Novel Dynamic Apertures for Reducing Spot Sizes in Proton Therapy
J Holmes1*, S Patel1, W Wong1, R Foote2, W Liu1, (1) Mayo Clinic, Phoenix, AZ, (2) Mayo Clinic, Rochester, MN
Presentations
PO-GePV-T-126 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To improve nearby normal tissue protection in proton therapy, spot sizes must be controlled and reduced if possible, which can be achieved by a novel concept of dynamic pinhole aperture (DPA). In this work, we investigate the clinical viability of the DPA based on its ability to reduce spot sizes while retaining a reasonable dose rate.
Methods: The DPA is a small device mounted to a motor assembly, capable of moving along the beam axis and two lateral directions. The DPA has a cylindrical opening with a range shifter attached that follows and trims each spot. The DPA alters the dose distribution by changing its positions instead of changing its shapes. MCsquare, an open-source Monte Carlo code, has been enhanced to support DPAs and used to calculate the dose distributions with varying parameters of DPAs to find the optimal parameters for clinical use in shallow tumors. The resulting spot sizes are compared among the DPA, an optimal thickness range shifter, and the extended range shifter (ERS). The beam transmission ratio is characterized for the DPA, on- and off-axis, to indicate the reduction of the dose rate.
Results: Compared to ERS, spot sizes are reduced 50% when the DPA is placed 5 cm from the patient at depths less than 50 mm. For a DPA of 3 mm radius positioned 5 cm from the patient, the spot size is reduced by 30% compared to an optimal range shifter placed at the same position. The beam transmission ratio is between 10-50% for a 3 mm radius DPA.
Conclusion: The device is simple, compact, and effective, especially for shallow tumors. Since the DPA adds treatment time for each spot, its use should be limited to a subset of spots of the treatment plan where the reduction in spot sizes is most beneficial.
Funding Support, Disclosures, and Conflict of Interest: Funding support: Arizona Department of Health Science Arizona Investigator Award and The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research
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