ESTRO 2021 Abstract Book

S1605

ESTRO 2021

Figure 1: Dose distributions typically produced by each planning method where A) original IMRT plan B) continuous VMAT C) bowtie VMAT D) MWT IMRT. Conclusion The results show that treatment to the IMC is achievable for both left and right sided breast cancer with DIBH while maintaining OAR doses within the RCR recommended constraints. With the MWT IMRT technique, CTV coverage was also maintained compared to what is currently achieved while keeping the OAR doses low, meeting all optimal constraints except for ipsilateral lung. PO-1885 Evaluation of a hybrid 3DCRT-VMAT technique for free breathing whole breast RT during COVID-19 G. Antorkas 1 , P. Doolan 1 , K. Ferentinos 2 , N. Zamboglou 2 1 German Oncology Center, Medical Physics, Limassol, Cyprus; 2 German Oncology Center, Radiation Oncology, Limassol, Cyprus Purpose or Objective Compared to 3DCRT, VMAT plans offer improved coverage and conformality, but there is a concern that in whole breast irradiation there could be an interplay effect with the breathing cycle of the patient. The prevalence of breath-hold techniques, that overcome this problem, have decreased during the COVID-19 pandemic due to the increased sterilization requirements. The aim of the study was therefore to evaluate the dosimetric results of a hybrid 3DCRT-VMAT (h3DVM) technique for free breathing whole breast irradiation, which it was hypothesized could offer the benefits of VMAT without a

large risk of interplay. Materials and Methods

Four female breast cancer patients, two left-sided and two right-sided, were selected in this retrospective planning study. A dose prescription of 42 Gy in 15 fractions was used and organ at risk constraints adhered to the RTOG 1005 protocol. Six treatment plans were generated for each patient, each with different weighting between the 3DCRT and VMAT fields (100%- 0%; 80%-20%; 60%-40%; 40%-60%; 20%-80%; and 0%-100%). The plans were created with the MONACO Treatment Planning

System. Results

Using this hybrid technique a significant increase in dose coverage (Figure 1) and conformity index (Figure 2) was achieved compared to 3DCRT alone. Conversely, an increase in doses to V20, V10, V5 of ipsilateral lung and mean dose to the heart was unavoidable. There were no significant differences in maximum dose and V1.86 in contralateral breast when the proportions of 3DCRT and VMAT were altered. By combining the two techniques it was possible to lower the mean dose of the target closer to the prescription dose, which led to a cooler plan compared to 3DCRT alone.