(Saturday, 3/30/2019)
Room: Exhibit Hall
Purpose: Superficial dose is an important parameter in breast cancer radiotherapy. When treated with conventional linacs, bolus is commonly applied to improve target coverage near the surface while also managing the risk of severe skin reactions and negative cosmesis. With the introduction of the new Halcyonâ„¢ linac with exclusively 6X flattening filter free (FFF) photon beams, the impact on superficial dose and the need for bolus in breast irradiation must be studied.
Methods: In-vivo measurements of superficial dose were made with optically stimulated luminescence dosimeters on 11 breast cancer patients treated with the Halcyonâ„¢ 6X FFF linac. Additionally, measurements were made of 6X FFF and 6X beams delivered to an anthropomorphic phantom. A planning study was carried out in which 16 patients treated on the Halcyon were re-planned with a conventional linac to determine the difference in superficial dose predicted by the treatment planning system (TPS). Measures were taken to optimize the accuracy of the TPS surface dose.
Results: The use of 6X FFF beams led to higher superficial dose compared with 6X beams. The in-vivo measurements show an average superficial dose of 84.2 ± 0.5% which is an increase of approximately 13 % compared with published measurements for a 6X linac with flattening filter. Comparison of surface dose for 6X and 6X FFF beams in the phantom measurements show an increase from 70 % ± 1.3% to 84.7 ± 1.3%, which is consistent with the in-vivo measurements. The planning comparison shows an increase in skin structure receiving >70% Rx from 59 ± 4.3 % to 78 ± 2.9 % for Halcyon compared with the standard c-arm linac with flattening filter.
Conclusion: The use of 6X FFF beams were associated with substantially greater superficial dose and may obviate the need for bolus as used with traditional linacs.
Funding Support, Disclosures, and Conflict of Interest: JMM is on the advisory board of,obtained personal fee from, and received grant funding unrelated to this work from Varian Medical Systems, LD received grant funding unrelated to this work from Varian Medical Systems, and CK received honoraria for speaking engagements from Varian Medical Systems.
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