Analysis of enhancements to IRREG program by comparison with measurements in blocked and unblocked Co-60, 4-MV and 10-MV photon beams

An IRREG-type irregular field dose computation program containing alternative calculation options was obtained from the Northwest Center for Radiological Physics. The authors improved the program, added other options, and compared the results with cross-beam scan measurements in water. The following options were studied: (1) field size at SSD versus field size at depth, (2) slant depth versus vertical depth, (3) geometric versus extended source penumbra, (4) minimum radius versus sector integration, (5) analytical TPR/sub o/ versus table lookup, (6) off-axis TPR/sub o/ derived from off-axis HVL, (7) off-axis TPR/sub o/ derived from OAF and OCR, (8) weighting of net primary by measured OAF, (9) weighting of net scatter by net primary, (10) weighting of sector scatter by sector primary, and (11) use of revised TPR and SPR definitions (Kahn). The effect of the calculation options applied individually and together is reported. The results show that significant improvement can be achieved by applying several of these enhancements to the Cunningham IRREG model