  PURPOSE: Permanent breast seed implant (PBSI) using 103Pd is emerging as an
effective adjuvant radiation technique for early-stage breast cancer. However,
clinical dose evaluations follow the water-based TG-43 approach with its
considerable approximations. Towards clinical adoption of advanced TG-186
model-based dose evaluations, this study presents a comprehensive investigation
for PBSI considering both target and normal tissue doses. METHODS: Dose
calculations are performed with the free open-source Monte Carlo (MC) code,
egs_brachy, using 2 types of virtual patient models: TG43sim (simulated TG-43
conditions: all water with no interseed attenuation) and MCref (heterogeneous
tissue modelling from patient CT, interseed attenuation, seeds at implant
angle) for 35 patients. Sensitivity of dose metrics to seed orientation and the
threshold for glandular/adipose tissue segmentation are assessed. RESULTS: In
the target volume, D90 is 14.1% lower with MCref than with TG43sim, on average.
Conversely, normal tissue doses are generally higher with MCref than with
TG43sim, e.g., by 22% for skin D1cm2, 82% for ribs Dmax, and 71% for heart
D1cm3. Discrepancies between MCref and TG43sim doses vary over the patient
cohort, as well as with the tissue and metric considered. Doses are sensitive
to the glandular/adipose tissue segmentation threshold with differences of a
few percent in target D90. Skin doses are sensitive to seed orientation.
CONCLUSIONS: TG-43 dose evaluations generally underestimate doses to critical
normal organs/tissues while overestimating target doses. There is considerable
variation in MCref and TG43sim on a patient-by-patient basis, suggesting that
clinical adoption of patient-specific MC dose calculations is motivated. The
MCref framework presented herein provides a consistent modelling approach for
clinical implementation of advanced TG-186 dose calculations.
