Design and Implementation of a Head & Neck Phantom (HANK) for System Audit and Verification of IMRT

Abstract

Introduction 

Head and neck is a challenging environment for IMRT and thorough testing of planning and treatment delivery systems is required. The phantoms used must be anatomically realistic, have similar radiological properties to the tissues concerned, and allow verification of dose and dose distribution with a variety of devices in a number of key positions throughout the target and normal tissues.

 Phantom design 

A phantom has been designed and built to meet these criteria with the specific purpose of being used to verify treatments for IMRT in the head and neck region. The phantom (HANK) is semi-anatomical Perspex with the facility to insert heterogeneities to simulate air cavities in a range of fixed positions. Chamber inserts are manufactured to incorporate either a standard plotting tank chamber (volume 0.125cm³) or a smaller Pinpoint chamber (volume 0.015cm³) and measurements can be made with both in a range of positions throughout the phantom. Coronal films can also be acquired within the phantom and additional solid blocks of Perspex allow transverse films to be acquired within the head region.

 Results 

Initial studies using simple conventional head and neck plans established the reproducibility of the phantom and measurement devices to within a setup error of <1mm. Subsequent verification of 9 clinical head and neck IMRT plans has demonstrated the efficacy of the phantom for allowing a range of patient-specific dose measurements to be made in regions of dosimetric and clinical interest. Agreement between measured values and those predicted by the Pinnacle Treatment Planning System have generally been found to be good: mean error of +0.2% (range -4.3 to +2.2, N=9) for the primary PTV, -0.1% (range -1.5 to +2.0, N=8) for the nodal PTV and +0.2% (range -1.8 to +4.3, N=9) for the spinal cord. The use of the phantom to measure combined dose distributions using radiographic film is ongoing.

 Conclusions 

The phantom has proved to be a valuable tool in the development of clinical head and neck IMRT, allowing accurate verification of absolute dose in regions of clinical and dosimetric interest.