ESTRO 2021 Abstract Book

S498

ESTRO 2021

Results Mean ITV volume 219cc (SD 66cc) and 6 ITVs were right sided. The median mean heart dose (MHD) was significantly lower in PBSP plans compared to photon plans (7Gy v 14Gy v 17Gy, p<0.001). CAA was an average of 15% (SD 2%) of the heart volume. Maximum D1cc to CAA was significantly lower with both photon and PBSP cardiac avoidance plans compared to standard plans without cardiac avoidance (19Gy v 22Gy v 41Gy, p=0.003) (figure 2). There was no difference in ITV D95% between the 3 planning strategies. Median mean lung dose (MLD) was 16Gy for standard and cardiac avoidance photon plans; MLD was significantly lower at 9Gy with PBSP (p<0.01). Ability to achieve D1cc<23Gy to CAA depended on the location of the ITV. If CAA>5mm from ITV then D1cc was already <23Gy in standard photon plans. No reduction to CAA D1cc was possible with either PBSP or photons if ITV and CAA overlapped. 9 plans where ITV was on the same axial plane but not overlapping the CAA were analysed. The mean dose reduction in D1cc to the CAA in these patients was 24Gy (SD10Gy) with photons and 26Gy (SD 13Gy) with PBSP.

Conclusion In this cohort, dose reduction to a cardiac avoidance area can be achieved with photons and protons without compromising ITV coverage or increasing MLD. Protons did not allow more reduction to D1cc than photons. The location of the ITV determines the maximum dose reduction that can be achieved, allowing selection of patients that could benefit from this strategy.

OC-0634 Lymphocyte Sparing Radiation Therapy for stage III NSCLC: a dosimetric study P. Fernandes 1 , Y. Jourani 1 , W. Birkfellner 2 , F. Charlier 3 , A. Ferreira 3 , G. Van de Ven 3 , L. Moretti 3 , E. Fardeau 3 , D. Van Gestel 3 , N. Reynaert 1