title: Dependence of the safe rectum dose on the CTV-PTV margin size and treatment technique.
abstract: BACKGROUND
  Late rectal injury is a common side effect of external beam radiotherapy for prostate cancer.
  
  
  OBJECTIVE
  The aim of this study was to evaluate what total dose may be safely delivered for prostate patients for 3DCRT and IMRT techniques and the CTV-PTV margin.
  
  
  METHODS
  3DCRT and IMRT plans were prepared for 12 patients. For each patient PTV was defined with CTV-PTV margins of 0.4, 0.6, …, 1.0 cm, and total doses of 70, 72, …, 80 Gy, with 2 Gy dose fraction. NTCP values for the rectum were calculated using the Lyman model. Both techniques were compared in terms of population mean DVH.
  
  
  RESULTS
  Significantly smaller NTCPs for IMRT were obtained. For both techniques diminishing the margin CTV-PTV of 2 mm leads to decreasing the NTCP of about 0.03. For total dose of 80 Gy the NTCP was smaller than 10% for the 4 mm margin only. The QUANTEC dose volume constraints were more frequently fulfilled for the IMRT technique than for the 3DCRT technique.
  
  
  CONCLUSIONS
  The IMRT technique is safer for prostate patients than the 3DCRT. If very high total doses are applied the CTV-PTV margin of 0.4 cm and the IMRT technique should be used. If the CTV-PTV margin of 0.6 cm is applied, the NTCP is smaller than 10% for the 3DCRT and IMRT techniques for the total doses smaller than 74 Gy and 78 Gy, respectively.
authors.affiliation: The Maria Skłodowska - Curie Memorial Cancer Center and Institute of Oncology, Medical Physics Department, Warsaw, Poland.
authors.forename: Pawel
authors.initials: P
authors.lastname: Kukołowicz
authors.affiliation: The Maria Skłodowska - Curie Memorial Cancer Center and Institute of Oncology, Medical Physics Department, Warsaw, Poland.
authors.forename: Helena
authors.initials: H
authors.lastname: Kukołowicz
authors.affiliation: The Maria Skłodowska - Curie Memorial Cancer Center and Institute of Oncology, Medical Physics Department, Warsaw, Poland.
authors.forename: Izabela
authors.initials: I
authors.lastname: Tyburska
keywords: 3DCRT and IMRT; CTV–PTV margin; Normal Tissue Complication Probability; Rectum injury