ESTRO 38 Abstract book

S325 ESTRO 38

medical (e.g. prescribed suppositories) and Grade 2B (GR2B) was classified as procedural (included minor cautery and topical formalin application). Model parameters were derived for both the rectum and the rectal wall by maximum likelihood estimation, and profile likelihood estimation was used to calculate confidence intervals for the parameters. Patients were also sorted by NTCP values (low to high), and divided into 12 groups of 86-87 patients to allow for a comparison between the calculated NTCP and the clinically observed frequency of morbidity. Results Late GR2A+2B rectal bleeding was observed in 155/1036 patients (15%) and GR2B in 45/1036 patients (4%). The volume parameter n was low (0.04-0.14) both for GR2A+2B and GR2B, and lowest for GR2B for the rectum. For uniform irradiation of one-third of the rectum (V = 1/3) an NTCP of 5% was found at 59 Gy for GR2A+2B and at 75 Gy for GR2B (Fig. 1). There was a close to 1:1 relation between the calculated NTCPs and the observed morbidity across the 12 groups (Fig. 2). However, there were large variations between the groups, most pronounced for GR2B (Fig. 2). The area under the curve values ranged between 0.62-0.63 for GR2A+2B and 0.54-0.55 for the GR2B.

for RP patients was 26 Gy, and for

The mean dose in S 0.05

patients maps p IMRT

without

RP

was

11

Gy.

The

and p PSPT

showed a similar spatial pattern

of p EC DIV[ p EC

, quantified by both the Areas Under the

, p IMRT

] and DIV[ p EC

, p PSPT

] Curves of 0.86.

Conclusion Similar significance patterns highlighted by the 3 VBAs justify the choice to analyze the 2 sub-cohorts as a whole in search of valuable insights into the thoracic RS. The completely different dose patterns of PSPT provide an unprecedented chance to look at the results of a VBA, with a sensible reduction of a potential bias related to the spatial correlation of IMRT dose distributions. This lays more robust foundations to the previously reported hypotheses that the lower parts of the lungs and the heart play a prominent role in the development of RP [Monti et al. , SciRep 2018]. OC-0614 NTCP models of late rectal morbidity after proton therapy in 1036 prostate cancer patients J. Pedersen 1 , S. Flampouri 2 , C. Bryant 2 , Z. Li 2 , N. Mendenhall 2 , L.P. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark ; 2 University of Florida Health Proton Therapy Institute, University of Florida Health Proton Therapy Institute, Jacksonville- Florida, USA Purpose or Objective Normal tissue complication probability (NTCP) models may be used to guide patient selection and treatment evaluation for proton therapy (PT), but current models are derived from outcomes following photon-based radiotherapy. Since photons and protons have fundamentally different properties (reduced dose bath with PT and a higher relative biological effectiveness, RBE), NTCP models for photons might not be applicable to PT. The aim of this study was therefore to derive parameters of an established NTCP model when using prospectively recorded late morbidity data from PT, focusing on rectal morbidity and prostate cancer. Material and Methods Dose volume histogram data for the rectum and rectal wall from 1036 prostate cancer patients (with no pre- treatment anti-coagulation medication) treated with PT between 2006 and 2010 were used. Patients were prescribed target doses of 78-82 Gy (RBE = 1.1) in 2 Gy fractions. Lyman-Kutcher-Burman model parameters were derived for two alternative late grade 2 rectal bleeding endpoints (CTCAE v3.0): Grade 2A (GR2A) was classified as