Dose Evaluation in Moving/Deforming Anatomy: Methods and Clinical Implications




The aim of any radiotherapy is to tailor the radiation dose to the tumoricidal target volume and to deliver as low radiation as possible to all other normal tissues. However, the motion and deformation induced in human tissue by the ventilatory motion is a major issue since the standard practice usually uses only one computed tomography (CT) scan (and hence one instance of the patient's anatomy) for treatment planning. Even if the planning CT images are free of motion artifacts, the interfraction movement that occurs due to physiological processes within time scales shorter than the delivery of one treatment fraction leads to differences between the planned and delivered dose distributions during a fractionated treatment regimen. Due to the influence of these differences on tumors and normal tissues, the tumor control probabilities (TCPs) and normal tissue complication probabilities (NTCPs) are likely to be impacted in the face of organ motion. Moreover, as the recent advances in radiation therapy provide improved segmentation and dose calculation methods, the dose distributions conform more closely to targets and the uncertainties associated with organ motion become more apparent and they may limit the success of the conformal radiotherapy thereby leading to local control failure and/or normal tissue complications. It is thus not only desirable, but also necessary to generate, prior to treatment, dose distributions that include the effects of the treatment geometric uncertainties by using the time-varying anatomical information as a substitute for the somewhat artificial (although useful) PTV. The methods used to achieve this goal depend on the model used to describe the patient's anatomy. The dose and fluence convolution approaches for rigid organs will be first discussed. For non-rigid behavior a dose reconstruction method that allows the accumulation of the dose to the deforming anatomy will be presented