Radiation and Cancer Physics

PD 19 - Physics 12 - Poster Discussion - Treatment Planning

1162 - Dosimetric Predictors For Attaining Rectal V3600cGy <1cc During SBRT for Prostate Cancer

Wednesday, October 24
3:15 PM - 3:21 PM
Location: Room 217 A/B

Dosimetric Predictors For Attaining Rectal V3600cGy <1cc During SBRT for Prostate Cancer
S. R. Blacksburg1, R. Sheu2, T. J. Carpenter1, G. Demircioglu1, J. Morgenstern1, A. Mirza1, M. R. Witten1, P. Endres1, and J. A. Haas1; 1NYU Winthrop Hospital, Mineola, NY, 2Icahn School of Medicine at Mount Sinai, New York, NY

Purpose/Objective(s): With increased utilization, Stereotactic Body Radiation Therapy (SBRT) remains an emerging method of treating men with localized prostate cancer. Despite over a decade of use, there exists considerable variability in prescriptions and planning objectives for prostate SBRT between institutions and collaborative groups. One commonly cited dosimetric parameter involves the Rectal V3600cGy<1cc. This study characterizes dosimetric predictors of incidentally exceeding this parameter when not actively constrained.

Materials/Methods: Between April 2, 2010 and January 6, 2018, 1645 consecutive patients with localized prostate cancer were treated with definitive robotic-based SBRT at our institution. The median prescription dose was 3500cGy (3500-3625) delivered in 5 fractions. The median age was 67 years (41-93). Based on NCCN risk categories, 31.6%, 56.4%, and 12.0% had low, intermediate, and high risk disease, respectively. Neoadjuvant and concurrent Androgen Deprivation Therapy (ADT) was utilized for 16.3% of cases. CTV was defined as prostate with 1cm of proximal seminal vesicle. An anisotropic expansion of 5mm with 3mm in the posterior direction was utilized for PTV. The mean CTV and PTV sizes were 81.9cc’s (10.25-247.4) and 140.9cc’s (37.51-358.2). 99.8% of plans had a PTV D95>3500cGy. Institutional dose objectives have been previously described. Pearson's chi-squared test was conducted for univariate regression.

Results: The mean Rectal V3600cGy was 0.7cc’s (0-6.24), and 387 (22.3%) treatment plans exhibited incidental Rectal V3600cGy >1cc. Attaining a PTV D95 >3625cGy predicted for a high probability of exceeding this objective (64.0% vs. 21.2%, p<.0001). CTV size >100cc’s also predicted for this finding (24.7% vs. 19.6%, p=.04). Plans for patients with intermediate and high risk disease had a higher incidence than those with low risk disease (25.8% and 24.2% vs. 17.8%, p=.004). Plans for patients on ADT also had an increased risk (30.7% vs. 22.2%, p=.003). On multivariate analysis, the lone predictor for incidental Rectal V3600cGy >1cc was PTV D95 >3625cGy (OR 5.991, CI 3.778-9.501, p<.001).

Conclusion: Institutional dosimetric objectives vary widely for SBRT for prostate cancer. With tight margins and rigid internal objectives, the incidental Rectal V3600cGy >1cc was relatively high on retrospective analysis of over 1600 treatment plans treated with prescribed doses of 3500cGy-3600cGy. With a 3mm posterior margin, most plans that attained a PTV D95 >3625cGy failed to meet this objective. Prospective investigation into dose prescriptions and optimal normal tissue objectives should be encouraged to attain consensus regarding justifiable future planning parameters.

Author Disclosure: S.R. Blacksburg: None. R. Sheu: None. T.J. Carpenter: None. A. Mirza: None. M.R. Witten: Consultant; Accuray. P. Endres: None. J.A. Haas: Consultant; Accuray.

Seth Blacksburg, MD, MBA

NYU Winthrop Hospital

Disclosure:
Employment
none: none

Compensation
Accuray: Speaker's Bureau

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