly shaped lesions. Although its growing development, the AAPM protocols do not comprehensively cover surface brachytherapy and, currently, no Quality Assurance (QA) guideline are available. The safety and accuracy of delivery is crucial, since of the lack of well-defined procedures, assuring the real dose to target correspondence. Skin-brachytherapy workflow may be error prone, with a wide liability related to the staff experience, especially with personalized surface mould. Dedicated phantoms for QA are not available, especially for curved surfaces. Further, in the TPS planning phase, an automatic pre-set for catheters reconstruction is not available, a manual approach is required. Materials and Methods Aim of this work was to present a primary approach for a QA skin-plan. Ten patients (pts) were select for cutaneous brachytherapy with 2÷7 Gy of dose per fraction. The moulds, holding up to 8 catheters, were settled on a pt specific thermoplastic mask, to guarantee treatment reproducibility. For each pt a 3D plan was realized with TPS Oncentra Brachy, based on the TG 43 dose calculation formalism. A manual reconstruction was used in personalized mould planning. Treatments were delivered with Flexitron Unit by 192 Ir HDR source, driven in the mould. For all pts, two pre-treatment QAplan were replicated, holding the original reconstruction. In QAplan1 three random catheters were selected for the activation of their first dwell position with 15s of transit time. In QAplan2 all the first positions of catheters were switched- on, with 15s each. The EBT3 radiochromic films, covering all the catheter tips, were stuck below the tips of catheters and exposed to each QAplan. Three plans were realized to simulate brachytherpy skin (homogenous, asymmetric, concave) targets on a water-equivalent phantom, aiming to compare planar doses with the exposed films placed on the top of the phantom. EBT3 films were calibrated with 6MV photons and exposed. Results QAplan1 tests the correctness of channel mapping, while QAplan2 shows the match of blurring outline to the 3D plan rendering, together with the shape of catheter tips in the mould. The tests performed confirmed the safety of treatments for all pts and the accuracy of the reconstruction method for all the plans. An agreement of 10% between measured and planned dose distribution was found for all the simulation plans. Conclusion Waiting for a higher specific QA procedure committed for skin-brachytherapy, those checks, if performed in a pre-treatment phase, provide a simple, fast and inexpensive method, that can significantly reduce serious errors occurrence. This approach can contribute to improve patient safety and increase the agreement with the required dose distribution.

PD-0564 Feasibility of real-time in vivo dosimetry for HDR gynaecological brachytherapy using a MOSFET.

F. Mahmood 1 , J. Mason 1 , R. McLauchlan 1

1 Imperial College Healthcare NHS Trust, Radiation Physics, London, United Kingdom

Purpose or Objective High dose rate (HDR) multi fraction treatments are currently used in gynaecological brachytherapy treatments. To ensure correct dosage for patient safety and treatment efficacy, it is important to monitor dose accurately. Use of in vivo dosimetry (IVD) for brachytherapy involves complexities such as measuring doses from different dwell positions in the applicators, measuring dose accurately in steep dose gradients and difficulties that arise from positioning a detector into the patient such that it is close enough to the treatment site to measure the dose to target. Therefore, IVD is not routinely performed in brachytherapy. Real time IVD measurements allow the treatment to be interrupted if an issue is detected. Previous real time IVD in HDR prostate brachytherapy demonstrated good agreement with predicted dose within measurement uncertainties [1] . This research focuses on the feasibility of real time IVD for HDR gynaecological brachytherapy. Materials and Methods Stage one followed the commissioning of the metal-oxide semiconductor field effect transistor (MOSFET) devices using an in-house phantom placed in a water tank to immobilise the MOSFET device within a treatment catheter at a fixed distance from the HDR source. Stage two consisted of calibration measurements to determine the dose (in Gy) each MOSFET reading