A Comparison of physical and dosimetric properties of lung substitute materials Kwo-Ping Chang a),b) and Shang-Ho Hung Institute of Radiological Sciences and Department of Radiological Technology, Tzu Chi College of Technology, Taiwan Yu-Huang Chie Department of Radiation Therapy, Mennonite Christian Hospital, Taiwan. An-Cheng Shiau Department of Radiation Therapy, Far Eastern Memorial Hospital, Taiwan Ruey-Jing Huang Institute of Radiological Sciences and Department of Radiological Technology, Tzu Chi College of Technology, Taiwan and Department of Radiology, Hualien Armed Forces General Hospital, Taiwan (Received 4 June 2011; revised 22 February 2012; accepted for publication 22 February 2012; published 20 March 2012) Purpose: The need for an accurate estimate of absorbed doses within and around irradiated thorax tissues necessitates the use of carefully selected materials from which phantoms are constructed. A lung substitute is more difficult to establish mostly due to its low physical density. Although many researchers have used cork as a lung substitute, very little research data address cork’s characteris- tics to determine which type of cork is optimal as a substitute for lung tissue. Methods: Natural cork, composition cork, rubber cork, ATOM, RANDO, and a reference lung material (ICRU-44 lung tissue) were investigated to establish comparisons of physical properties. Following the determination of the respective physical properties, the dose distributions from 6 MV photon beams in water/lung substitute/water phantoms were assessed using the Monte Carlo method. Physical and electron densities affecting the dose distributions through lung tissues in different field size conditions were investigated Results: The physical properties (physical density, electronic density, and effective atomic number) of the composition cork are the most similar to those of the ICRU-44 lung, and the CT number of the composition cork is very similar to that of humans aged 30–60. PDD of the composition cork and the RANDO phantom are the most comparable to that of ICRU-44 lung in 1  1 cm 2 field size due to the combined properties of physical density (PD) and electron density per gram (EDG) of the studied lung materials. PD and EDG affect the lung dose primarily in small field size. The effects of PD are minimal in large fields, having a more rapid lateral electron equilibrium. EDG dominates PDD pattern in lung material when large fields are applied. Combined effects of PD and EDG are nonlinear for all field sizes. Conclusions: The composition cork is the preferred lung substitute based on physical and dosimetric properties. V C 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.3694097] Key words: cork, lung substitute, physical properties, dosimetry, Monte Carlo simulation I. INTRODUCTION Optimization of radiotherapy relies on maximizing the dose to the target volume while minimizing the dose to the sur- rounding tissues. Dosages for treatment of lung tissue are more critical due to the perturbations in absorbed dose distri- butions produced by the lung when the thorax is irradiated. For treatment of lung cancer, dose heterogeneity corrections remain controversial. ETAR (equivalent tissue-air ratio), modified Batho law, are the algorithms often used in the clini- cal treatment planning system (TPS) for the inhomogeneity correction. In some TPSs, dose calculation is performed by using a convolution/superposition algorithm. Unfortunately, these approaches cannot accurately simulate some interac- tion events in the lung (i.e., low density medium) region, e.g., the recoil electrons and multiple scatterings of the sec- ondary electrons. 1 Monte Carlo algorithms model radiation transport through a detailed description of the physics of the interactions of radiation and is therefore accepted as the most accurate algorithm for dose calculation in heterogene- ous media. Monte Carlo calculations provide a benchmark for analytic calculations used in the treatment planning sys- tems and a bridge between measurements and analytically based numerical calculations. The measurements are more practically derived from a variety of phantom materials. As such, the need for an accurate estimate of absorbed doses within and around irradiated thorax tissues necessitates the use of carefully selected materials from which phantoms are 2013 Med. Phys. 39 (4), April 2012 0094-2405/2012/39(4)/2013/8/$30.00 V C 2012 Am. Assoc. Phys. Med. 2013