WE‐C‐108‐09: Patient‐Specific Monte Carlo Dosimetry for I–125 Prostate Brachytherapy

Purpose: To investigate dose distributions in the prostate and surrounding tissues for I–125 brachytherapy using Monte Carlo (MC) calculations with patient‐specific phantoms derived from CT images. We explore the effectiveness of different techniques to mitigate streaking artifacts due to brachytherapy sources in post‐implant CT images. Methods: Nine patients (45–87 sources) are considered. Streaking artifacts in post‐implant CT images are mitigated using various metallic artifact reduction (MAR) techniques: raw sinogram, fan beam virtual sinogram and 3D median filter. Segmented structures (CTV, rectum and bladder) guide the assignment of tissues (air, muscle, prostate, calcification, average tissue and bone) to develop patient‐specific MC phantoms. The EGSnrc user‐code Brachydose and GEANT4 user‐code ALGEBRA are employed for MC dose calculations using patient‐specific phantoms derived from CT images. Dose distributions generated with patient models derived using different MAR techniques and different calculation methods (MC, TG‐43) are compared directly, and using recommended dose metrics. Results: Application of each MAR technique to patient CT data results in comparable mitigation of streaking artifacts within the treatment volume in CT images. MC calculations based on uncorrected CT data Result in high dose spikes (>200%) in the treatment volume and lower doses (40–70%) in surrounding tissues compared to MC calculations with corrected phantoms. Dose distributions within the prostate from MC simulations using phantoms generated with different MAR techniques are comparable to each other but differ from TG‐43 calculations with significant (10%+) local differences. Conclusion: Mitigation of streaking artifacts in CT images is necessary for patient‐specific MC dose calculations; however, dose distributions and clinical metrics in the target and OAR are insensitive to the particular MAR technique applied. Differences between MC and TG‐43 dose distributions within the target volume and surrounding organs for I‐125 prostate brachytherapy underline the importance of patient‐specific model‐based dose calculations for treatment planning and evaluation. NSERC, CCSRI, The Canada Research Chairs Program