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OAR Dose Tolerance Recommendations for Prostate and Cervical HDR Brachytherapy: Dose Versus Volume Metrics

S Geneser*, J Cunha , J Pouliot , I Hsu , UC San Francisco, San Francisco, CA

Presentations

SU-C-16A-5 Sunday 1:00PM - 1:55PM Room: 16A

Purpose: HDR brachytherapy consensus dose tolerance recommendations for organs at risk (OARs) remain widely debated. Prospective trials reporting metrics must be sufficiently data-dense to assess adverse affects and identify optimally predictive tolerances. We explore the tradeoffs between reporting dose-metrics versus volume-metrics and the potential impact on trial outcome analysis and tolerance recommendations.

Methods: We analyzed 26 prostate patients receiving 15 Gy HDR single-fraction brachytherapy boost to 45 Gy external beam radiation therapy and 28 cervical patients receiving 28 Gy HDR brachytherapy monotherapy in 4 fractions using 2 implants. For each OAR structure, a robust linear regression fit was performed for the dose-metrics as a function of the volume-metrics. The plan quality information provided by recommended dose-metric and volume-metric values were compared.

Results: For prostate rectal dose, D2cc and V75 lie close to the regression line, indicating they are similarly informative. Two outliers for prostate urethral dose are substantially different from the remaining cohort in terms of D0.1cc and V75, but not D1cc, suggesting the choice of reporting dose metric is essential. For prostate bladder and cervical bladder, rectum, and bowel, dose outliers are more apparent via V75 than recommended dose-metrics. This suggests that for prostate bladder dose and all cervical OAR doses, the recommended volume-metrics may be better predictors of clinical outcome than dose-metrics.

Conclusion: For plan acceptance criteria, dose and volume-metrics are reciprocally equivalent. However, reporting dose-metrics or volume-metrics alone provides substantially different information. Our results suggest that volume-metrics may be more sensitive to differences in planned dose, and if one metric must be chosen, volume-metrics are preferable. However, reporting discrete DVH points severely limits the ability to identify planning tolerances most predictive of adverse effects. Thus, we recommend that full OAR DVH reporting be required for future prospective trials.

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