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

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.