Activity-dependent transit dose assessment in ¹⁹²Ir high-dose-rate brachytherapy surface applicator using...

To characterize the newly released GafChromic^TM EBT4 film for transit dose measurement in high-dose rate brachytherapy (HDR-BT) surface applicators and to systematically investigate the relationship between transit dose and ¹⁹²Ir source activity.

EBT4 film was used to measure dose at 3 mm depth from a horizontally oriented Leipzig-style surface applicator connected to a ¹⁹²Ir HDR-BT remote afterloading unit. Transit dose was systematically characterized across eight different source activities ranging from 3.372 to 9.716 Ci (13,724 U to 39,544 U) using measurements with seven nominal dwell times (5-100 seconds, scaled to 10 Ci (40,700 U)). Transit dose was derived from linear extrapolation to zero dwell time and compared with dose calculated (10-Ci source with 100-s nominal dwell time, excluding transit contributions) using the treatment planning system (TPS) Eclipse (Version 16.1).

EBT4 film demonstrated excellent dose-response linearity with nominal dwell time (adjusted R² > 0.99) across all source activities, confirming its suitability for transit dose measurement. Transit dose contributions ranged from 0.3% to 4.3% of TPS dose (100-s nominal dwell time at 10 Ci), with maximum contribution from the 9.543 Ci source (0.225±0.040 Gy) and minimum from the 5.502 Ci source (0.017±0.025 Gy). Transit dose showed poor linear correlation with source activity (adjusted R² = 0.497), indicating that factors beyond source activity influence transit dose magnitude.

This study validates EBT4 film as a reliable tool for transit dose measurement in surface brachytherapy. The observed non-linear relationship between transit dose and source activity reveals the critical influence of experimental setup variables, particularly source guide tube geometry, which affects the source-to-target distance during transit. These findings demonstrate that transit dose is impacted by multiple interdependent factors including source activity, guide tube configuration, and setup geometry. For clinical applications, this highlights the importance of standardized setup protocols and empirical measurement for accurate transit dose assessment.