title: Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam.
abstract: BACKGROUND
  Accurate dosimetry at the interface of two different density medium (e.g., air cavity in the head and neck cancers and lungs in thoracic region) is a major cause of concern in external beam radiation therapy. It has been observed that there is dose variation in and around air cavities, which occur as a result of the loss of both longitudinal and lateral electronic equilibrium. Heterogeneous structures with spatial differences in functionality and sensitivity for radiation pose challenge to radiation dosimetry. This study is an attempt to evaluate the dose perturbations produced at the interface of two medium for C0-60 gamma radiation.
  
  
  METHODS
  Low density polyethene foam has been used to mimic air cavity. GAFCHROMIC EBT2 dosimetry film was used for the measurement of dose at different locations. Simulation studies were performed using DOSRZnrc user code that comes with EGSnrc V4 2.4.0. Cylindrical geometry is used for all the simulations.
  
  
  CONCLUSIONS
  We observed significant variation in dose for smaller fields. There is a dose build down in the backward region and a dose build up in the forward direction. In the region of electronic disequilibrium, dose reduction near interface (proximal end) will have negative impact if target region is embedded there, on the contrary, it would be beneficial if there is normal tissue/critical organ adjacent to it.
authors.affiliation: Department of Radiotherapy, King George Medical University, Lucknow, India.
authors.forename: Nirmal Kumar
authors.initials: NK
authors.lastname: Painuly
authors.forename: Navin
authors.initials: N
authors.lastname: Singh
authors.forename: Teerth Raj
authors.initials: TR
authors.lastname: Verma
authors.forename: Arun
authors.initials: A
authors.lastname: Chairmadurai
authors.forename: Lalit Narendra
authors.initials: LN
authors.lastname: Chaudhari
authors.forename: Archana
authors.initials: A
authors.lastname: Saily
authors.forename: Mohan Chand
authors.initials: MC
authors.lastname: Pant
mesh_terms: D003037:Cobalt Radioisotopes; D005720:Gamma Rays; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D009010:Monte Carlo Method; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage
chemical_list: D003037:Cobalt Radioisotopes