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Nasopharyngeal tumors are commonly treated with intensity-modulated radiotherapy techniques. For photon dose calculations, problems related to loss of lateral electronic equilibrium exist when small fields are used. The anisotropic analytical algorithm (AAA) implemented in Varian Eclipse was developed to replace the pencil beam convolution (PBC) algorithm for more accurate dose prediction in an inhomogeneous medium. The purpose of this study was to investigate the accuracy of the AAA for predicting interface doses for intensity-modulated stereotactic radiotherapy boost of nasopharyngeal tumors. The central axis depth dose data and dose profiles of phantoms with rectangular air cavities for small fields were measured using a 6 MV beam. In addition, the air-tissue interface doses from six different intensity-modulated stereotactic radiotherapy plans were measured in an anthropomorphic phantom. The nasopharyngeal region of the phantom was especially modified to simulate the air cavities of a typical patient. The measured data were compared to the data calculated by both the AAA and the PBC algorithm. When using single small fields in rectangular air cavity phantoms, both AAA and PBC overestimated the central axis dose at and beyond the first few millimeters of the air-water interface. Although the AAA performs better than the PBC algorithm, its calculated interface dose could still be more than three times that of the measured dose when a 2 × 2 cm(2) field was used. Testing of the algorithms using the anthropomorphic phantom showed that the maximum overestimation by the PBC algorithm was 20.7%, while that by the AAA was 8.3%. When multiple fields were used in a patient geometry, the dose prediction errors of the AAA would be substantially reduced compared with those from a single field. However, overestimation of more than 3% could still be found at some points at the air-tissue interface.

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Inhaled progeny of 222Rn (radon progeny) are the most important source of irradiation of the human respiratory tract. Their attachment to atmospheric aerosols follows a well-established relationship between the activity size distribution (ASD) and the number size distribution. Recent studies have shown that indoor aerosols are derived primarily from outdoor sources, so it is pertinent to study the effects of different ambient environments on the indoor radon dose (in terms of the dose conversion coefficient or DCC, in units of mSv WLM-1). Commonly encountered ambient aerosols were studied here, which included the traffic-, urban-, and marine-influenced aerosols. The ASDs of attached radon progeny for all three studied ambient environments were well-represented by normal distributions. From these ASDs, the DCCs were calculated using the ICRP66 model and the scaled Yeh-Schum model. All other employed parameters were adopted from original references or authoritative reports. The DCCs for a nominal home calculated using the James model and the Yeh-Schum model were 12 and 8 mSv WLM-1, respectively. The DCCs were largest for urban-influenced ambient environments and smallest for marine-influenced ambient environments, and those for traffic-influenced ambient environments were close to that for a nominal home. If we adopt the stochastic model, the probability of contracting radon-induced lung cancer by a person living with a marine-influenced ambient environment will be half that of a person living with an urban-influenced ambient environment.

No abstract