A new approach to assess transit dose impact in gynecological and prostate brachytherapy with effective transit time

The aim of this study was to evaluate the dosimetric impacts of transit doses in various brachytherapy techniques, e.g., intra-cavitary, interstitial, and hybrid brachytherapy, on prostate and gynecological cancers using commercially available treatment planning system (TPS).

For the evaluation, 36 gynecological cancer and 12 prostate cancer treatment plans were investigated, and transit doses were calculated using effective transit time to the middle of dwell positions in TPS. Effective transit times varied with step size, and movements were determined with a high-speed camera. The increment of Δ in dose-volume histogram parameters for clinical target volume (CTV) V_100% and CTV D_90%, rectum D_2cc and D_1cc, and bladder D_2cc and D_1cc were evaluated, after which dwell time distribution for each technique was analyzed to examine relationships between dosimetric change in CTV D_90% and dwell time.

The median Δ for interstitial technique in prostate cancer was larger than those in other techniques: that of CTV V_100% was 0.7%, and those of bladder and rectum D_2cc and D_1cc were over 1.0%. In CTV D_90%, the median Δ values for interstitial technique in gynecological and prostate cancers were 1.0% and 1.6%, respectively. The interstitial techniques displayed a positive correlation between Δ of CTV D_90% and the percentage of dwell times of < 2 s of the total dwell times.

In this study, the dosimetric impacts of transit doses in various brachytherapy techniques were comprehensively clarified. A relatively large impact was observed in interstitial techniques, and the percentage of dwell times of less than 2 s could be a rough indicator of the necessity of considering the transit dose.