3216 - Dosimetric Feasibility of Genomically Stratified Dominant Lesion Boost In Two-Fraction Prostate SBRT
Presenter(s)
N. Aghdam1, V. Santos2, C. Mendez3, A. Sanchez3, A. Katz4, A. Corcoran4, H. Lepor5, W. Huang5, D. R. Wise6, T. J. Carpenter3, A. Tong7, M. J. Zelefsky8, J. W. Lischalk2, and J. A. Haas3; 1Department of Radiation Oncology, Beth Israel Deaconess Medical Center, Boston, MA, 2Department of Radiation Oncology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, New York, NY, 3Department of Radiation Oncology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, Mineola, NY, 4Department of Urology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, Mineola, NY, 5Department of Urology, Perlmutter Cancer Center at New York University Grossman School of Medicine, New York, NY, 6Department of Medicine, Perlmutter Cancer Center at NYU Langone Medical Center, New York, NY, 7Department of Radiology, NYU Langone Health and New York University, Grossman School of Medicine, New York, NY, 8NYU Langone Health, New York, NY
Purpose/Objective(s): Extreme hypofractionation to two fractions enables substantial radiobiologic escalation in localized prostate cancer. In this Phase I/Ib, Single Arm Study of Two Fraction SBRT, we evaluated whether addition of a genomically guided dominant intraprostatic lesion (DIL) simultaneous integrated boost (SIB) to 28 Gy affects organ-at-risk dosimetry compared with 25 Gy whole-gland treatment alone.
Materials/Methods: Twenty-two patients were analyzed, including 12 treated with SIB and 10 treated without SIB. Two fractions of SBRT were delivered to a total dose of 25 Gy in 2 weekly treatment to the whole prostate with a 28 Gy SIB to the dominant lesion in patients with unfavorable intermediate risk disease or favorable intermediate risk with an elevated decipher score. All patients underwent MRI based SBRT treatment planning after fiducial marker and rectal spacer placement. Median and mean dosimetric parameters were compared between cohorts, including urethral, bladder, rectal, CTV, and PTV metrics.
Results:
In the overall cohort, 64% (n=14) of patients had 1 lesion, 27% (n=6) had 2 lesions, and 4.5% had either 3 or 4 lesions (n=1, respectively). Approximately 82% (n=18) of patients had at least 1 lesion in the peripheral zone, and 18% (n=4) with at least 1 lesion in the transition zone. Median rectum maximum dose was 2558 cGy, bladder maximum dose was 2626 cGy, and prostatic urethra maximum dose was 2712 cGy. Median PTV mean dose was 2643 cGy, demonstrating consistent target coverage. Of the 12 SIB lesions, 9 were located in the peripheral zone and 3 in the transitional zone. SIB plans demonstrated lower prescription isodose lines (median 83.1% vs 86.6%), reflecting focal dose intensification. Median SIB volume was 1.84 cc (range 0.42–8.88 cc), with focal mean dose of 2953 cGy, confirming effective dose escalation. Despite escalation to 28 Gy within the dominant lesion, organ-at-risk exposure was comparable between cohorts. Median rectum maximum dose was 2572 cGy in the SIB group versus 2531 cGy in the non-SIB group. Median bladder maximum dose was 2626 cGy versus 2625 cGy, respectively. Median prostatic urethra maximum dose was 2697 cGy in the SIB cohort compared with 2716 cGy in the non-SIB cohort. Mean rectal, bladder, and urethral doses were similarly not increased with SIB. PTV mean dose was nearly identical between groups (2644 vs 2645 cGy), with consistent CTV and PTV coverage.Conclusion: In this 22-patient preliminary cohort, 2 fraction SBRT with genomically stratified DIL SIB to 28 Gy achieved radiobiologic escalation without increasing urethral, bladder, or rectal dose exposure, supporting dosimetric feasibility and organ-at-risk preservation.