title: Change in prostate volume during extreme hypo-fractionation analysed with MRI.
abstract: BACKGROUND
  Hypo-fractionated external beam radiotherapy with narrow CTV-PTV margins is increasingly applied for prostate cancer. This demands a precise target definition and knowledge on target location and extension during treatment. It is unclear how increase in fraction size affects changes in prostate volume during treatment. Our aim was to study prostate volume changes during extreme hypo-fractionation (7 × 6.1 Gy) by using sequential MRIs.
  
  
  METHODS
  Twenty patients treated with extreme hypo-fractionation were recruited from an on-going prospective randomized phase III trial. An MRI scan was done before start of treatment, at mid treatment and at the end of radiotherapy. The prostate was delineated at each MRI and the volume and maximum extension in left-right, anterior-posterior and cranial-caudal directions were measured.
  
  
  RESULTS
  There was a significant increase in mean prostate volume (14%) at mid treatment as compared to baseline. The prostate volume remained enlarged (9%) at the end of radiotherapy. Prostate swelling was most pronounced in the anterior-posterior and cranial-caudal directions.
  
  
  CONCLUSIONS
  Extreme hypo-fractionation induced a significant prostate swelling during treatment that was still present at the time of last treatment fraction. Our results indicate that prostate swelling is an important factor to take into account when applying treatment margins during short extreme hypo-fractionation, and that tight margins should be applied with caution.
authors.affiliation: Department of Oncology, Skåne University Hospital, Lund University, 22185 Lund, Sweden. adalsteinn.gunnlaugsson@skane.se.
authors.forename: Adalsteinn
authors.initials: A
authors.lastname: Gunnlaugsson
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Kjellén
authors.forename: Oskar
authors.initials: O
authors.lastname: Hagberg
authors.forename: Camilla
authors.initials: C
authors.lastname: Thellenberg-Karlsson
authors.forename: Anders
authors.initials: A
authors.lastname: Widmark
authors.forename: Per
authors.initials: P
authors.lastname: Nilsson
mesh_terms: D000230:Adenocarcinoma / Q000175:diagnosis* / Q000473:pathology / Q000532:radiotherapy*; D000368:Aged; D001918:Brachytherapy / Q000009:adverse effects; D019583:Dose Fractionation, Radiation; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009929:Organ Size / Q000528:radiation effects; D011379:Prognosis; D011467:Prostate / Q000473:pathology* / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000175:diagnosis* / Q000473:pathology / Q000532:radiotherapy*; D016896:Treatment Outcome; D047368:Tumor Burden / Q000528:radiation effects