UBC Theses and Dissertations
A four dimensional volumetric modulated arc therapy planning system for stereotactic body radiation therapy in lung cancers Chin, Erika Ming Yee
A novel 4D volumetric modulated arc therapy (4D VMAT) planning system is presented where radiation sparing of organs at risk (OARs) is enhanced by exploiting relative target and healthy tissue motion induced by patient respiration. In conventional radiation therapy, a motion encompassing margin is normally added to the clinical target volume (CTV) to ensure the tumour receives the planned treatment dose. For lung tumours which display large displacements due to patient breathing, this results in a substantial increase in dose to the OARs. These wider margins are incompatible with the growing clinical use of stereotactic body radiation therapy (SBRT) in lung cancer treatment. The ablative dose fractions of SBRT are correlated to significantly better survival rates but also increase the risk of serious normal tissue injury. The 4D VMAT system aims to reduce OAR dose by incorporating 4D CT information on volumetric target and OAR motions directly into the optimization process. The resulting treatment plans have respiratory phase-optimized radiation beam apertures whose deliveries are synchronized to the patient’s respiratory cycle. The performance of the 4D VMAT system was evaluated by comparing against other tumour motion compensation techniques such as 3D VMAT, gated VMAT and tracked VMAT for a range of tumour motions in both phantom simulations and on SBRT eligible patient 4D CT data. Results showed that 4D VMAT's ability to spare healthy tissue is superior to 3D VMAT and tracked VMAT. 4D VMAT treatment plan quality relative to gated VMAT is similar and in certain cases, depending on the spatial relationship between anatomical structures, it can be superior. Further dose escalation is possible with 4D VMAT and gated VMAT, but 4D VMAT has the advantage of faster treatment times which are only 11-25% longer than 3D VMAT, whereas gated VMAT are 77-148% longer. Lastly, although 4D VMAT is a respiration synchronized technique, preliminary tests showed that treatment plan quality can be robust to some desynchronization delivery errors caused by irregular patient breathing. This thesis concludes with a detailed discussion on the subsequent investigative tasks that must be conducted to bring 4D VMAT nearer to clinical implementation.
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