Four-dimensional radiation therapy for thoracic carcinoma : Dosimetric evaluation using deformable image registration

Respiratory motion remains a significant challenge for radiation therapy in targeting the tumour. The use of planning margins to avoid geometrical miss of the target volume during respiration results in excessive lung tissue irradiation that limits the prescribed dose to be safely delivered and escalated for better therapeutic gain. The purpose of this study was to develop effective dose planning techniques for treatment to be performed under natural patient breathing. The techniques accounted for the dosimetric influences of tumour movement and aimed to provide an optimized treatment volume by minimizing the internal target volume (ITV) without compromising the target coverage. In the study, the accumulated 4D dose distribution over the tumour volume was calculated using deformable image registration (DIR). A DICOM-RT based tool-box was specially developed for automated 4D dose calculation and evaluations. A new concept of defining the internal target volume from 4D dose coverage, namely inverse ITV (iITV) was introduced via the dose volume enclosed by the minimum accumulated dose in the tumour during the respiratory cycle. The dosimetric advantages of using this iITV with reference to the conventional ITV were confirmed in nine clinical cases by an average dose volume reduction of 16.4% (ranging from 2.3% to 29.9%). 4D radiotherapy involves complex dose distribution which was found to be affected by a number of factors including tumour size, magnitude of tumour displacement, tumour motion characteristics and the reference phases selected for dose planning. Our findings indicate that optimal dose planning was generally, but not always, achieved with the planning CT performed at the temporal mean tumour position and the degree of target coverage maximization strongly depends on the nature of tumour movement. Moreover, the conventionally geometric defined treatment margin could over estimate the treatment volume for a required target coverage. In conclusion, 4D dose calculation based on DIR offers realistic dose estimation, as both geometric and temporal factors are considered, and also provides optimal dose plans by minimizing the treatment volume. However, 4D radiation planning involves a number of factors resulting from the properties of tumours (eg. tumour size, amplitude and characteristics of tumour motion, etc) and from the procedure of treatment planning (eg. reference phase for dose planning, penumbra of dose beam, employed treatment volume etc) that interactively affect the resultant dosimetry. Since these factors vary patient-by-patient, there is no single formula or universal solution that can be used to obtain optimal dose planning. The 4D dose toolbox developed in this study could however provide a user friendly platform for 4D dose calculation and analysis, and allow the optimal treatment modalities and planning techniques to be determined for individuals