ESTRO 2023 - Abstract Book

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Digital Posters

ESTRO 2023

Materials and Methods The established program involved four main steps:

1) The template definition in the brachytherapy software of the US device and its calibration against the physical template, used to insert the needles, and the TPS grid. 2) The validation of the source specification data (Bebig Model I25.S17 plus I-125) available in Variseed TPS v.9.0 (Varian), through comparison to the consensus datasets provided in AAPM TG 43; 3) The completion of a set of tests to assess the TPS dose calculation accuracy and plan evaluation tools. These tests included, first, the verification of the dose calculated by TPS for a single source and multiple sources, against reference values provided in AAPM TG 43, and manual calculations using the 2D formalism. Then, the assessment of the dose display by creating five isodose lines and checking if they appeared in the correct location. Finally, the evaluation of the accuracy of the volume and DVH calculation by considering objects with known dimensions; 4) The performance of an end-to end test, using the prostate phantom model 053S (CIRS), to simulate the entire process of a typical LDR prostate treatment, from US image acquisition, to treatment planning and seeds implant. This step aimed at reducing the time spent in the operating room by defining and training the workflow to be adopted. Results The source data provided by the vendor, including the dose rate constant, the radial dose function, the anisotropy function, active length and half-time, was matched to the consensus datasets. The dose values calculated by TPS were in close agreement (<2%) with the reference values provided in TG 43 and manual calculations, for various positions (up to 4 cm) along and away of the source. The correct sum of the dose from two sources was also verified (difference <2%), for points at multiple locations. The isodose lines test confirmed the correct dose display. A good consistency between DVH data and dose display was also checked. The difference between the structure volume calculation and the known value was less than 1%, for the considered geometrical shapes. The end-to-end test went through smoothly, being the whole process successfully simulated. The correct calibration of the templates was confirmed, the continuous image acquisition was performed through the US transversal probe, the contours and the source placement tools were tested and the seeds implant was checked in live mode. Conclusion A comprehensive quality assurance program for implementation of LDR prostate brachytherapy procedures has been established, and will be used for QA or training purposes. 1 Riga East University Hospital, Therapeutic Radiology and Medical Physics, Riga, Latvia; 2 Childrens Clinical University Hospital, Medical Devices and Safety, Riga, Latvia; 3 Riga East University Hospital, Therapeutic Radiology and Medical Physics, Riga, Latvia; 4 Riga East University Hospita, Therapeutic Radiology and Medical Physics, Riga, Latvia Purpose or Objective The aim of brachytherapy is to deliver a precise and safe dose to the target volume, while protecting healthy tissue from undue exposure. As institution have had a complete check for different types of new design MRI compatible RING applicators (Bravos, Varian), measurements were performed to determinate source positioning errors and compare with the treatment planning Ring template implemented in the TPS (Eclipse V15.6) Materials and Methods In this study dwell position errors of RING applicators with two different ring diameters (26mm and 30mm) and three different ring angles (45 ⁰ , 60 ⁰ and 90 ⁰ ) were measured. For each applicator geometry combination (diameter and angle) two same applicators (in total, twelve RING applicators were tested) were evaluated to see if source position in same applicator types differ. To evaluate source dwell position, three treatment plans for each applicator were performed, to irradiate radiochromic films: source dwell time was 10sec nominally for each third position starting from first, second and third position in each plan, rest position dwell times were set 1sec nominally, to force source to stop and simulate dwell positions as in real treatment plan. In this case, only each third position was visibly irradiated on radiochromic film. After film irradiation, each visible dwell position was marked, and all three films was fused together to get correct dwell positions with step 5mm (as in treatment plans). After dwell position determination, angles to each position were measured and error was calculated. The aim was to get real dwell positions for RING applicators and compare with plan template for treatment calculation. Results For all RING applicators there was significant difference of average errors between dwell positions. As the acceptable error of ±1mm was determined but measured position errors were within the range between 0 and 9.5mm for different dwell positions, a decision was made to create separate templates for each type of applicators with average error correction for each dwell position. (Image) However, after template dwell position and measurement comparison, only 60 ⁰ and 45 ⁰ RING applicators was within acceptable error range. For 90 ⁰ applicators dwell position errors in each position were more than 1mm between measurements (for 26mm and 30mm diameter applicators max 2.8mm 3.2mm respectively). PO-2177 Study of dwell positions discrepancies from TPS in brachytherapy RING applicators I. Balode 1 , V. Dmitrijeva 2 , G. Boka 3 , Z. Rudusa 4