3349 - Genitourinary Dosimetry and Early Urinary Quality of Life in a Randomized Prostate SAbR Trial with Planned Dose Heterogeneity
Presenter(s)
K. Pithadia1, H. Zhang1, C. Y. Liao1, R. T. Dess2, R. Hannan1, K. L. Stephans3, R. D. Tendulkar3, T. P. Robin4, A. N. Slade5, J. P. Christodouleas6, D. E. Spratt7, A. Garant1, W. C. Jackson2, C. Gonzalez1, Y. Yan1, Y. Gonzalez1, M. Lee8, R. D. Timmerman1, N. B. Desai9, and M. H. Lin1; 1Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, 2Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 3Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, 4Department of Radiation Oncology, University of Colorado School of Medicine, Aurora, CO, 5Department of Radiation Oncology, Stony Brook University Hospital, Stony Brook, NY, 6Department of Radiation Oncology, Perelman School of Medicine of the University of Pennsylvania, Philadelphia, PA, 7University Hospitals Seidman Cancer Center, Case Western Reserve University, Cleveland, OH, 8University of Texas Houston McGovern Medical School, Houston, TX, 9University of Texas Southwestern Medical Center, Dallas, TX
Purpose/Objective(s): Prospective datasets with substantial planned dose heterogeneity are uncommon in prostate stereotactic ablative radiotherapy (SAbR). We leveraged a randomized phase II trial allowing 40–45 Gy prescriptions with neurovascular-sparing dose de-escalation to characterize genitourinary dosimetry, evaluate incidental urethral and bladder wall sparing, and assess associations between genitourinary dose and early urinary quality of life (QoL).
Materials/Methods: This retrospective analysis included patients enrolled on a patient-blinded, multicenter phase II trial of prostate SAbR (40–45 Gy/5 fractions by institution preference) with or without neurovascular bundle (NVB) dose de-escalation to 30 Gy. Randomization was stratified by institution and thus indirectly prescription dose. Bladder wall, urethral, NVB, and internal pudendal artery (IPA) dosimetric parameters were extracted from approved plans. We compared dosimetry between arms using Wilcoxon rank-sum test. Associations between dosimetry and 3-month change in Expanded Prostate Cancer Index Composite (EPIC) urinary QoL were evaluated using Spearman rank correlation coefficient (?), and dosimetry was compared between patients with and without a minimal important difference (MID) using Wilcoxon rank-sum test.
Results:
Among evaluable patients, protocol-directed neurovascular sparing in the experimental arm resulted in significantly lower NVB D0.03 cc compared with control (experimental: 56/59 evaluable, median 30.6 Gy [IQR 30.0–31.18] vs control: 52/59 evaluable, median 44.0 Gy [IQR 42.3–47.1]; p<0.001). Similarly, IPA D0.03 cc was lower in the experimental arm (median 14.9 Gy [IQR 13.5–16.9] vs 19.8 Gy [IQR 17.1–22.3]; p<0.001). In contrast, bladder wall and urethral dose metrics were similar between arms, demonstrating no incidental genitourinary sparing. Median bladder wall V18.3 Gy was 17.23% (IQR 13.92–22.9) in the experimental arm versus 16.64% (IQR 12.41–19.43) in control (p=0.13). Median urethral D0.03 cc was 42.24 Gy (IQR 41.7–45.8) in the experimental arm versus 42.0 Gy (IQR 41.5–45.79) in the control arm (p=0.89). Bladder wall and urethral dosimetry were not significantly correlated with 3-month change in EPIC urinary incontinence or obstructive/irritative outcomes, when assessed as continuous dose–response (all |?|<0.15). These parameters also were not associated with urinary MID (all p>0.05).Conclusion: In this prospective randomized prostate SAbR trial, neurovascular sparing was achieved without varying urethral or bladder wall dosimetry. Despite substantial genitourinary dose heterogeneity, no clear early dose–response relationship with urinary QoL was observed at 3 months. With 2-year QoL as the primary endpoint, longer follow-up will clarify clinically meaningful urethral and bladder dose associations.