Evaluation of the treatment planning system decimal ElectronRT and custom-made bolus for FLASH-irradiation
Abstract: Introduction: The novel external beam therapy technique using ultra-high dose rates known as FLASH has become known for its normal tissue sparing effect. Currently in Lund, veterinary clinical trials are carried out on canine patients using electrons with ultra- high dose rates. By implementing a treatment planning system for FLASH-irradiation, the dose distribution can be visualized and the gantry and the couch angle can be optimized. Moreover, an optimized thickness bolus can be added to the treatment plan for a more conformal and homogeneous dose distribution inside the patient. The aim of this master thesis is to evaluate the accuracy of the decimal ElectronRT, eRT, treatment planning system by .decimal for the purpose of implementing the eRT treat- ment planning system in future FLASH-irradiation treatments. In addition, the goal is to employ a custom-made optimized thickness bolus to a treatment plan inside the treatment planning system to determine if the treatment can be made more conformal. The custom- made optimized thickness bolus is provided by the treatment planning system Material & Methods: Firstly, percentage depth dose curves and dose profiles were mea- sured in a solid water phantom for different field sizes and at a source-to-surface distance (SSD) = 70 cm. The data was used to create a model of the modified linear accelerator used for FLASH-irradiation in the treatment planning system. Then, the model of the modified linear accelerator was used to create treatment plans for a solid water and an Alderson phantom. An SSD of 70 cm could not be used to create treatment plans due to a current limitation of the system. Hence, only an SSD of 100 cm was used and the plans were evaluated by new phantom measurements. Lastly, an optimized thickness bo- lus was added to the Alderson phantom treatment plan. Then, a new measurement on the Alderson phantom was made using the custom-made bolus with optimized thickness. The measurement was compared to the calculated dose profile exported from the treat- ment planning system. Results: The identical repeated measurements of the percentage depth dose at SSD = 70 cm had their highest deviation values around 3%, except the percentage depth dose curve for the field size d = 3 cm. The comparison of the phantom measurements at SSD = 100 cm and the exported data from the treatment planning system showed good agree- ment. Although some differences could be seen for large field sizes, the deviation values of the full-width at half-maximum (FWHM) were not higher than 3%. The bolus mea- surement verified that a peak in dose, previously seen in measurements with no bolus, was effectively removed. Despite some differences that could be seen visually between the measured inline beam dose profile in Alderson phantom and the exported beam dose profile from the treatment planning system, the deviation value of the FWHM was not higher than 2%. Conclusion: The eRT treatment planning system was evaluated and phantom measure- ments were performed for comparison purposes. Although the results indicate good agreement, further measurements are needed to verify the accuracy of the eRT treatment planning system at SSD = 70 cm, as the model data are based on an SSD = 70 cm. The results using optimized thickness bolus seem promising although further measurements are needed.
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