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UBC Theses and Dissertations

Respiratory-gated liver stereotactic radiotherapy in flattening filter free photon mode Karan, Tania


Respiratory motion makes radiotherapy of abdominal targets, such as the liver difficult. Respiratory gating is used to treat tumours in select portions of the respiratory cycle where their location is well known, increasing target coverage precision and reducing normal tissue damage. The tradeoff is an increase in treatment time, which can be partially overcome by using a higher dose rate, provided by flattening filter free (FFF) photon mode. To validate that increasing dose rate through removal of the flattening filter does not decrease cell kill, radiobiological experiments were carried out with three cell lines. Both the clonogenic survival assay and γH2AX double strand break assay indicated that FFF mode does not negatively affect cell kill. To form dosimetric and radiobiological bases for gated radiotherapy in FFF mode, a phantom was used to simulate respiratory traces of 10 patients previously treated for hepatic disease. Following 4DCT imaging, four plans were created and delivered for each respiratory trace; one nongated and three with the following gating windows: 80-20%, 40-60% and 30-70%. Treatments were delivered in 10MV FFF mode at 2400 MU/min, with optically stimulated luminescence dosimeters (OSLD) used for dosimetric measurements. OSLD doses were compared to those calculated by Eclipse Treatment Planning System. The plans were delivered to vials of cells to confirm whether dose to a moving target is compromised, changing cell survival. Dosimetry and survival was compared between the gated groups and also against the baseline – nongated treatment of a static phantom. Discrepancies of up to 7% were observed between the planned and measured doses. Most gated plans showed Eclipse overestimating the physical dose to the OSLDs, however the reverse was true for nongated plans. The best correlation between Eclipse and OSLD dose was seen in nongated plans and plans with a short duty cycle during expiration. Significant differences in cell survival and metabolic activity were seen between the gated groups and could not be attributed to dosimetric differences. No trend was established for the gated plan that results in the highest cell kill. Dosimetrically this work concludes that gating in FFF mode results in acceptable variations in target coverage.

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