Design of a heterogeneous thorax phantom for remote verification of three-dimensional conformal radiotherapy

The recent proliferation of clinical trials using three-dimensional conformal radiotherapy (3DCRT) has created a need for the Radiological Physics Center (RPC) to remotely evaluate the planning and delivery of 3DCRT treatments. The RPC is developing a heterogeneous, anthropomorphic phantom as a quality assurance tool for tumor dose delivery in 3DCRT. In response to the RTOG 3D lung trials, the designed phantom simulates a thorax. The phantom's dimensions have been determined from a sample of cases submitted to RTOG 3D lung protocol 93-11. Commercially available materials have been evaluated for use as suitable inhomogeneities in the phantom. Radiochromic film and TLD have been evaluated to assess field localization and absolute dose delivery. Expected tolerances are within 3 mm for field placement, and within 5% for absolute dose determination. The phantom consists of a water-fillable shell that encloses a bone-equivalent spinal column, two lung-equivalent structures, and a target within the left lung. Dose contours are measured on two orthogonal planes within the target. Absolute dose is measured in the target, the spinal cord, and the heart. An insert containing the bulk of the phantom's left lung and a target is used for imaging. To verify the treatment, the imaging insert is replaced with a treatment insert, which contains dosimeters and is indexed so that position will be reproducible.