Can a 10 MV-FFF Photon Beam be Used in Stereotactic Thalamotomy to Reduce Beam Delivery Times: A Dosimetric Evaluation

Abstract

Stereotactic radiosurgery (SRS) relies on small fields to ablate lesions. Treating functional diseases, such as essential tremor, requires a significantly higher dose (150 Gy) to a small target in a single fraction. Currently, Linac based treatment is delivered via pinhole-shaped fields formed by a 4 mm circular cone using a 6 MV photon beam. Utilizing a higher energy, high dose rate beam such as a 10 MV flattening-filter-free (FFF) beam with dose rates up to 2400 cGy/min could reduce long radiation delivery times by a factor of 4 comparing to the standard 6 MV beam with dose rate up to 600 cGy/min. This study aims to assess the benefit and feasibility of using a 10 MV-FFF beam by evaluating the dosimetric effects between the proposed 10 MV-FFF and commonly used 6 MV beam and comparing the dose to organ-at-risk (OARs) based on a typical real patient tremor treatment plan. The planning tool codes BEAMnrc/DOSXYZnrc were used to generate beams of 6 MV and 10 MV-FFF for a medical linear accelerator. Calculated patient dose distributions were compared between two energy beams based on a realistic treatment plan for thalamotomy. Additionally, dose to OARs were evaluated by using the dose-volume histograms (DVH) for the same target dose coverage. The mean energies of photons within the 4 mm cone projected area were 1.73 MeV and 2.09 MeV and the values of percentage depth-dose curve (%dd) at d = 5 cm and SSD = 95 cm were 75.5% and 78.3%, for the 6 MV and 10 MV-FFF beams, respectively. A patient delivered treatment plan with a prescribed dose of 15,000 cGy utilizing the 6MV-SRS beam was developed which used a 4 mm cone with a 21-arc delivery. Using that plan as the template, doses to OARs were evaluated comparing the 6MV to the 10MV-FFF energies. The maximum doses to the nearest OAR, brainstem, was 733 cGy and 822 cGy for the 6 MV and 10 MV-FFF beams, respectively. The details of maximum and mean doses to OARs are presented in Table 1 between the two energies. Compared to the 6 MV delivery, there is only a 12% increase in the maximum dose to the brain stem when using the 10 MV-FFF delivery based on the patient treatment plan. If the mild dose increase to OAR is acceptable for using 10 MV-FFF, the treatment time with 10 MV-FFF beam can be reduced to less than 1 hour from 3 hours with the 6 MV beam. The shorter treatment time can increase the accuracy of treatment target positioning as well patient comfort. Table 1 Calculated doses to OARs based on the real patient plan to deliver the same 15,000 cGy target dose between using 6 MV and 10 MV-FFF beams.