SU-E-T-314: Evaluation of Dosimetric Impact of Catheter-Position Uncertainty in Balloon High Dose Rate Brachytherapy of Breast Cancer.

Abstract

To evaluate the dosimetric impact of catheter-position uncertainty prior to each fraction in balloon high dose rate (HDR) brachytherapy for breast cancer. For 30 balloon HDR patients, each dwell position of the catheters was manually shifted distally (+) and proximally (-) with a magnitude of ±1 mm, ±2 mm, ±3 mm and ±4 mm. A total of 240 plans were retrospectively produced and compared to clinical treatment plans to simulate catheter-position uncertainty. The following dosimetric data were evaluated: PTV_EVAL V90[%] after subtracting air/seroma volume, skin and rib maximal dose (Dmax[%]) and normal breast tissue V200[cc]. PTV_EVAL V90 was decreased in 93% of cases while increased with maximum value of < 0.7% in 7% of cases. Average/maximal reduction was increased from 0.3%/1.2% (±1 mm), 1.0%/3.5% (±2 mm) and 2.6%/6.2% (±3 mm) to 5.0%/9.2% (± 4 mm) as catheter-position error was increased. Change of skin and rib Dmax values was case-specific. They were increased in 52% of cases while decreased in 48% of cases. As catheter-position error was increased, the average/maximal deviation was increased from 1.6%/9.3% (±1 mm), 3.1%/19.1% (±2 mm) and 4.6%/29.1% (±3 mm) to 6.3%/40.2% (± 4 mm). Normal breast tissue V200 was increased in 90% of cases while decreased with maximum value of < 0.4cc in 10% of cases. Average/maximal increase was elevated from 0.3cc/1.2cc (±1 mm), 0.8cc/2.9cc (±2 mm) and 1.8cc/4.8cc (±3 mm) to 2.9cc/6.7cc (± 4 mm) as catheter-position error was increased. The catheterposition tolerance of ±2 mm set by the AAPM TG 56 is clinically acceptable for most clinical cases. However, in a case where the dosimetric data of treatment plan are close to the dosimetry limits of the clinical protocol, smaller tolerances such as ±1 mm or zero tolerance is clinically recommended to minimize delivered dose discrepancy from the planned dose.