Radiation Physics and Chemistry 61 (2001) 593–595 Interface dosimetry: measurements and Monte Carlo simulations of low-energy photon beams Indra J. Das a, *, Alireza Kassaee a , Frank Verhaegen b , Vadim P. Moskvin c a Department of Radiation Oncology, University of Pennsylvania, 3400 Spruce Street, 2 Donner Building, Philadelphia, PA 19104, USA b Department of Physics, Institute of Cancer Research, Royal Marsden Hospital, London, UK c Department of Radiation Oncology, Indiana University, Indianapolis, IN, USA Abstract A comparison of measured and simulated dose perturbations at high-Z interfaces with Monte Carlo (MC) codes, EGS4, MCNP4B, and PENELOPE, having varied algorithms is presented. The measured dose perturbations strongly depend on the chamber design and are always lower than the MC data. The EGS4 data are closer to the ion chamber values. The other two codes, MCNP4B and PENELOPE, predict relatively higher magnitude. The simulated secondary electron spectra from high-Z interfaces are different but cannot explain the differences in magnitude. It is concluded that MC codes capable of handling low-energy transport and better boundary crossing algorithms are needed for interface effects. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Interface dosimetry; Dose perturbation; Monte Carlo simulation; Low-energy photon 1. Introduction Low-energy photons are used for superficial and intracavitary tumors in external and brachytherapy treatments. In kilovoltage (kV) beams, interface effects associated with dose perturbations at high atomic number (Z) interfaces with soft tissue are intense, localized, and significant due to production and trans- port of photoelectrons and fluorescent radiation (Das, 1997; Das and Chopra, 1995). Dose perturbation factor has been defined as ratio of doses with and without interface and separated into back-scatter and forward- dose perturbation factor, BSDF and FDPF, respec- tively. Most radiation measuring devices have significant problems at low energies, which was elaborated recently (Das et al., 1999) and visible by published dose perturbation factors (Das, 1997; Regulla et al., 1998; Verhaegen and Seuntjens, 1995). In this study, Monte Carlo (MC) simulations are used for interface dosimetry and compared with measurements. 2. Materials and methods The experimental setup is similar to the one reported earlier (Das, 1997; Das and Chopra, 1995) for the kV beams. Additional data were taken with specially designed thin window ion chambers (1 and 5 mm) in kV, 137 Cs, and 60 Co beams. Three different MC codes: MCNP4b, EGS4, and PENELOPE (Breiesmeister, 1997; Nelson et al., 1985; Salvat et al., 1996) were used for simulations. All these MC codes are class II type that can simulate experimental conditions with various high- Z (Al, Ti, Cu, Zn, Ta, Pb) materials used clinically. 3. Results A comparison of BSDF and FDPF for kV beams for a Pb/polystyrene interface is shown in Fig. 1. Large discrepancies are noted among the MC codes and *Corresponding author. Tel.: +1-215-662-6472; fax: +1- 215-349-5978. E-mail address: das@xrt.upenn.edu (I.J. Das). 0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S0969-806X(01)00342-5