Optical surface scanning of breast cancer patients in radiotherapy – an investigation of inter- and intrafractional motion effects
Abstract: Purpose: The overall purpose of this master thesis was to investigate if the patient positioning could be improved for breast cancer patients with nodal involvement, using the optical scanning system Catalyst during patient setup. Patient motion during treatment and the effect it has on the dose distribution was also investigated. Materials and Methods: Eleven patients positioned according to the Catalyst and 10 patients positioned according to the conventional laser and skin markings based setup (LBS) were enrolled in this study. To evaluate if positioning was improved with the surface based setup (SBS), the setup deviations arising from matching daily verification images to the reference images were acquired for both positioning methods. The setup deviations were then compared between the different methods by studying the distribution of deviations. The systematic and random setup deviations and PTV margin were also determined for both groups. In total, 127 fractions of patients positioned with LBS and 93 fractions of patients positioned with SBS were analysed. To investigate what impact the offline correction strategy used at Skåne University Hospital (SUS) has on the positioning, the data was analysed both with and without the offline corrections applied. The dosimetric effect of patient motion was evaluated for one patient by analysing recorded motion data during beam delivery by the Catalyst system in terms of isocenter shift. All isocenter shifts that occurred when the beam was on were extracted and added to the original patient position to get the total isocenter shift, including both inter- and intrafractional motion. The mean isocenter shift for each fraction was determined and the isocenter position was shifted in the original treatment plan for each fraction in the treatment planning system (TPS). The dose was then re-calculated for each fraction and summed to one plan that was compared to the original dose distribution. Results: The obtained results indicate that SBS is a better method of positioning patients than LBS. For LBS, with offline corrections included, the amount of fractions where setup deviation exceeded the clinical setup deviation threshold of 4 mm was 22% / 21% / 28% in vrt/lng/lat. The corresponding values for SBS were 7.5% / 6.5% / 20% in vrt/lng/lat. The systematic and random setup error was smaller with Catalyst setup in all directions and the PTV margin could be reduced in the lng direction for SBS, compared to LBS. The setup deviations were also evaluated for LBS and SBS, both with and without the offline corrections applied. The results showed that although it is important to use the correction strategy for LBS, SBS did not depend on the correction strategy and the positioning was not improved as much for this method. No large impact on the DVH could be seen for the PTV of the summed treatment plan where the isocenter was shifted to a mean position for each fraction. D98% of the original plan was 46.6 Gy. D98% for the plan where the isocenter was shifted was 46.5 Gy. The absorbed dose to the heart and left lung was lower than in the original plan and the absorbed dose to the spinal cord was slightly increased. The difference between the plans was not clinically relevant. Conclusions: The amount of setup deviations exceeding 4 mm was decreased in all directions for SBS compared to LBS. Also, both the systematic and random setup error was decreased for SBS. The correction strategy improved the positioning for LBS, while the SBS did not depend as much on the corrections. The DVH of one patient with considerable motion during treatment indicated that the dose distribution was not affected to any large extent, when inter- and intrafractional motion was considered. However, to be able to draw any definite conclusions about the dosimetric effect due to patient motion more patients have to be evaluated.
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