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Reduction of Treatment Times for Breath-Hold Patients in a Spot-Scanning Proton Machine Through the Use of Mini-Ridge Filters


L Courneyea

L Courneyea*, H Wan Chan Tseung, J Yu, C Beltran, M Herman, Mayo Clinic, Rochester, MN

TU-A-108-9 Tuesday 8:00AM - 9:55AM Room: 108

Purpose:

Study the feasibility of synchrotron-based spot-scanning proton therapy for breath-hold patients by reducing treatment time through the use of a Mini-Ridge Filter (MRF) to increase the width of the Bragg peaks.

Methods:

Five spot-scanning nozzles were simulated in TOPAS[1]: four with MRFs of varying maximal thicknesses (6.15mm-24.6mm) and one without an MRF. The MRF ridges were aligned transverse to the beam, with the number of ridges (4-16) proportional to MRF thickness. The material thickness of these ridges approximately followed a Gaussian distribution. Using these simulations, Monte Carlo data was generated for Eclipse machine commissioning. For each nozzle, standard and stereotactic (SR) lung phantom treatment plans were created and assessed for delivery time and plan quality.

Results:

Use of an MRF resulted in a reduction of the number of energy layers needed in treatment plans, thereby decreasing the number of synchrotron spills needed and hence treatment time. For normal plans, treatment time without an MRF was 65 seconds per field, whereas all four of the MRF plans had treatment times of less than 30 seconds per field--sufficiently low for breath-hold. For SR plans, the shortest treatment time achieved was 47 seconds per field, compared to 94 seconds without an MRF. The time reduction had diminishing gains as the MRF thickness increased. PTV coverage and dose uniformity was comparable across all plans; however, the average dose to the surrounding lung increased from 45% of the prescription dose with no MRF up to 54% (worst case with MRF).

Conclusion:

An MRF can be used to reduce treatment time to aid breath-hold treatments without compromising plan quality. To balance treatment time and normal tissue dose, the ideal choice was shown to be the thinnest MRF which achieves the timing necessary for breath-hold.

References:

J. Perl et al., Med. Phys. 39, 6818


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