Accelerated Partial Breast Irradiation dosimetry for a conventional versus an integrated MR-Linac
MO-0150
Abstract
Accelerated Partial Breast Irradiation dosimetry for a conventional versus an integrated MR-Linac
Authors: Jenna Dean1, Farshad Foroudi1, Michael Chao1, Felicity Height1, Sandra Fisher1, Drew Smith1, Sweet Ping Ng1
1Olivia Newton-John Cancer, Wellness and Research Centre, Radiation Oncology, Heidelberg, Australia
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Purpose or Objective
Accelerated Partial Breast Irradiation (APBI) is an accepted short-course treatment option for patients with early breast cancer. This study evaluates our early experience for APBI patients treated on the integrated Magnetic Resonance Linear accelerator (MR-Linac) as part of the departmental Feasibility of Imaging and Radiation Treatment Delivery on the MR Linac (FIRM) Study.
Material and Methods
As part of the FIRM study, nine suitable patients were parallel planned for APBI in the supine position (30Gy in 5 fractions, delivered on alternate days) for both conventional and MR-Linac. Protocol constraints and patient selection were based on the Florence APBI Trial. The cohort included left and right sided patients. A 6MV volumetric modulated arc therapy (VMAT) plan was generated for the conventional linac and a 15 field 7MV FFF step-and-shoot intensity modulated radiation therapy (IMRT) plan for the MR-Linac. Beam angles were limited to the ipsilateral side for both techniques. Target and OAR (heart, lung(s), skin, chest wall and contralateral breast) doses were collected and analysed.
Results
All plans in the study met protocol criteria. Target coverage was comparable (average D95% of 29.6Gy on the MR-Linac vs 29.2Gy for conventional). MR-Linac plans were marginally hotter on average (D2% of 31.4Gy vs 31.1Gy). Ipsilateral and contralateral lung doses were similar. Maximum dose (0.035cc) to the contralateral breast was also comparable. The volume of heart receiving 3Gy was lower for MR-Linac plans, but mean doses were slightly higher. The percentage of 15Gy to the uninvolved ipsilateral breast was almost 10% higher for the MR-Linac plans, but still well under protocol tolerance. Maximum doses to skin evaluation structures were on average 0.5Gy cooler for VMAT plans. Target position provided more challenges for MR-Linac plans due to physical machine limitations as well as proximity to skin and chest wall.
Conclusion
Our experience indicates that clinically acceptable APBI plans can be generated for patients for the MR-Linac that are comparable to conventional VMAT plans. Additional planning considerations must be applied to ensure robust MR-Linac dosimetry.