ESTRO 36 Abstract Book

S922 ESTRO 36 _______________________________________________________________________________________________

Sweden 2 Uppsala University, Experimental and clinical oncology, Uppsala, Sweden Purpose or Objective To investigate a Gleason driven dose painting approach for high risk prostate cancer patients based on outcome for conventional treatments, and using apparent diffusion coefficient (ADC) MR images for dose prescription. Material and Methods We based our retrospective study on a total of N =122 high- risk prostate patients treated with radiotherapy, with inclusion criteria to have a pre-treatment PSA<60 µg/L and biopsies analyzed a Uppsala University Hospital. The 5- year local tumor control probability was estimated with Kaplan Meier analysis to TCP obs =94.7% (CI 86.4-98.0%). The PSA inclusion condition was used to exclude patients with possible pre-treatment spread. The homogeneous treatment dose D h was estimated to 91.6 Gy EQD 2 based on α/β=1.93 for the given proton boost (20Gy in 4 fractions, RBE=1.1) and photon dose (50 Gy in 25 fractions). All patients underwent androgen deprivation therapy. We parameterized the populations dose-response TCP pop ( D ) with a logistic function with the parameter γ 50 =2.01 and D 50 chosen so that TCP pop ( D h )= TCP obs . The patients’ biopsy statements were used to construct simulated prostates with voxelized distributions of Gleason scores G varying per voxel. Voxel specific dose-response functions TCP vox ( D , G ) were derived with the logistic parameters γ 50,eff and D 50 ( G ) set so that the average TCP pat for all patients equals TCP obs at D h , and the average slope for the patients TCP pat equals the slope for TCP pop ( D ) at D h . Hence, the voxel specific dose-response functions are be described by TCP vox ( D , G )=1/(1+( D 50 ( G )/ D ) 4γ50,eff ), where D 50 ( G ) and γ 50,eff , for D = D h , reconstructs TCP vox ( D h , G <6)= C and TCP vox ( D h , G ≥6)= C - k ×( G -6). For G <6 TCP vox was set to not vary with Gleason scores since ADC-MRI likely not distinguish G <6 from normal tissue. We used 3 different values of C , a high value C high =1 resulting in zero desired dose for G <6 voxels, a low value C min resulting in a homogeneous dose distribution ( k =0), and an intermediate C im for a certain minimum dose. ADC images for a high-risk patient were translated into a 3D-map of Gleason scores based on results published by Turkbey et al. We used the above functions for dose painting to minimize the average dose while keeping the TCP pat equal to that for a homogeneous dose of D h . Results For the C high scenario the average dose decreased by 9 Gy (max dose 98 Gy). For the intermediate C im scenario the average dose decreased by 2 Gy with doses in the range of 74 to 98 Gy. Fig. 1 shows resulting Gleason score to TCP mappings normalized for a 50cc prostate while Fig. 2 shows a dose painted prostate for the C im scenario.

radiosensitivity RT. Additional sets of imaged-derived parameters will be investigated and further cell-lines be measured to identify relations with radiosensitivity for the development of a multiparametric prediction model for personalized RT in HNSCC. before the start of

Fig 1. TCP vs Gleason scores comprising a 50cc prostate volume and corresponding dose-response functions for the intermediate C im scenario.

EP-1689 Gleason driven dose painting based on ADC MR imaging E. Grönlund 1 , S. Johansson 2 , T. Nyholm 1 , A. Ahnesjö 1 1 Uppsala University, Medical radiation sciences, Uppsala,