The contribution from transit dose for¹⁹²Ir HDR brachytherapy treatments

Abstract

Brachytherapy treatment planning systems that use model-based dose calculation algorithms employ a more accurate approach that replaces the TG43-U1 water dose formalism and adopt the TG-186 recommendations regarding composition and geometry of patients and other relevant effects. However, no recommendations were provided on the transit dose due to the source traveling inside the patient. This study describes a methodology to calculate the transit dose using information from the treatment planning system (TPS) and considering the source's instantaneous and average speed for two prostate and two gynecological cases. The trajectory of the (192)Ir HDR source was defined by importing applicator contour points and dwell positions from the TPS. The transit dose distribution was calculated using the maximum speed, the average speed and uniform accelerations obtained from the literature to obtain an approximate continuous source distribution simulated with a Monte Carlo code. The transit component can be negligible or significant depending on the speed profile adopted, which is not clearly reported in the literature. The significance of the transit dose can also be due to the treatment modality; in our study interstitial treatments exhibited the largest effects. Considering the worst case scenario the transit dose can reach 3% of the prescribed dose in a gynecological case with four catheters and up to 11.1% when comparing the average prostate dose for a case with 16 catheters. The transit dose component increases by increasing the number of catheters used for HDR brachytherapy, reducing the total dwell time per catheter or increasing the number of dwell positions with low dwell times. This contribution may become significant (>5%) if it is not corrected appropriately. The transit dose cannot be completely compensated using simple dwell time corrections since it may have a non-uniform distribution. An accurate measurement of the source acceleration and maximum speed should be incorporated in clinical practice or provided by the manufacturer to determine the transit dose component with high accuracy.

HDR 192Ir source speed measurements using a high speed video camera

“…The transit dose component of a brachytherapy source movement from the safe to the dwell positions, between the dwell positions, and when returning to the safe has been studied previously, [1][2][3][4][5][6][7] reporting differences up to a factor of 10 for the source speed for the same afterloader. 8 The effect of these differences on the transit dose component for clinical brachytherapy treatments performed with a HDR 192 Ir source was evaluated recently. 8 The study demonstrated potentially significant dose variations depending on the speed profile.…”

Impact of source position on high-dose-rate skin surface applicator dosimetry

“…HDR source transit dose has long been recognized and explored by many researchers using various methods [26,38–42]. Among them, Fonseca, et al have extensively evaluated the transit dose of the 192 Ir HDR afterloader, with respect to the source speed profiles, for a more realistic dose correction [38–40]. For the skin applicator however only one dwell position is used and the absolute transit dose contribution is a function of source activity only.…”

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

“…To assess the accuracy of dwell-times it is important to take into account that most manufacturers include the transit time in the dwell time of the next dwell position, except for the first dwell-position [23,24]. Note that the clinical effects of the transit dose can be substantial (see appendix 2) [25].…”

“…The transit dose component increases with increasing number of catheters, reducing the total dwell time per catheter or increasing the number of dwell positions with low dwell times. This contribution may become significant (>5%) if it is not corrected for appropriately [25]. Transit dose correction based on average source speed fails to accurately correct the dose, indicating that accurate measurement of the source acceleration and speed profile should be considered [58].…”