Purpose: Traditionally, an ultrasound-based volume study is performed during an in-person visit for patients who choose to receive PPB. This study is to investigate the feasibility of using diagnostic staging MR images for pre-planning of PPB in replacement of ultrasound-based volume study to minimize patients’ in-person visits.
Methods: Diagnostic staging MR images of fifteen previous PPB patients were retrospectively retrieved. High resolution T2 images (0.5mmx0.5 mm in plane, 3mm thickness) were imported into brachytherapy treatment planning system. Prostate volumes were delineated and compared with volumes from both ultrasound-based pre-treatment volume study and intra-operative planning study. Treatment plans using I-125 seeds were generated by the same physician/physics team. The prescription dose, seeds activity and dosimetric criteria of MRI-based plans were kept the same as their corresponding ultrasound-based plans. The number of seeds required in MRI-based plans were compared with ultrasound-based plans. The patterns of needles and seeds were visually compared too.
Results: The prostate volumes of 15 patients from MRI-based volume study range from 21.5cc to 76.2cc with average of 36.1cc ± 15.1cc (1SD). The volume difference between MRI-based pre-plan and ultrasound-based pre-plan is 2.0cc ± 1.5cc (1SD) and not statistical significant (p value=0.77). The similar small difference 2.9cc ± 2.2cc (1SD), (p value=0.88) was observed between MRI-based pre-plan and ultrasound-based intra-operative plan. The number of seeds used in MRI pre-plan is very close to the number used in ultrasound-based plans with average difference 3±2 (1SD), (p value=0.81). The implant pattern, particularly the seeds pattern in each needle can be very different due to probe caused gland deformation and imaging slicing orientation between MRI imaging and ultrasound study.
Conclusion: MRI-based virtual volume study can generate similar plan as ultrasound-based volume study and be used for ordering loosed seeds. However, using MRI-based plan to order pre-loaded needles need to be further studied.