title: Dosimetric evaluation of internal shielding in a high dose rate skin applicator.
abstract: OBJECTIVE
  The Valencia HDR applicators are accessories of the microSelectron HDR afterloading system (Nucletron) shaped as truncated cones. The base of the cone is either 2 or 3 cm diameter. They are intended to treat skin lesions, being the typical prescription depth 3 mm. In patients with eyelid lesions, an internal shielding is very useful to reduce the dose to the ocular globe. The purpose of this work was to evaluate the dose enhancement from potential backscatter and electron contamination due to the shielding.
  
  
  METHODS
  Two methods were used: a) Monte Carlo simulation, performed with the GEANT4 code, 2 cm Valencia applicator was placed on the surface of a water phantom in which 2 mm lead slab was located at 3 mm depth; b) radiochromic EBT films, used to verify the Monte Carlo results, positioning the films at 1.5, 3, 5 and 7 mm depth, inside the phantom. Two irradiations, with and without the lead shielding slab, were carried out.
  
  
  RESULTS
  The Monte Carlo results showed that due to the backscatter component from the lead, the dose level raised to about 200% with a depth range of 0.5 mm. Under the lead the dose level was enhanced to about 130% with a depth range of 1 mm. Two millimeters of lead reduce the dose under the slab with about 60%. These results agree with film measurements within uncertainties.
  
  
  CONCLUSIONS
  In conclusion, the use of 2 mm internal lead shielding in eyelid skin treatments with the Valencia applicators were evaluated using MC methods and EBT film dosimetry. The minimum bolus thickness that was needed above and below the shielding was 0.5 mm and 1 mm respectively, and the shielding reduced the absorbed dose delivered to the ocular globe by about 60%.
authors.affiliation: Sección de Radiofísica en Radioterapia, Hospital Universitario LA FE, Valencia, Spain.
authors.forename: Françoise
authors.initials: F
authors.lastname: Lliso
authors.affiliation: Servicio de Radiofísica, ERESA, Hospital General Universitario, Valencia, Spain.
authors.forename: Domingo
authors.initials: D
authors.lastname: Granero
authors.affiliation: Sección de Radiofísica en Radioterapia, Hospital Universitario LA FE, Valencia, Spain; Radiotherapy Department, Clínica Benidorm (Alicante), Spain.
authors.forename: Jose
authors.initials: J
authors.lastname: Perez-Calatayud
authors.affiliation: Sección de Radiofísica en Radioterapia, Hospital Universitario LA FE, Valencia, Spain.
authors.forename: Vicente
authors.initials: V
authors.lastname: Carmona
authors.affiliation: Sección de Radiofísica en Radioterapia, Hospital Universitario LA FE, Valencia, Spain.
authors.forename: M Carmen
authors.initials: MC
authors.lastname: Pujades
authors.affiliation: Departament de Física Atòmica, Molecular i Nuclear, Universitat de Valencia, València, Spain.
authors.forename: Facundo
authors.initials: F
authors.lastname: Ballester
keywords: Monte Carlo methods; film dosimetry; skin brachytherapy