title: Hypofractionation for radiotherapy of prostate cancer using a low alfa/beta ratio--possible reasons for concerns? An example of five dimensional radiotherapy.
abstract: It is very attractive, due to the assumed low alfa/beta ratio of prostate cancer (PC), to construct new treatment schedules for prostate cancer using only a few large fractions of radiation (hypofractionation). This will widen the therapeutic window since the ratio for PC might be lower than that of the organs at risk (OAR). PC is an extremely variable disease and often contains both highly and poorly differentiated cells. It is reasonable to assume that different cells have different patterns of radiosensitivity, i.e. alfa/beta ratios and proliferation. In this study we will simulate the effect on the outcome of the treatment with different fractionations and different ratios.
  
  
  METHODS
  In this simulation we use an extension of the Linear Quadratic (LQ)/Biological Effective Dose (BED) formula called the dose volume inhomogeneity corrected BED (DVIC-BED). In the formula the tumour volume is divided in 50 subvolumes (step of 2%) and it is possible to calculate the relative effect of the treatment with different ratios (1.5, 4 and 6.5) in different subvolumes.
  
  
  RESULTS
  The simulations demonstrate that only a small portion (5-10%) of cells with a higher ratio will dramatically change the effect of the treatment. Increasing the total dose can compensate this, but this will on the other hand increase the dose to the OAR and also the risk for severe side effects.
  
  
  CONCLUSIONS
  These simulations highlight possible reasons for concerns about the use of hypofractionation for pathologically heterogeneous tumours, such as prostate cancer, and also demonstrate the need for testing new treatment schedules using both high and low ratios.
authors: Department of Oncology, Sahlgrenska Hospital and Academy, University of Gothenburg, Gothenburg, Sweden. bo.lennernas@telia.com
authors: Bo
authors: B
authors: Lennernäs
authors: Sten
authors: S
authors: Nilsson
authors: Seymour
authors: S
authors: Levitt