Spatial and dosimetric resolution for intensity -modulated radiation treatment of targets containing biological subvolumes

Intensity-modulated radiation therapy (IMRT) is advocated for treatment of head and neck cancer due to its effective sparing of organs at risk. Studies show that the area where relapses predominantly occur is within the clinical target volume receiving the highest prescription dose. One approach to improve local control may be to identify patients with radioresistant subvolumes and modulate the dose distribution to treat these subvolumes with appropriately escalated doses. Developments in biological imaging will soon allow fusion of biological images with high resolution anatomic images resulting in what is referred to as a "bioanatomic" data set. These images would allow treatment plans to be designed to escalate dose to radioresistant hypoxic subvolumes.;The purpose of this research was to investigate the spatial and dosimetric resolution for IMRT as applied to dose escalation to radioresistant subvolumes within the target. A 3D model simulating the dose delivery of IMRT was developed. A toolset was created in Matlab containing several functions and virtual head/neck phantom image sets designed to investigate basic physics as well as the clinical implementation of IMRT.;Results from the model indicate radiobiologically significant dose escalation can be achieved using IMRT to increase dose to subvolumes in the GTV for the head and neck. Radiation biology suggests that 2.5 to 3 times the dose required to kill normoxic cells is needed to kill hypoxic cells. Doses of greater than 220% of the dose prescribed to the GTV were achieved. Dose escalation at this level could potentially lead to an improvement in local control for head and neck cancers. Dose distributions with gradients of 25 cGy/mm (standard deviation 2 mm) were demonstrated by the model for subvolumes 5mm in size. Dose gradients of 15 cGy/mm (standard deviation 2 mm) were attained across 1 cm subvolumes. Dose escalation was achieved without increasing normal tissue toxicity beyond current standards.;The beam model was validated in 2D, and the results of the 3D model were validated through comparison to dose computations performed in a commercial treatment planning system and through treatment delivery to a solid water phantom. Reasonable agreement was observed for these comparisons.