SU‐E‐T‐464: Dosimetric Assessment of a New Multi‐Center Protocol for Radiotherapy of Multiple Ipsilateral Breast Cancers

Purpose: To assess dosimetric planning feasibility for enrolling patients into a new prospective multi‐center protocol (ACOSOG Protocol Z11102) of breast conservation surgery with radiotherapy (RT) of multiple ipsilateral breast cancers (MIBC) including multicentric and/or multifocal breast cancers. Methods: CT data for 9 representative breast patients with two lumpectomy cavities in a breast treated in prone position were retrospectively analyzed. The boost PTV for lumpectomy cavity was delineated based on the seroma and/or surgical clips with 1 to 2 cm non‐uniform 3D expansion, not including chestwall, skin, and bone. A whole breast irradiation (WBI) of 50 Gy in 25 fractions was planned using 3D tangential beams of 6 or 15 MV photons with necessary wedges and/or field‐in‐field techniques to provide uniform dose coverage. No nodal irradiation was planned. The dose was calculated with heterogeneity correction using a treatment planning system (Xio v4.80, Elekta). For each boost PTV, additional 10 Gy in 5 fractions was planned by using one single photon beam, either 6 or 15 MV depending on the depth of the PTV, oriented in the direction with shortest depth to the PTV. All plans were evaluated to check whether all the dose‐volume criteria required by the protocol can be met. Results: All plans generated for patients in our study meet the dose‐volume criteria required by the protocol. For all dose‐volume parameters, the discrepancies between the patient averages and dose‐volume goals were separated by at least 1.4 sigmas (standard deviation). The average for the percentage breast volume receiving 60 Gy, a major concern because of the large boost PTVs, was (29.7±7.3)%, well below the required value of 40%. Conclusion: The dose‐volume criteria required for the new multi‐center protocol of radiotherapy for multiple ipsilateral breast cancers can be met with no major difficulties with the standard planning techniques.