Partial Breast Extracranial Radiosurgery: A Dosimetric Evaluation of Treatment Delivery Accuracy

Abstract

Partial breast extra-cranial radiosurgery was proposed to treat early stage breast cancer patient preoperatively. Cone-beam CT (CBCT) acquired prior to treatment represents the actual patient position during the treatment. This study evaluates dosimetric variations between the delivered plan (DP) based on CBCT and the original plan (OP) based on the planning CT. Seven patients treated to 15 Gy in the prone position were included in this study. The gross tumor volume (GTV) (volume: 0.5 cc to 2.7 cc) was defined based on fused CT and breast MR images. The clinical target volume (CTV) encompassed the GTV + a 1.5-cm isotropic margin. An additional 0.3-cm isotropic margin was added to form the planning target volume (PTV). Both CTV and PTV excluded the outermost 0.5 cm of the patient surface. A 3 mm thick region along the ipsilateral breast surface was defined as skin to analyze dose to the subcutaneous tissue. Four to five non-coplanar photon beams were arranged to avoid heart, lungs and contralateral breast and intensity modulated radiation therapy (IMRT) provided further optimization. The plans were normalized to deliver 100% of the prescription to at least 95% of the CTV. For treatment, target localization was achieved based on both skin surface and soft-tissue tumor and/or biopsy markers using CBCT. To standardize the CBCT images, the CT numbers were assigned - breast to -122 HU, lungs to -719 HU, and all others to 40 HU. The GTV, CTV, and PTV were copied to CBCT based on the target localization while the skin, breasts, and lungs were manually contoured. The delivered dose distribution was re-calculated on the CBCT. Dosimetric parameters of OP and DP were compared. The average CTV receiving 100% of the prescription (V100%) was 95.5% ± 0.4 vs 93.2% ± 5.7 (OP vs. DP). The difference in V100% between OP and DP was > 10% for 2 patients (DP ≤ OP). However, the difference in V96% between OP and DP was < 1% for all 7 patients. The average maximum dose to skin was 76.7% ± 13.9 vs. 77.4% ± 11.7. The difference in maximum skin dose between OP and DP was > 8% for 3 patients (DP ≥ OP for 2 patients and ≤ OP for 1 patient). The lung V5Gy was ≤ 1% for all patients in both OP and DP. The contralateral breast V1Gy was 8.1% vs 12.9% for 1 patient, 6 patients had V1Gy < 1% in both OP and DP. The heart V5Gy was zero in OP for all patients, and it was not delineated in CBCT due to difficulties with image quality. Partial breast radiosurgery in this cohort was delivered with a high level of accuracy to the target coverage (CTV V96% achieved for all patients within 1%). Similarly, doses to critical organs were essentially as planned. However, small differences in skin doses were seen due to the effect of set-up variation on breast shape.