Isolating the impact of tissue heterogeneities in high dose rate brachytherapy treatment of the breast

Background and purpose

Clinical brachytherapy treatment planning is performed assuming the patient is composed entirely of water and infinite in size. In this work, the effects of this assumption on calculated dose were investigated by comparing dose to water in water (D_w,w) in an unbound phantom mimicking TG-43 conditions, and dose to medium in medium (D_m,m) for breast cancer patients treated with high dose rate brachytherapy.

Materials and methods

Treatment plans for 123 breast cancer patients were recalculated with a Monte Carlo-based treatment planning software. The dwell times and dwell positions were imported from the clinical treatment planning system. The dose was computed and reported as D_w,w and D_m,m. Dose-volume histogram (DVH) metrics were evaluated for target volumes and organs at risk.

Results

D_w,w overestimated the dose for most studied DVH metrics. The largest median overestimations between D_m,m and D_w,w were seen for the planning target volume (PTV) V_200% (5.8%), lung D_{0.1 cm}³ (6.0%) and skin D_{0.1 cm}³ (4.2%). The differences between D_m,m and D_w,w were statistically significant for all investigated DVH metrics_. The PTV V_90% had the smallest deviation (0.7%).

Conclusion

There was a significant difference in the DVH metrics studied when tissue heterogeneities and patient-specific scattering are accounted for in high dose rate breast brachytherapy. However, for the studied patient cohort, the clinical coverage goal (PTV V_90%), had the smallest deviation.