Genitourinary Cancer

PV 04 - Poster Viewing Q&A - Session 4

TU_24_2598 - Real-time MRI-guided Prostate Stereotactic Body Radiation Therapy: Impact on Treatment Delivery and Implications

Tuesday, September 17
2:45 PM - 4:00 PM
Location: ASTRO Innovation Hub

Real-time MRI-guided Prostate Stereotactic Body Radiation Therapy: Impact on Treatment Delivery and Implications
V. Agrawal1, R. F. Farjam1, H. Nagar1, D. Margolis1, J. Babich1, S. C. Formenti2, and J. Kang1; 1NewYork-Presbyterian Hospital/Weill Cornell Medicine, New York, NY, 2Cornell University, New York, NY

Purpose/Objective(s): The MRI-guided real-time on-table adaptive radiotherapy system integrates a linear accelerator with real-time magnetic resonance imaging (MRI), allowing superior delineation and tracking of soft tissue during radiation treatment (RT). In this study, we investigate the imaging characteristics of patients undergoing prostate SBRT using an MR imaging system, as part of prospective protocol (NCT03664193). We hypothesize that the high doses of radiation delivered during SBRT may result in prostate swelling and distortion, and aim to quantitate the degree of change in the volume of the prostate during treatment. The results of this study may have implications on treatment planning and delivery, and guide appropriate adaptation of planning target volumes (PTVs) of MR-guided RT for prostate cancer patients over the course of treatment.

Materials/Methods: We reviewed treatment plans of patients enrolled on an IRB-approved study for MR-guided SBRT with focal dose escalation for localized prostate cancer. All patients received a dose of 36.25 Gy in 5 fractions to the prostate +/- proximal seminal vesicles, with a PTV expansion of 3-5 mm. Patients also received a simultaneous integrated boost to visible intraprostatic nodules to dose of 37.5-45 Gy. Treatments were delivered 2-3 times per week. Prostate and bladder size before SBRT, at mid-point and at the time of last SBRT, were measured using a treatment planning system. Contouring was blinded to the original planning volumes. P values were calculated using paired t-test.

Results: A total of 35 individual SBRT delivery sessions for 7 patients were evaluated. The median prostate gland size prior to treatment was 46.3 cm3 (range, 30-75 cm3). The prostate volume significantly increased with a median increment of 10.0 cm3 (Range 3.5 cm3 to 20.7 cm3) by mid-treatment corresponding to a change of 30.6% (Range 5.2% to 41.3%), p=0.002. Baseline to end of treatment prostate size increased by a median volume of 6.1 cm3 (Range -1.7 cm3 to 14.0 cm3) corresponding to an increase of 20.8% (Range -3.9% to 30.5%), p=0.01. As a result of these changes in volume, the dose received by 95% of the planning target volume (D95) decreased by 9.6% in patients with the largest volumetric changes. Dose to 10% of the bladder increased by median of 5.3%.

Conclusion: Standard, non-MR guided RT may be unable to detect and account for the expansion and contraction in prostate volume during prostate SBRT. MR-guided prostate SBRT enables the tracking of alterations in prostate volume before, during and at the end of SBRT treatment. The preliminary results suggest a significant disparity between prostate size at time of treatment planning and during treatment with an initial expansion of the prostate by mid-treatment followed by a contraction near the end of treatment. A SBRT should adapt for prostate, bladder and rectum deformation. We have modified our protocol to enable adaptive planning and MR-guided RT for prostate cancer.

Author Disclosure: V. Agrawal: None. R.F. Farjam: None. H. Nagar: None. D. Margolis: Consultant; Blue Earth Diagnostics. J. Babich: Stock; Fusion Pharmaceuticals, Ground Fluor Pharmaceuticals, Noria Therapeutics, Inc. S. Formenti: Advisory Board; BMS, Smith Kline, Astra Zeneca, EISAI. Consultant; Regeneron Pharmaceuticals, Eli Lilly. Honoraria; Regeneron Pharmaceuticals. Research Grant; Janssen Pharmaceutials (J&J). Speaker's Bureau; Regeneron Pharmaceuticals, Varian. Travel Expenses; Regeneron Pharmaceuticals. J. Kang: None.

Vishesh Agrawal, MD

NewYork-Presbyterian/Weill Cornell Medical Center

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