title: Receiver operating curves and dose-volume analysis of late toxicity with stereotactic body radiation therapy for prostate cancer.
abstract: OBJECTIVE
  The 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 for prostate cancer.
  
  
  METHODS
  Seventy-eight patients diagnosed with low- to intermediate-risk prostate cancer and treated with stereotactic body radiation therapy alone were reviewed retrospectively. All patients received a total dose of 38 Gy in 4 fractions with a planning target volume expansion of 2 mm. 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.
  
  
  RESULTS
  The median age at treatment was 69 years with a median prostate volume of 46.2 mL. The median prescription isodose line was 67% (interquartile range, 65, 70). The median clinical follow-up was 35.49 months. 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 to isodose line (P = .009) and normalized volumes for heterogeneity ≥46 Gy. The ROC method successfully produced thresholds for dose-volume recommendations for both prostate and urethra, including normalized prostate volumes from 46 to 50 Gy, such as volume of target tissue receiving 46% of the prescribed dose (V46) Gy of 36.7% (sensitivity, 71%; specificity, 61%; area under the curve, 0.67) with an associated probability of late GU grade 2+ toxicity of 21%.
  
  
  CONCLUSIONS
  Intraprostatic heterogeneity should be controlled with potential thresholds at V46 Gy <36.7%, V48 Gy <21%, and V50 Gy <9.5% of the normalized prostate volume to keep late grade 2+ GU toxicity ≤20% with 4-fraction schemes. This may be facilitated with a higher prescription isodose line (>69%).
authors.affiliation: Helen Diller Family Comprehensive Cancer Center, University of California at San Francisco, San Francisco, California; Department of Epidemiology and Biostatistics, University of California at San Francisco, San Francisco, California.
authors.forename: Li
authors.initials: L
authors.lastname: Zhang
authors.affiliation: Department of Radiation Oncology, University of California at San Francisco, San Francisco, California.
authors.forename: Julian
authors.initials: J
authors.lastname: Johnson
authors.affiliation: Department of Radiation Oncology, University of California at San Francisco, San Francisco, California.
authors.forename: Alexander R
authors.initials: AR
authors.lastname: Gottschalk
authors.affiliation: Department of Radiation Oncology, University of California at San Francisco, San Francisco, California.
authors.forename: Albert J
authors.initials: AJ
authors.lastname: Chang
authors.affiliation: Department of Radiation Oncology, University of California at San Francisco, San Francisco, California.
authors.forename: I-Chow
authors.initials: IC
authors.lastname: Hsu
authors.affiliation: Department of Radiation Oncology, University of California at San Francisco, San Francisco, California; Department of Urology, University of California at San Francisco, San Francisco, California.
authors.forename: Mack
authors.initials: M
authors.lastname: Roach
authors.affiliation: Department of Radiation Oncology, Oakland University William Beaumont School of Medicine, Beaumont Health System, Royal Oak, Michigan. Electronic address: zachary.seymour@beaumont.edu.
authors.forename: Zachary A
authors.initials: ZA
authors.lastname: Seymour
mesh_terms: D000368:Aged; D006801:Humans; D008297:Male; D008875:Middle Aged; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D012372:ROC Curve; D016634:Radiosurgery / Q000009:adverse effects*; D011879:Radiotherapy Dosage; D012189:Retrospective Studies; D014566:Urogenital System / Q000528:radiation effects*