3250 - The Utilization of Dosimetry with Lu-177-PSMA-617 Radiopharmaceutical Treatments: Does Dosimetry Affect Treatment Change?
Presenter(s)
C. B. C. Cobb1, V. Elangovan2, A. C. Olson3, A. C. Mueller4, A. Saoudi5, J. E. Ligorski6, D. Dimitriadou5, K. Zia7, R. J. Lalonde8, K. Bennet9, H. D. Skinner10, and R. B. Patel5; 1Department of Radiation Oncology, University of Pittsburgh, UPMC Hillman Cancer Center, Pittsburgh, PA, 2Department of Radiation Oncology, University of Pittsburgh, Pittsburgh, PA, 3UPMC-Shadyside Hospital, Pittsburgh, PA, 4Department of Radiation Oncology, UPMC Hillman Cancer Center, University of Pittsburgh, Pittsburgh, PA, 5Department of Radiation Oncology, University of Pittsburgh School of Medicine; UPMC Hillman Cancer Center, Pittsburgh, PA, 6University of Pittsburgh Medical Center, Pittsburgh, PA, 7Department of Radiology, Division of Nuclear Medicine, UPMC, Pittsburgh, PA, 8Department of Radiation Oncology, Medical Physics, UPMC Shadyside, UPMC Hillman Cancer Center, University of Pittsburgh, Pittsburgh, PA, 9Department of Radiology, Division of Nuclear Medicine, University of Pittsburgh/UPMC, Pittsburgh, PA, 10Department of Radiation Oncology, UPMC Hillman Cancer Center, Pittsburgh, PA
Purpose/Objective(s):
Lu-177-PSMA-617 radiopharmaceutical is approved to treat metastatic castration-resistant prostate cancer. However, the use of dosimetry with Lu-177-PSMA-617 is not universally utilized in clinical practice. We aim to demonstrate the feasibility of doing dosimetry, to examine the treatment changes for patients who have dosimetry calculated versus those who do not, and to examine the utilization of external beam radiation therapy (EBRT) in patients treated with Lu-177-PSMA-617 who have and have not had dosimetry done.Materials/Methods:
A retrospective analysis was conducted of all patients treated with Lu-177-PSMA-617 in a large integrated cancer center network, in both Radiation Oncology clinics and Nuclear Medicine clinics, from January 2024 to February 2026. Single-time-point dosimetry was done on day four after Lu-177-PSMA-617 treatment, with dose calculations completed from a single imaging SPECT scan. Patient characteristics that were analyzed include age, treatment department, Lu-177-PSMA-617 injection dates, number of treatment cycles, dosimetry completion, number of dosimetry assessments, treatment changes and if EBRT was done within 3 months either before, during or after Lu-177-PSMA-617 treatment. A treatment change from Lu-177-PSMA-617 was defined as having a good response and being on hold, having low uptake, changing to systemic therapy and/or treatment with dose escalated EBRT. Toxicity, palliative radiation, hematologic malignancy, fragility, thrombocytopenia, laboratory changes and hospice were excluded as a treatment change.Results:
161 patients were treated in this study. 70 patients (43.5%) had dosimetry done, with the average number of dosimetry assessments being 2.9, and 91 patients (56.5%) did not. 94.6% of patients who were offered dosimetry were able to have imaging done in order to have dosimetry calculated. Of the 40 patients who had treatment changes, 33 patients (82.5%) had dosimetry. This includes treatment changes due to: EBRT (57.5%), on hold due to good response (30%), low uptake (7.5%) and changing to systemic therapy (7.5%). Of the patients who had treatment changes, 7 patients did not have dosimetry (17.5%). These treatment changes include: EBRT (5%), on hold due to good response (2.5%) and changing to systemic therapy (10%). Please see Table 1 which includes Chi-squared analysis results.Conclusion:
Dosimetry for Lu-177-PSMA-617 is feasible in routine clinical practice and associated with an increased rate of treatment changes and a higher utilization of EBRT. Future directions include prospective validation of response-based EBRT for suboptimal response to Lu-177-PSMA-617. Table 1. Results for Dosimetry vs. No Dosimetry with Treatment Change and EBRT utilization| Dosimetry | No Dosimetry | p-value | |
| Treatment Change | 82.5% | 17.5% | <0.0001 |
| EBRT | 92% | 8% | <0.0001 |