Major reduction of monitor units with the avoidance of leaf-sequencing step by direct aperture based IMRT optimisation.

Background And Purpose: Compared with conventional 3D conformal radiotherapy (3D-CRT) the use of intensity-modulated radiation therapy (IMRT) has increased monitor units (MUs) in the delivery of prescribed dose to the patient and thus a potential risk of radiation-induced secondary cancer. Due to the elimination of the leaf-sequencing step in direct aperture based IMRT optimisation (DABO) the MUs in the beam delivery can be reduced. We compared MUs calculated by DABO with other IMRT techniques and 3D-CRT.

Material And Methods: Treatment plans for five head and neck cancer patients using dynamic IMRT technique (DMLC) and step-and-shoot (SMLC) technique (Varian Helios Cadplan), 3D-CRT (Varian Eclipse) and a home-made DABO were produced. The total number of MUs, dose coverage and standard deviation of prescribed dose in planning target volume (PTV) between different techniques were compared.

Results: In all patients the PTV coverage and sparing of critical structures between the DABO, Helios DMLC and SMLC IMRT techniques was equivalent. Average MUs for beam delivery were 883 MU, 683 MU, 379 MU and 411 MU for DMLC, SMLC, DABO and 3D-CRT, respectively.

Conclusions: The DABO IMRT technique is able to produce treatment plans equivalent in target coverage, dose uniformity and normal tissue sparing compared with the commercial IMRT techniques. The number of MUs with DABO were reduced with a factor of 2.3 (DMLC) and 1.8 (SMLC). The study suggests that with DABO IMRT technique the risk of secondary cancer can be reduced without compromise in the quality of treatment plans.