Computer and Approximation Methods of Calculating Depth Dose in Irregularly Shaped Fields

Abstract

A computer program was developed to calculate depth dose in irregularly shaped fields. The program is general for any shaped field and includes corrections for variations of exposure rate within the field, body contour, and non-nominal source-skin distances. Depth doses were calculated in several actual cases involving irregular fields such as mantle, split, L-shaped, and inverted Y fields. Data were examined to determine the degree of dose inhomogeneity in these fields. An approximation method used in the irregular field dosimetry was evaluated with regard to its accuracy and limitations. Comparison of depth dose determinations using both methods showed a discrepancy of only 2 to 3%. INDEX TERMS: Computers. Dosimetry. Therapeutic Radiology, treatment planning Radiology 106:433-436, February 1973 Radiation Physics I R R E GU LAR FIELDS are encountered in radiotherapy when radiation sensitive structures are shielded from the main beam or when the field extends beyond the irregularly shaped patient body contour.. In such fields, the calculation of depth dose is often accomplished by approximation methods. These methods fall into two main categories; (a) those based on phantom measurements with correction factors to accommodate variations in the actual treatment conditions from the phantom set-up (1, 2), and (b) those involving geometric approximations in which the given irregular field is approximated by a rectangular field. Whereas these methods are often helpful in routine dosage calculations, their validity must be checked for various treatment conditions. So far the scatter calculation method as described by Clarkson (3) and Johns and Cunningham (4) has proved to be most general in its application and may be considered as a standard of comparison for approximation. methods (5).A computer program was developed embodying Clarkson's principle of separating 'the primary and scattered components of dose at a point in a phantom. The program was written in Fortran for the CDC-3300 computer. Depth doses computed for several cases involving irregular fields were compared with those calculated by geometric approximations. The results were analyzed to evaluate the accuracy and limitations of these approximations. COMPUTER PROGRAMThe use of scatter-air ratios (SAR) in calculating depth dose in irregular fields has been discussed in detail in the literature (4, 5). In principle, the same method was adopted for the computer program used in this study. The computer divides the field into elementary sectors using the coordinates of the field contour and the point of dose calculation. The radii of the sectors are calculated using mathematical equations. The SAR values for the sectors are assigned by interpolation of the SAR table for circular fields stored in the computer. The values are summated to give average scatter-air ratio (SAR) for the field at the given point at depth.The computed SAR is converted to average tissue air ratio (TAR) by the equationwhere TAR (0) is the tissue-air ratio for 0 X 0 field, i.e., TAR (0) = ...

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