ESTRO 2023 - Abstract Book

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Digital Posters

ESTRO 2023

Ruthenium plaques are used to treat small ocular melanomas. Treatment planning typically assumes that a plaque will be surgically centred under the tumour base. However, sometimes, surgical constraints require a plaque to be placed eccentrically. In this work, we investigate the dosimetric consequences of eccentric plaque placement by calculating the dose delivered to 98% of geometrically modelled tumour volumes (D98). D98 has been demonstrated to be related to tumour control probability [1]. Materials and Methods Dose distributions from two commonly-used plaque models (CCA and CCB) were calculated using Plaque Simulator (version 6.6.9 (current release is 6.8.9), EyePhysics, LLC, Los Alamitos, CA, USA; PS) [2] (fig 1). For the CCA plaque (diameter 15.3 mm), a 7 mm base diameter tumour was modelled; for the CCB plaque (diameter 20.3 mm), 7 and a 10mm base diameter tumours were modelled. The treatment time required to deliver a prescription dose of 100 Gy to the tumour apex for centred plaque placements was determined for each plaque and tumour. D98 was then calculated for plaque placements of increasing eccentricity, in 1 mm steps. Tumour heights of 2-6mm were modelled. For each combination (tumour height, tumour base dimension and plaque), “safety margins” were calculated. These safety margins represent the eccentricities at which D98 drops below 95% and 90% of the prescription dose from plaque edge alignment with tumour edge. The 95% safety margins range from 1.00 (CCB plaque, 4mm tall tumour with base diameter of 10 mm) to 4.00 mm (CCA plaque, 5 mm tall tumour with a base diameter of 7 mm). The 90% safety margins range from 0.33mm (CCB plaque, 4mm tall tumour with a base diameter of 10 mm) to 3.09mm (CCA plaque, 5 mm tall tumour with a base diameter of 7 mm) (table 1). For the simple geometries studied, the safety margin is clearly related to plaque model, tumour base dimensions and tumour height. Safety margins are likely to depend on the geometric shape of the tumour base as well. Conclusion D98 in tumours treated with Ru-106 plaque brachytherapy decreases with increasing plaque eccentricity, and the severity of this decrease increases with tumor height. Treatment times that were calculated assuming central plaque placement should be modified to account for eccentric plaque placement in order to maintain the originally anticipated probability of tumour control. Results D98 decreases as plaque eccentricity increases for all scenarios.

References

[1] Acta Oncologica, 59:8, 918-925, DOI: 10.1080/0284186X.2020.1762925

[2] Int J Radiat Oncol Biol Phys 2005 , 61, 4, 1227–1242, 2005

PO-2164 Measurement of the force braking down the source in its movement through brachytherapy applicator

V. Stserbakov 1

1 North-Estonia Medical Center Foundation, Radiotherapy, Tallinn, Estonia

Purpose or Objective Brachytherapy QA Programme suppose verification accuracy of applicator library built-in into treatment planning system, which is provided by the manufacture. One method to create the user` own model of brachytherapy applicator is to measure mechanical characteristics of afterloader and applicators parts separately and based on that create simulation, which describe behavior of joined the afterloader-applicator connection. The purpose of this work is do all required measurements for applicator side of Flexitron (Elekta) system. Or, at this step of QA project, to measure the force braking down the source when it moves inside the brachytherapy applicator. Materials and Methods In figure you can see scheme of measurement setup to study strength in cable when dummy source 2 is moving inside brachytherapy applicator 1. Cable 6 attached to load cell 4 of digital scale 5 (Digital scales 0.01-200g, MH-200) pinned on chamber holder block of positioning assembly 7 of water phantom WP-3840 (CNMC). Movement of the system can be done manually/remotely 8. Reading of the scale in grams corresponds to the “resistance force” of dummy movement through brachytherapy applicator. Only care should be taken about “cable free space” A, where should be avoided creating the “snaking/arcing” of the cable.