Direct reconstruction and associated uncertainties of¹⁹²Ir source dwell positions in ring applicators using gafchromic film in the treatment planning of HDR brachytherapy cervix patients

Abstract

This paper details a practical method for the direct reconstruction of high dose rate (192)Ir source dwell positions in ring applicators using gafchromic film in the treatment planning of brachytherapy cervix patients. It also details the uncertainties associated with such a process. Eight Nucletron interstitial ring applicators-Ø26 mm (×4), Ø30 mm (×3) and Ø34 mm (×1), and one 60 mm intrauterine tube were used in this study. RTQA2 and XRQA2 gafchromic films were irradiated at pre-programmed dwell positions with three successive (192)Ir sources and used to derive the coordinates of the source dwell positions. The source was observed to deviate significantly from its expected position by up to 6.1 mm in all ring sizes. Significant inter applicator differences of up to 2.6 mm were observed between a subset of ring applicators. Also, the measured data were observed to differ significantly from commercially available source path models provided by Nucletron with differences of up to 3.7 mm across all ring applicator sizes. The total expanded uncertainty (k = 2) averaged over all measured dwell positions in the rings was observed to be 1.1 ± 0.1 mm (Ø26 mm and Ø30 mm rings) and 1.0 ± 0.3 mm (Ø34 mm ring) respectively, and when transferred to the treatment planning system, equated to maximum %dose changes of 1.9%, 13.2% and 1.5% at regions representative of the parametrium, lateral fornix and organs at risk respectively.

Automatic tandem and ring reconstruction using MRI for cervical cancer brachytherapy

“…In this study, we used an idealized source path model provided by the manufacturer to assess DPE provided by the algorithm. Previous work has shown that this idealized source path model may poorly model the true source path, particularly within the ring, leading to DPEs on the order of 6 mm . In addition to the idealized source path, the brachytherapy TPS also provides a measured source path for each ring size and angle, intended to more closely model the true ring dwell positions, which we have verified in our clinic using autoradiography .…”

NCS Report 30: Code of Practice for Quality Assurance of Brachytherapy with Ir-192 Afterloaders

“…In certain applicators path deviations up to 6 mm have been found [20,21]. Because the path deviation effect is dependent on the type of applicator used, it has to be determined for each type of applicator.…”

“…In curved geometries this extra space causes path deviations between the intended source position and the actual one. In particular in the case of gynaecological ring-applicators, the trajectory of the source wire along the circular lumen results in a deviation from the expected nominal source path [20,21,30]. See figure 3.5 for an illustration.…”

Dosimetric audit in brachytherapy

“…Eight peer-reviewed journal publications were obtained that presented results of fully completed audits or pilot audits at several centres, involving dosimetric measurement, 18,[37][38][39][40][41][42][43] and one concerned with a geometric applicator reconstruction audit. 44 Two more journal publications presented methodologies for brachytherapy audit currently in progress, 45,46 and the authors are aware of another two audits currently in progress in the UK that are not yet described in the literature: a well-type chamber intercomparison and an alanine dosimetry phantom measurement of source strength, discussed later. Each of these above audits deals with afterloading equipment.…”

“…The final brachytherapy audit being conducted at several centres in the UK is based on a design described by Awunor et al, 46 who have developed an audit process for the direct reconstruction of dwell positions in ring applicators using a universal jig and radiochromic film. Whilst this is not a dosimetric audit, in that no dose measurements are made, the accuracy of dwell positions in rings is a subject much debated and might impact on the accuracy of clinical dose delivery.…”