Independent assessment of source transit time for the BEBIG SagiNova® cobalt-60 high dose rate brachytherapy afterloader.

Abstract

Independent verification of transit time and the methodology employed in commercial high dose rate (HDR) afterloaders to compensate its effect is an important part of their commissioning and quality assurance. This study aimed to independently evaluate the Co-60 source transit time of the new BEBIG SagiNova® HDR afterloader unit by employing a dosimetric approach using a well-type ionization chamber. The source was placed at three dwell positions (DPs) to mimic a variety of clinical situations with different distances from the afterloader unit. The distances of the DPs to the afterloader were 129.37cm, 124.50cm and 118.57cm. Plans were generated using the SagiPlan® treatment planning system to produce 3, 5, 10, 15, 20, 30, 40, 60 and 120s dwell times (DTs). The residual transit times (following any possible system compensation) were assessed using the ESTRO-recommended approach of obtaining transit time compensation factors and another strategy established for teletherapy sources. The mean residual transit time depended on the distance between the afterloader and the DP, ranging from 0.43 to 1.10s. The transit dose contribution was case-specific, ranging from 0.4% for a 60s DT at the nearest DP to the afterloader up to 15.6% for a 3s DT at the furthest DP from the unit. The results show that currently SagiNova® afterloader does not apply transit time compensation and suggest a 0.2-0.5s compensation for each arrival and departure DP from/to the afterloader, depending on position in an 11cm active length.