title: Analytical Consideration of Surface Dose and Kerma for Megavoltage Photon Beams in Clinical Radiation Therapy.
abstract: BACKGROUND
  In radiation therapy, estimation of surface doses is clinically important. This study aimed to obtain an analytical relationship to determine the skin surface dose, kerma and the depth of maximum dose, with energies of 6 and 18 megavoltage (MV).
  
  
  METHODS
  To obtain the dose on the surface of skin, using the relationship between dose and kerma and solving differential equations governing the two quantities, a general relationship of dose changes relative to the depth was obtained. By dosimetry all the standard square fields of 5x5cm to 40x40cm, an equation similar to response to differential equations of the dose and kerma were fitted on the measurements for any field size and energy. Applying two conditions: a) equality of the area under dose distribution and kerma changes in versus depth in 6 and 18 MV, b) equality of the kerma and dose at x=dmax and using these results, coefficients of the obtained analytical relationship were determined. By putting the depth of zero in the relation, amount of PDD and kerma on the surface of the skin, could be obtained.
  
  
  RESULTS
  Using the MATLAB software, an exponential binomial function with R-Square >0.9953 was determined for any field size and depth in two energy modes 6 and 18MV, the surface PDD and kerma was obtained and both of them increase due to the increase of the field, but they reduce due to increased energy and from the obtained relation, depth of maximum dose can be determined.
  
  
  CONCLUSIONS
  Using this analytical formula, one can find the skin surface dose, kerma and thickness of the buildup region.
authors.affiliation: Deprtment of Medical Physics, School of Medicine, Ahavaz Jundishapur University of Medical sciences, Ahvaz, Iran E-mail : abdalvandneda@yahoo.com.
authors.forename: Mohammad Javad
authors.initials: MJ
authors.lastname: Tahmasebi Birgani
authors.forename: Mohammad Ali
authors.initials: MA
authors.lastname: Behrooz
authors.forename: Sasan
authors.initials: S
authors.lastname: Razmjoo
authors.forename: Mansour
authors.initials: M
authors.lastname: Zabihzadeh
authors.forename: Jafar
authors.initials: J
authors.lastname: Fatahiasl
authors.forename: Reza
authors.initials: R
authors.lastname: Maskni
authors.forename: Neda
authors.initials: N
authors.lastname: Abdalvand
authors.forename: Zeynab
authors.initials: Z
authors.lastname: Asgarian
authors.forename: Azin
authors.initials: A
authors.lastname: Shamsi
mesh_terms: D006801:Humans; D010315:Particle Accelerators; D017785:Photons; D011829:Radiation Dosage; D011878:Radiotherapy; D011879:Radiotherapy Dosage; D012867:Skin / Q000528:radiation effects*; D012984:Software