A Clinical Process for Imaging, Planning, and Delivery of Dose Escalation for Intraprostatic Gross Tumor Volumes

Abstract

Conventional prostate cancer radiation therapy uniformly irradiates the entire prostate gland. This practice ignores evidence that the gross tumor volume (GTV) within the gland impacts local recurrence and metastatic spread. We have developed a clinical process to image, plan and deliver an escalated dose to an intraprostatic GTV. Patients undergo an MRI study with endorectal coil, MRI guided biopsy, and fiducial marker insertion, followed by CT/MRI simulation. The coil causes anatomical distortions that necessitate deformable registration. Images are imported into the treatment planning system. The prostate and GTV are contoured on T2 and ADC images and exported to a biomechanical-based deformable registration program. Deformable registration is performed and the resulting GTV contours are exported to the planning system. Volumetric modulated arc therapy plans are developed to deliver 95 Gy in 38 fractions to the GTV, while delivering 76 Gy to the remaining prostate. Planning criteria include PTV D99 of at least 95% of the prescription, and no more than 90 Gy to 0.5 cc of rectum or bladder. Six patients to date have been enrolled on a prospective research ethics board approved study and planned as described above. Patients has 1-3 GTVs with a median GTV volume of 1.8 cc (range, 0.1 - 4.4 cc). The accuracy of the deformable registration was assessed to be systematically better than 0.5 mm, with a standard deviation of 1.6 mm or less in any direction. The deformation vector field was available for four of the patients. The average maximum GTV deformation (3D vector) was 5 mm (range, 3 - 9 mm). Four patients met all target and normal tissue criteria, while 2 patients required compromise of the high dose PTV (D99 reduced to 86 and 87 Gy) to spare rectum. The median dose to 0.5 cc of rectum and bladder were 83 Gy and 79 Gy, and the median rectum D30 was 37 Gy. A clinical process that images intraprostatic GTVs, applies a deformable registration, and develops a treatment plan to deliver escalated dose to the GTV while minimizing normal tissue doses is feasible. Deformation caused by the endorectal coil is substantial for these small GTVs, demonstrating the necessity of deformable registration. Further investigations will attempt to incorporate MRI-based image guidance for daily target localization.