Marker-less Intra-Fraction Organ Motion Tracking - A Hybrid ASM Approach

External beam radiation therapy attempts to deliver a high dose of ionizing radiation to destroy cancerous tissue, while sparing healthy tissues and organs at risk (OAR). Advances in intensity modulated radiotherapy treatment (IMRT) call for a greater understanding of uncertainties in the treatment process and more rigorous protocols leading to greater precision in treatment delivery. The degree to which this can be achieved depends largely on the cancer site. The treatment of organs comprised of soft tissue (e.g. in the abdomen) and those subject to rhythmic movements (e.g. lungs) cause inter and intra-fraction motion artifacts that are particularly problematic. Various methods have been developed to tackle the problems caused by organ motion during radiotherapy treatment, e.g. real-time position management (RPM) respiratory gating (varian) and synchronized moving aperture radiation therapy (SMART), developed by researchers at Harvard medical school. The majority of the work focuses on tracking the position of the pathologic region, with the intra-fraction shape variation of the region being largely ignored. This paper proposes a novel method that addresses both the position and shape variation caused by the intra-fraction movement. This approach is seen able to reduce the margin of clinical treatment volume (CTV), hence, spare yet more surrounding healthy tissues from being exposed to radiation and limiting irradiation of OAR.