Dose calculations for asymmetric fields defined by independent collimators using symmetric field data.

Abstract

Several methods have been developed for the dosimetry of asymmetric radiation fields formed by independently moving collimator jaws. Three of these methods, based on different principles and modified to comply with our set of available data, are utilized for the calculation of asymmetric field dose profiles. All three methods use output factors and per cent depth doses or tissue maximum ratios of symmetric fields. In the first method, calculation of the off-centre ratio (OCR) of the asymmetric field is based on the symmetric field from which the asymmetric is originated, by setting the one jaw in an asymmetrical position. In the second method the OCR of the symmetric field is used for the OCR calculation of the asymmetric field of the same size; whereas the third method does not allow for the asymmetric OCR calculation. The results obtained using data for the 6 MV photon beam of a Philips SL-20 linear accelerator indicate that both the first and second method can accurately reproduce asymmetric field profiles from symmetric field data; the third method does not allow for penumbra reproduction, but it is accurate at the central part of the asymmetric field. The problems encountered in the application of the three methods are reported and their accuracy is compared.

Analytical approach for determining beam profiles in water phantom of symmetric and asymmetric fields of wedged, blocked, and open photon beams

“…In addition, the correction factor of jaw, , acts similarly not only for symmetric fields but also for asymmetric fields. The depth dose differences between symmetric and asymmetric field at any point are indicated by unlike the previous studies (8–12) …”

“…3–7, the comparison with measurements using γ‐index shows that the accuracy of the calculated dose distributions fits well in a 3% error in low‐dose gradient region, except for asymmetric fields. In general, the primary dose rate at shallow depths in the phantom may actually increase at distances away from the central axis (called horns) as a result of flattening filter effect on the radiation beam (10–12) . A flattening filter correction that depends on depth in a phantom and radial distance from the central axis is required to model the increase in dose rate away from the central beam axis that is not included in this paper.…”

“…In the 1990s, some studies have addressed asymmetric and wedged asymmetric fields using symmetric field data (8–11) . The method based on work by Thomas and Thomas (10) generates asymmetric field profiles by computing the off‐center ratio (OCR) of the asymmetric field while using output factors and PDDs of the equivalent symmetric field.…”