Room: Karl Dean Ballroom B1
Purpose: To assess the feasibility of using synthetic CT images for treatment planning of dominant intraprostatic lesions (DILs), a high risk region of interest that offers potential for increased local control.
Methods: An IRB-approved, dosimetric study was performed on fifteen prostate cancer patients who underwent MR imaging using TSE T2-weighted, GRE mDixon T1-weighted, and diffusion-weighted imaging (DWI) sequences. A radiologist interpreted the MR images, and suspected DILs were contoured based on the T2 and DWI images. Air, bone, fat, and soft tissue were segmented and assigned HU values to create a synthetic CT from the water and fat mDixon T1 sequences. Simultaneous integrated boost (SIB) and standard RapidArc treatment plans were created for each patient. Total dose was 79.2Gy to the non-boosted planning target volume (PTV) for both plans with a boost of 100Gy for the DIL in the SIB plan. A radiobiological model was created to determine individualized dose response curves based on the patientâ€™s apparent diffusion coefficient maps. Tumor control probability (TCP) and rectal normal tissue complications probability (NTCP) were also assessed.
Results: Maximum and mean doses to the non-boost PTV were 87.4Â±0.8Gy and 81.2Â±0.3Gy with the SIB and 85.2Â±0.7Gy and 81.0Â±0.4Gy without the SIB. For the DIL, maximum and mean doses were 105.1Â±1.1Gy and 102.6Â±0.6Gy. Femoral heads, rectum, bladder, and penile bulb were within established QUANTEC guidelines for both plans. The average TCP was 94% with the SIB (99% for the DIL) compared to 78% for the standard plan (83% for the DIL). The average rectal NTCP was 6.2% with the SIB and 5.3% without the SIB.
Conclusion: We demonstrated the feasibility of implementing MR-only treatment planning for prostate cancer with an SIB for DILs. Dose to the DIL can be escalated to 100Gy on the synthetic CTs while maintaining tumor control and minimizing normal tissue toxicities.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by a Research Scholar Grant, RSG-15-137-01-CCE from the American Cancer Society. The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189.