Background and purpose: With three-dimensional conformal intensity-modulated radiotherapy (3D-c-IMRT) a heterogeneous dose distribution can be achieved in both planning treatment volume and in adjacent normal tissues and organs to be spared. 3D-c-IMRT demands for modified photon fluence profiles which can be accomplished with different techniques. This report deals with the commissioning of metal compensators and the first experiences in clinical use. Dosimetric accuracy, dose coverage and practical experience like treatment delivery time, monitor units and dose outside the treated volume are evaluated.
Patients and methods: From January 2002 to April 2004, 24 patients with head-and-neck cancers were treated with 3D-c-IMRT using tin-wax compensators. The dose prescription included a simultaneously integrated boost (SIB). High-dose volume was irradiated with 60-70 Gy (median 66 Gy), low-dose volume with 48-54 Gy (median 52 Gy) administered by a standardized seven- portal coplanar beam arrangement. Dose at one parotid gland was aimed at 26 Gy. The compensators used consisted of tin granules embedded in wax; recalculation was performed with compensators made of the alloy MCP96 as well.
Results: In 21 of 24 patients 3D-c-IMRT with tin-wax compensators reduced the median dose to one parotid gland to < 30 Gy. Recalculation with compensators with higher density which allowed higher attenuation revealed better protection of the parotid gland. The treatment delivery time per fraction was between 6 and 12 min (plus time for patient positioning), approximately 300 MU per 2 Gy were applied. The dose outside the treated volume was increased with regard to open fields and comparable with a physical wedge of 15-30 degrees . Quality assurance and treatment of patient were fast and simple. It was shown, that calculated dose distribution corresponded to measured dose distribution with high accuracy.
Conclusion: The described method offers facilities for a good dose coverage of irregular target volumes with different prescribed doses and a considerable dose reduction in adjacent organs at risk. The dose sparing of organs at risk can be further improved, if a compensator material with higher density is used.