The tradeoff between treatment plan quality and required number of monitor units in intensity-modulated radiotherapy

Purpose: To provide a mathematical approach for quantifying the tradeoff between intensity-modulated radiotherapy (IMRT) complexity and plan quality. Methods and Materials: We solve a multi-objective program that includes IMRT complexity, measured as the number of monitor units (MU) needed to deliver the plan using a multileaf collimator, as an objective. Clinical feasibility of plans is ensured by the use of hard constraints in the formulation. Optimization output is a Pareto surface of treatment plans, which allows the tradeoffs between IMRT complexity, tumor coverage, and tissue sparing to be observed. Paraspinal and lung cases are presented. Results: Although the amount of possible MU reduction is highly dependent on the difficulty of the underlying treatment plan (difficult plans requiring a high degree of intensity modulation are more sensitive to MU reduction), in some cases the number of MU can be reduced more than twofold with a <1% increase in the objective function. Conclusions: The largely increased number of MU and irradiation time in IMRT is sometimes unnecessary. Tools like the one presented should be considered for integration into daily clinical practice to avoid this problem.