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Implication of Spot Position Error On Plan Quality in Pencil-Beam Proton Therapy

J Yu

J Yu*, C Beltran, M Herman, Mayo Clinic, Rochester, MN

MO-D-108-11 Monday 2:00PM - 3:50PM Room: 108

Purpose: To assess the dosimetric implication of spot position errors on intensity modulated proton therapy (IMPT) treatment plan-quality and develop an analytical routine to aid in that assessment.

Methods:IMPT plans with different spot spacing (SS) (3-6mm) were created on a water phantom and a patient CT using a commercial planning system. Spot position errors (PE) (1-2mm) were simulated by shifting spots away from their planned location using a custom built script. Various perturbations of single-spot shift and multiple-spot shift in a correlated and random manner were simulated. The resulting dose perturbations were compared to the original plans to calculate percent dose error (PDE (%)).

Results:Spot shifts created hot and cold spots adjacent to the location of the error. In the phantom study of the single-spot error, the PDE at the largest dose difference has been mapped out in SS and PE space to be a quadratic 3D surface, and the slope of the surface became less steep at the deeper depth. As the global dose distribution reduced from volume to plane and line the PDE increased substantially.
For clustered multiple-spot error the dose perturbation was significantly larger than single-spot error, but there is saturation for the magnitude of PDE though the perturbation volume increased. In a patient plan, 5% of total spots were clustered which resulted in the PDE of 5 to 6 % and is decreased to 2% when the spots and shift direction are randomly chosen.

Conclusion:The dose perturbation is a complex function of PE, SS, depth and global dose distribution. To quantitatively and systematically evaluate the impact of position errors on plan-quality a simulation tool for both selective and random spot(s) position errors was developed.

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