A commercial imrt treatment-planning dose-calculation algorithm

Purpose: The dose-calculation algorithm for a commercial arc-based intensity modulated radiation therapy (IMRT) treatment-planning and delivery system (Peacock, NOMOS Corporation) is described. Methods and Materials: The IMRT delivery system uses a dynamically controlled multileaf collimator with 40 leaves that project on our accelerator to either 1.0 x 0.84 cm² or 1.0 x 1.68 cm² at isocenter arranged in two banks of 20 leaves each. The dose- calculation algorithm uses tissue-phantom ratios derived from percent depth dose measurements, measured relative output data, and single leaf profiles. Some compromises are made in the algorithm terms to enable more straightforward dosimetry measurements and to reduce dose computation times. The dose calculation algorithm is presented, and consequences of the approximations are investigated using previously published 4 MV photon beam data. Results: Most of the approximations lead to dose errors of a few percent. However, the use of depth-invariant single leaf profiles results in errors as large as 9% for 4 MV fixed beams. Conclusions: Large dosimetric errors are possible for small fixed fields using this algorithm. However, the algorithm is designed for tomotherapy dose delivery, where doses are delivered from multiple directions and depths. Investigations of the algorithm in more clinically relevant conditions have been conducted and show that the algorithm accuracy is 1.3% and therefore is clinically acceptable for tomotherapy.