Dosimetry Validation of MR-Only Treatment Planning Based upon the Synthetic MRCAT Pelvis with Continuous Hounsfield Units in Prostate Cancer

Abstract

MR-based radiotherapy delivery systems are increasingly encouraging treatment planning based upon an AI generated synthetic CT created from an MR image. Concerns remain concerning the dosimetric accuracy of this approach. to evaluate and compare the feasibilities and dosimetry of magnetic resonance (MR) image-based planning using pelvic synthetic computed tomography (sCT) versus conventional computed tomography (cCT) based planning. We used the new MRCAT pelvis MRI-SIM (1.5T) with continuous Hounsfield units for dose calculation. The hypothesis was that planning based upon sCT vs cCT would not lead to a difference in dose delivery. A total of 10 consecutive patients with prostate cancer representing a range of treatment regimens underwent both a conventional CT based 3d simulation and also an MR simulation from which an MRI-simulation procedure for calculating attenuation (MRCAT) image was generated. A VMAT plan was generated based upon the cCT, and then applied to the sCT without re-optimization. Delivered dose was recalculated on the sCT plan using the fixed monitor units previously optimized for the cCT. Dose volume histograms (DVHs) were calculated for targets and organs at risk (OAR) and compared with those achieved with the cCT. In addition, dose values from 10 different point-coordinates and dose distributions volumes (i.e., volume receiving 80%, 50%, 30% prescribe dose etc.) were compared. All patients were receiving definitive RT to prostate: 2 prostate and seminal vesicles and 8 prostate with lymph nodes were treated. Median prostate volume was 34.3 cc (range 29 cc-84 cc), median number of fractions was 12 (range 5 fr-26 fr), median prescription dose was 51.6 Gy (range 40 Gy-70.2 Gy). The mean dose to the prostate planning target volume and six different OAR were calculated on sCT and cCT, differed by 0.7% ± 1.3% (p = 0.02). Three dose distribution line volume between 30%-80% of prescribed dose, differed by 1.8%±3% (p = 0.02). Ten point-coordinate dose values from different tissues including bone, fat, air and muscle around the planning treatment volume differed by 0.4% ±0.7% (p = 0.03). There was a very small (∼ 1%), but clinically insignificant, difference in the dose distribution results obtained using sCT from MRCAT compared with those obtained with cCT. The Philips pelvic MRCAT, even in the absence of a cCT, is suitable for clinical treatment planning in prostate radiotherapy.