Correction factors for ionization chamber measurements with the ‘Valencia’ and ‘large field Valencia’ brachytherapy applicators

Abstract

Treatment of small skin lesions using HDR brachytherapy applicators is a widely used technique. The shielded applicators currently available in clinical practice are based on a tungsten-alloy cup that collimates the source-emitted radiation into a small region, hence protecting nearby tissues. The goal of this manuscript is to evaluate the correction factors required for dose measurements with a plane-parallel ionization chamber typically used in clinical brachytherapy for the 'Valencia' and 'large field Valencia' shielded applicators. Monte Carlo simulations have been performed using the PENELOPE-2014 system to determine the absorbed dose deposited in a water phantom and in the chamber active volume with a Type A uncertainty of the order of 0.1%. The average energies of the photon spectra arriving at the surface of the water phantom differ by approximately 10%, being 384 keV for the 'Valencia' and 343 keV for the 'large field Valencia'. The ionization chamber correction factors have been obtained for both applicators using three methods, their values depending on the applicator being considered. Using a depth-independent global chamber perturbation correction factor and no shift of the effective point of measurement yields depth-dose differences of up to 1% for the 'Valencia' applicator. Calculations using a depth-dependent global perturbation factor, or a shift of the effective point of measurement combined with a constant partial perturbation factor, result in differences of about 0.1% for both applicators. The results emphasize the relevance of carrying out detailed Monte Carlo studies for each shielded brachytherapy applicator and ionization chamber.

On the use of the absorbed depth‐dose measurements in the beam calibration of a surface electronic high‐dose‐rate brachytherapy unit, a Monte Carlo‐based study

“…Nevertheless, the effect of the dose‐ratio uncertainty on the overall uncertainty is less than 0.2%. This is true even in an extreme case of 10% dose‐ratio uncertainty, which was conceived to include any unknown effect of the chamber performance . However, it is more realistic to consider that the absorbed dose‐ratio can be determined with uncertainties lower than 5% .…”

“…The simulations were performed using penEasy (v. 2015-05-30), 21 a modular program for PENELOPE2014, which simulates electrons and photons transport from 50 eV to 1 GeV. 22 PENELOPE has been successfully used in the study of both electronic 8,23 and radioisotope-based 18,[24][25][26] brachytherapy sources.…”

“…or inaccuracies in the system alignment . Other studies show that dosimeters, typically used in the absorbed depth‐dose determination for brachytherapy systems, would require the incorporation of additional (depth‐dependent) correction factors and modification of the effective point of measurement recommended by the manufacturer . Those are the issues that need to be considered in the uncertainty evaluation.…”

“…This is true even in an extreme case of 10% dose-ratio uncertainty, which was conceived to include any unknown effect of the chamber performance. 17,18 However, it is more realistic to consider that the absorbed dose-ratio can be determined with uncertainties lower than 5%. 8,17 Furthermore, we have considered a large chamber energy response of 2%, when the particular chamber used in our evaluation presents values of 0.1% and 0.7% for the k Q,Q0 and N k , respectively.…”

A Monte Carlo study of the relative biological effectiveness in surface brachytherapy

“…This fact becomes relevant when considering some evidence indicating that the attributes of the applicators could have an unexpected impact on the presumed DSB 19 . Furthermore, surface applicators may (or may not) include flattening filters (FFs), a critical part along the beam path, which can change the beam spectrum significantly 21,22 . For those reasons, this work aims to estimate the RBE through a hybrid MC approach, using realistic models of the sources and applicators used in surface brachytherapy treatments included in the TG‐253 report.…”