Room: Exhibit Hall
Purpose: To determine the feasibility of designing radiation plans that would cover the entire prostate, respect all dose constraints of the organs at risk (OAR), and also deliver an even higher dose to the visible tumor within the prostate.
Methods: In this study 32 patients with low-risk and intermediate-risk prostate cancer will be treated with Precise Cyberknife at 38Gy total dose, over 4 fractions. Additionally to the planning CT the patients will undergo mpMRI and PSMA-PET scans to determine the dominant intra-prostatic lesion (DIL) and simultaneously boost it to 44Gy. For three patients, this process has been performed and treatment plans were created. The plans were optimized based on the PTV (prostate and basis of the vesicles), CTV(MRI+PET) (intersection of GTV-PET and GTV-MR + 4mm), CTV(MRI) (GTV shown in MRI + 4mm), urethra, bladder, rectum, sigmoid and rectal mucosa. The standard SBRT protocol was applied regarding the dose constraints to the OARs.
Results: In all the cases, the prescribed dose to the PTV was achieved (D95=38-38.2Gy). The maximum dose to urethra was below the minor variation (40.2-41.4Gy) and the same holds for rectum (37.8-39.8Gy). For bladder, two dose constraints were used (Dmax and D10%) and both satisfied either the primary goal or the minor variation (43.4-47.6Gy) and (28.4-33.3Gy), respectively. For two of the patients, the DIL was in the center of the prostate including the urethra. In those cases, the priority was to respect the dose constraints of urethra. So, for CTV(MRI+PET), D95 was 38.2-42.2Gy and for CTV(MRI) it was 38.2-40.0Gy, respectively.
Conclusion: The results of the examined patients showed that it is possible to direct the high dose inhomogeneity that characterizes SBRT plans to the DIL volume. However, in given cases the dose coverage of the boost has to be compromised depending on the location of DIL against urethra.