Investigation of source position uncertainties & balloon deformation in MammoSite brachytherapy on treatment effectiveness

Dosimetric impact of source-positioning uncertainty in high-dose-rate balloon brachytherapy of breast cancer

Purpose

To evaluate the dosimetric impact of source-positioning uncertainty in high-dose-rate (HDR) balloon brachytherapy of breast cancer.

Material and methods

For 49 HDR balloon patients, each dwell position of catheter(s) was manually shifted distally (+) and proximally (–) with a magnitude from 1 to 4 mm. Total 392 plans were retrospectively generated and compared to corresponding clinical plans using 7 dosimetric parameters: dose (D₉₅) to 95% of planning target volume for evaluation (PTV_EVAL), and volume covered by 100% and 90% of the prescribed dose (PD) (V₁₀₀ and V₉₀); skin and rib maximum point dose (D_max); normal breast tissue volume receiving 150% and 200% of PD (V₁₅₀ and V₂₀₀).

Results

PTV_EVAL dosimetry deteriorated with larger average/maximum reduction (from ± 1 mm to ± 4 mm) for larger source position uncertainty (p value < 0.0001): from 1.0%/2.5%, 3.3%/5.9%, 6.3%/10.0% to 9.8%/14.5% for D₉₅; from 1.0%/2.6%, 3.1%/5.7%, 5.8%/8.9% to 8.7%/12.3% for V₁₀₀; from 0.2%/1.5%, 1.0%/4.0%, 2.7%/6.8% to 5.1%/10.3% for V₉₀. ≥ ± 3 mm shift reduced average D₉₅ to < 95% and average V₁₀₀ to < 90%. While skin and rib D_max change was case-specific, its absolute change (∣Δ(Value)∣) showed that larger shift and high dose group had larger variation compared to smaller and lower dose group (p value < 0.0001), respectively. Normal breast tissue V₁₅₀ variation was case-specific and small. Average ∣Δ(V₁₅₀)∣ was 0.2 cc for the largest shift (± 4 mm) with maximum < 1.7 cc. V₂₀₀ was increased with higher elevation for larger shift: from 6.4 cc/9.8 cc, 7.0 cc/10.1 cc, 8.0 cc/11.3 cc to 9.2 cc/ 13.0 cc.

Conclusions

The tolerance of ± 2 mm recommended by AAPM TG 56 is clinically acceptable in most clinical cases. However, special attention should be paid to a case where both skin and rib are located proximally to balloon, and the orientation of balloon catheter(s) is vertical to these critical structures. In this case, sufficient dosimetric planning margins are required.

The impact of uncertainties associated with MammoSite brachytherapy on the dose distribution in the breast

The MammoSite radiation therapy system is a novel technique for treatment of patients with early-stage breast cancer. It was developed to overcome the longer schedules associated with external-beam radiation therapy. It consists of a small balloon (4 cm in diameter) connected to an inflation channel and a catheter for the passage of a high dose rate 192Ir brachytherapy source. The device is placed into the tumor resection cavity and inflated with a mixture of saline and radiographic contrast agent to a size that fills the cavity. A high dose rate 192Ir source is driven into the balloon center using a remote afterloader to deliver the prescribed dose at a point 1 cm away from the balloon surface. There are several uncertainties that affect the dose distribution in the MammoSite brachytherapy. They include source position deviation, balloon deformation, and the concentration of the contrast medium inside the balloon. The purpose of this study is to investigate the extent of the dose perturbation for various concentrations of the contrast medium in a MammoSite balloon using Monte Carlo simulations and thermoluminescent dosimetry. This study also combines the impact of these uncertainties on the MammoSite treatment efficacy. The current study demonstrates that the combined uncertainties associated with the MammoSite brachytherapy technique--up to the value of 2 mm balloon deformation, 1 mm source deviation, and 15% contrast concentration--have no impact on the tumor control probability.