Receiver Operating Curves and Dose-Volume Analysis of Late Toxicity with Stereotactic Body Radiotherapy for Prostate Cancer

Publication date: Available online 17 July 2016
Source:Practical Radiation Oncology
Author(s): Li Zhang, Julian Johnson, Alexander R. Gottschalk, Albert J. Chang, I-Chow Hsu, Mack Roach, Zachary A. Seymour
PurposeThe purpose of this study was to evaluate a receiver operating characteristic (ROC) curve method to determine dose thresholds with late genitourinary (GU) toxicity after stereotactic body radiation therapy (SBRT) for prostate cancer.Methods and materialsSeventy-eight patients diagnosed with low- to intermediate-risk prostate cancer and treated with SBRT alone were reviewed retrospectively. All patients received a total dose of 38Gy in 4 fractions with a planning target volume expansion of 2mm. GU toxicity was documented according to the Common Terminology Criteria for Adverse Events (version 4). ROC analysis applied on a logistic regression model was used to determine optimal dosimetric parameters for GU toxicity.ResultsThe median age at treatment was 69years with a median prostate volume of 46.2mL. The median prescription isodose line was 67% (interquartile range: 65–70%). The median clinical follow-up was 35.49months. Late grade 1, 2, and 3 GU toxicity occurred in 21.8%, 19.2%, and 2.6% of cases, respectively. Late grade 2+GU toxicity was associated with prescription isodose line (p=0.009) and normalized volumes for heterogeneity ≥46Gy. The ROC method successfully produced thresholds for dose-volume recommendations for both prostate and urethra, including a normalized prostate volumes from 46–50Gy, such as V46Gy of 36.7% (sensitivity 71%, specificity 61%, AUC 0.67) with an associated probability of late GU grade 2+ toxicity of 21%.ConclusionsIntraprostatic heterogeneity should be controlled with potential thresholds at V46Gy <36.7%, V48Gy <21%, and V50Gy <9.5% of the normalized prostate volume to keep late grade 2+ GU toxicity ≤20% with four fraction schemes. This may be facilitated with a higher prescription IDL (>69%).



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