ESTRO 2021 Abstract Book

S1604

ESTRO 2021

Conclusion The study has demonstrated that the automated IMRT plan quality are competitive to TPS-2 plans in terms of healthy tissue sparing and PTV coverage. The simulated fractions demonstrate the TPS-1 plans robustness over the course of treatment. The TPS-1 built-in dose accumulation feature enables delivered dose daily evaluation and has clinical potential. Further studies are planned to confirm these results PO-1884 Determining a planning method for delivering Internal Mammary Nodal Chain radiotherapy. C. Bennett 1 , R. Bill 1 , J. Kirk 1 , D. Ledsom 2 , K. Williams 2 1 The Clatterbridge Cancer Centre, Physics, Liverpool, United Kingdom; 2 The Clatterbridge Cancer Centre, Radiotherapy, Liverpool, United Kingdom Purpose or Objective In July 2018, NICE updated their guidance on the management of early and locally advanced breast cancer. They recommended treatment of the Internal Mammary Nodal Chain (IMC) to be included in breast cancer radiotherapy for patients with node-positive invasive breast cancer. The evidence suggested treatment to the IMC gave clinically meaningful reductions in locoregional recurrence and improved overall survival. The aim of this planning study was to determine the most appropriate technique for including the IMC in the treatment volume while minimising the dose to the heart and lungs. Materials and Methods Twenty patients previously treated for breast cancer were included in this retrospective study. There were equal numbers of left and right sided patients and six also had SCF and axillary nodes treated. The current method for breast radiotherapy in the department uses tangential dynamic IMRT with opposing beams positioned to avoid the OARs. Three new treatment plans were created for each patient: 1) VMAT with dual continuous, 210° arcs positioned around the ipsilateral side of the patient, 2) tangential VMAT with two arcs of 80° each and 3) tangential dynamic IMRT with modified wide tangents to include the IMC. All patients were scanned in DIBH and the plans were created using Eclipse treatment planning system. The plans were evaluated for target coverage and against the recommended OAR dose constraints in the RCR guidelines and the current constraints in the local departmental breast protocol. There was also a comparison with the dosimetric properties achieved in the original clinical plan. Results There was significantly reduced V95% coverage of the whole breast CTV with both VMAT techniques compared with IMRT (p <0.02). All new techniques resulted in increased ipsilateral lung dose but there was a significant increase with VMAT techniques over MWT IMRT (p <0.01). The VMAT techniques reduced the high dose volume to the heart but there were significant increases in the low dose volume and mean heart dose with VMAT techniques compared to MWT IMRT (p <0.01). Continuous VMAT was the only technique able to achieve V90% > 95% to the IMC. However, all techniques were able to achieve V90% > 90% and V80% > 98%.