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

Introduction: The dwell time and transit time components contribute to the overall delivered dose to patients in high dose rate (HDR) brachytherapy treatments. The transit time results from source entry and exit as well as source movements between dwell positions. It depends on various parameters such as the source speed profile, source indexer distance to dwell position, and step size. Usually brachytherapy treatment planning systems do not apply a correction for transit time and the afterloader unit itself adjusts the final dwell times taking account of the transit time. The independent verification of transit time for each HDR afterloader is recommended before clinical use. Some published reports have used a video camera and/or stopwatch to perform this task. To the best of our knowledge, there is no published report of transit time measurement for the recently released BEBIG SagiNova® HDR afterloader unit. The goal of this study is to independently evaluate the corrected source transit time of this afterloader unit without using a video camera or stopwatch.   Materials and Methods: The assessed unit was a newly installed SagiNova® 60Co HDR afteroloader (software version 1.2.4; Eckert & Ziegler BEBIG GmbH). A dosimetric method was used. The measurements were performed using a previously commissioned PTW source check4п (type 33005) well- type ionization chamber and UNIDOS E® electrometer. A 30 cm plastic needle and a 100 cm transfer tube were used to accurately place the source at the point of maximum response (‘sweet spot’) of the well chamber. Plans were generated using SagiPlan version 2.0 to irradiate the well chamber for dwell times of 3, 5, 10, 15, 20, 30, 40, 60, and 120 s. All the measurements were repeated three times. The transit time following its correction by the afterloader software were assessed using the ESTRO-recommended approach of obtaining transit time correction factors and another strategy established for teletherapy sources. The results obtained using the two strategies were also compared.   Results: For the measured setup, the transit time was 0.7 s using both methods. The mean ESTRO transit time correction factor was 0.93 (range 0.88 to 0.99 for dwell times of 3 to 120 s).   Conclusion:   This work demonstrates an accurate dosimetry-based method to measure source transit time during the commissioning step of HDR BT afteroloaders, using a practical approach and available standard equipment. It also shows the consistency of the results between the two dosimetric methods of obtaining source transit time. The results suggest that the afterloader software correction does not completely eliminate this effect, which results in slightly longer    irradiation times.