Gachon University Gil Medical Center INCHEON, Incheon
J. G. Lee1, Y. T. Kim2, J. Sung3, Y. Choi4, I. J. Lee5, and J. W. Kim6; 1Gachon University Gil Medical Center, Incheon, Korea, Republic of (South), 2Department of Radiation Oncology, Yonsei Cancer Center, Yonsei University College of Medicine, Seoul, Korea, Republic of (South), 3Korea University, Seoul, Korea, Republic Of, 4Department of Radiation Oncology, Gangnam Severance Hospital, Yonsei University College of Medicine, Seoul, Korea, Republic of (South), 5Yonsei Cancer Center, Seoul, Korea, Republic of (South), 6Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada
Purpose/Objective(s): MRI is an imaging modality of choice for soft tissue and bone lesions, and MRI-guided radiotherapy (MRgRT) is increasingly utilized with the potential benefit of reducing PTV margins. The high specificity of prostate-specific membrane antigen (PSMA) PET can help identify active lesions in prostate cancer patients. We aim to determine the role of combining PSMA PET and MRgRT for prostate cancer patients who were treated with curative aim. Materials/
Methods: With the introduction of 1.5 T MR-Linac in August 2021 and PSMA PET-CT in October 2022 at our institution, prostate cancer patients underwent RT with PSMA PET-assisted targeting and MRI guidance at the physician’s discretion. A uniform margin of 4 mm was added to CTV for the prostate, tumor bed, and pelvic nodal chain, and 3 mm to metastatic lymph node(s) and bone lesion(s), showing PSMA uptake to define PTV. For daily plan adaptation, adapt-to-position (ATP) was used without recontouring, while adapt-to-shape (ATS) was utilized when recontouring of target or organ-at-risk was required. This study was approved by the institutional review board (IRB No. 3-2022-0413). Results: A total of 24 patients were treated with PSMA PET-assisted MRgRT for prostate cancer. The median interval between histologic diagnosis and the start of RT was 21.5 (range 2-177) months, and 14 patients received hormone therapy, nine of whom showed refractoriness prior to RT. Median follow-up after RT was 13.4 (range 0.3-29) weeks. Treatment was classified into five categories: prostate or pelvic RT with bone oligometastatic or nodal recurrence (57.6-60 Gy in 20-24 fractions, Group 1, n=7), salvage RT for solitary bone metastasis (30-42 Gy in 3-10 fractions, Group 2, n=3), salvage re-irradiation for primary, nodal or bone recurrence (30-54 Gy in 5-20 fractions, Group 3, n=5), salvage RT for biochemical recurrence without PSMA uptake (57.6 Gy in 24 fractions, Group 4, n=6), and definitive RT with intralesional boost (>107%) (60 Gy in 20 fractions, Group 5, n=3). The average pre-RT PSA was 5.6 (±13.2) ng/mL, and the average reduction of PSA was 4.9 (±13.3) ng/mL at the median 10 (range 0-21) weeks following RT. No grade 3 or higher toxicity (CTCAE 5.0) was reported. Conclusion: PSMA PET-assisted MRgRT can help reduce PTV margins and escalate RT dose to active lesions. Category-based outcome assessment with longer follow-up can help prioritize the needs for such highly specialized treatment.
