title: A modeling study of functional magnetic resonance imaging to individualize target definition of seminal vesicles for external beam radiotherapy.
abstract: BACKGROUND
  Pre-treatment magnetic resonance imaging (MRI) can give patient-specific evaluation of suspected pathologically involved volumes in the seminal vesicles (SV) in prostate cancer patients. By targeting this suspicious volume we hypothesize that radiotherapy is more efficient without introducing more toxicity. In this study we evaluate the concept of using MRI-defined target volumes in terms of tumor control probability (TCP) and rectal normal tissue complication probability (NTCP).
  
  
  METHODS
  Twenty-one high-risk prostate cancer patients were included. Pre-treatment CT images, T2 weighted (T2w) MRI and two multi-parametric MRI were acquired. Overlap between a suspicious volume in the SV observed on T2w images and a suspicious volume observed on either multi-parametric MRI was assumed to reflect a true malignant region (named 'MRI positive'). In addition the entire SV on the CT-scan was delineated. Three treatment plans of 2 Gy ×39 fractions were generated per patient: one covering the MRI positive volume in SV and prostate with margin of 11 mm to the MRI positive in the SV and two plans covering prostate and SV using 11 and 7 mm SV margin, respectively. All plans were prescribed the same PTV mean dose. Rectal NTCP grade ≥2 was evaluated with the Lyman-Kutcher-Burman model and TCP was estimated by a logistic model using the combined MRI positive volume in SV and prostate as region-of-interest.
  
  
  RESULTS
  Fourteen of twenty-one patients were classified as MRI positive, six of which had suspicious volumes in all three MRI modalities. On average TCP for the plan covering prostate and the MRI positive volume was 3% higher (up to 11%) than the two other plans which was statistically significant. The increased TCP was obtained without increasing rectal NTCP grade ≥2.
  
  
  CONCLUSIONS
  Using functional MRI for individualized target delineation in the SV may improve the treatment outcome in radiotherapy of prostate cancer without increasing the rectal toxicity.
authors.affiliation: a Department of Oncology , Section of Radiotherapy, Rigshospitalet , Copenhagen , Denmark.
authors.forename: Sidsel
authors.initials: S
authors.lastname: Damkjær
authors.affiliation: a Department of Oncology , Section of Radiotherapy, Rigshospitalet , Copenhagen , Denmark.
authors.forename: Jakob B
authors.initials: JB
authors.lastname: Thomsen
authors.affiliation: a Department of Oncology , Section of Radiotherapy, Rigshospitalet , Copenhagen , Denmark.
authors.forename: Svetlana I
authors.initials: SI
authors.lastname: Petersen
authors.affiliation: a Department of Oncology , Section of Radiotherapy, Rigshospitalet , Copenhagen , Denmark.
authors.forename: Jens Peter
authors.initials: JP
authors.lastname: Bangsgaard
authors.affiliation: a Department of Oncology , Section of Radiotherapy, Rigshospitalet , Copenhagen , Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: M Petersen
authors.affiliation: a Department of Oncology , Section of Radiotherapy, Rigshospitalet , Copenhagen , Denmark.
authors.forename: Ivan R
authors.initials: IR
authors.lastname: Vogelius
authors.affiliation: b Clinical Trial Service Unit - Nuffield Department of Population Health , University of Oxford , Oxford , UK.
authors.forename: Marianne C
authors.initials: MC
authors.lastname: Aznar
mesh_terms: D004307:Dose-Response Relationship, Radiation; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008954:Models, Biological; D058958:Organs at Risk / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012669:Seminal Vesicles / Q000473:pathology* / Q000528:radiation effects