Method
prostate
PROSTATE
Ten prostate radiotherapy patients were included in the study. These patients were recruited to the NINJA (Novel Integration of New prostate radiation therapy schedules with adjuvant Androgen deprivation) clinical trial (ACTRN12618001806257) which had local ethics approval (HREC/18/LPOOL/420), investigating stereotactic radiotherapy to the prostate comparing monotherapy against a virtual high dose rate brachytherapy boost regimen. This clinical trial also contains an MRI-only planning sub-study, demonstrating the ability to fully transition centres from CT- to MRI-based prostate radiotherapy planning, which these patients were enrolled into. Patients in this trial were prescribed either 40 Gy in 5 fractions or a stereotactic boost of 20 Gy in 2 fractions followed by a standard 36 Gy in 12 fractions, with treatment plans consisting of two VMAT arcs. The patients involved in the current study were part of the MRI-only planning sub-study, and scanned on a Siemens (Erlangen, Germany) Skyra 3T MRI with a flat radiotherapy couch and body coil mounted on coil mounts as per trial protocol. Patient age ranged from 60 to 72, and Body Mass Index (BMI) ranged from 23.1 to 32, with a mean of 27.1.The standard planning MRI sequence as used for the clinical trial was a 3D T2-weighted isotropic SPACE (Sampling Perfection with Application optimised Contrasts using different flip angle Evolution) sequence which covered the entire pelvis, with scan limits from L5/S1 to the pubic symphysis. This sequence was previously validated against CT by Dowling et al. [
MRI sequence parameters which differed between the standard and fast T2 SPACE and the average scan time for each sequence
MRI sequences were converted to sCT using a hybrid atlas-voxel method as described in Dowling et al. [
Treatment planning for these patients was completed on the S-sCT using the Pinnacle Treatment Planning System (v16.21; Philips Healthcare, Andover, MA) utilising the auto-planning module for beam optimisation. Patient treatment plans met all trial guidelines, with each treatment plan consisting of two full Volumetric Arc Therapy (VMAT) treatment beams. Each patient’s corresponding clinically approved treatment plan was copied to the F-sCT and recalculated for comparison of isocentre point dose, a 1%/1mm global gamma comparison and DVH analysis of the PTV, bladder and rectum.