Magnetic resonance imaging‐only‐based radiation treatment planning for simultaneous integrated boost of multiparametric magnetic resonance imaging‐defined dominant intraprostatic lesions

Abstract Objective To assess the feasibility of using synthetic computed tomography for treatment planning of the dominant intraprostatic lesion (DIL), a high‐risk region of interest that offers potential for increased local tumor control. Methods A dosimetric study was performed on 15 prostate cancer patients with biopsy‐proven prostate cancer who had undergone magnetic resonance imaging. DILs were contoured based on the turbo spin echo T2‐weighted and diffusion weighted images. Air, bone, fat, and soft tissue were segmented and assigned bulk‐density HU values of –1000, 285, –50, and 40, respectively, to create a synthetic computed tomography. Simultaneous integrated boost (SIB) and standard treatment plans were created for each patient. The total dose was 79.2 Gy to the non‐boosted planning target volume for both plans with a boost of 100 Gy for the DIL in the SIB plan. A radiobiological model was created to determine individualized dose–response curves based on the patient's apparent diffusion coefficient maps. Results Mean doses to the non‐boost planning target volume were 81.2 ± 0.3 Gy with the SIB and 81.0 ± 0.4 Gy without. For the DIL, the boosted mean dose was 102.6 ± 0.6 Gy. Total motor unit was 860 ± 100 with the SIB and 730 ±100 without. Femoral heads, rectum, bladder, and penile bulb were within established dose guidelines for either treatment technique. The average tumor control probability was 94% with the SIB compared with 78% without boosting the DIL. Conclusion This study showed the feasibility of magnetic resonance imaging‐only treatment planning for patients with prostate cancer with a SIB to the DIL. DIL dose can be escalated to 100 Gy on synthetic computed tomography, while maintaining the original 79.2 Gy prescription dose and the organ of interest clinical dose limits.