A Model for Secondary Monitor Unit Calculations of PBS Proton Therapy Treatment Plans.

Greg Schimke,Joseph Syh,Hsinshun Terry Wu

doi:10.14338/ijpt-18-00031.1

Abstract

Purpose:This article summarizes a volume-based method by which secondary monitor unit (MU) calculations may be performed for pencil beam scanning, single field uniform dose (SFUD) proton therapy treatment plans.Materials and Methods:Treatment planning system (TPS) simulations were performed by using the local beam model to define relationships between planning target volume (PTV) characteristics and the MUs required to deliver a uniform dose for a given beam orientation. Relevant target attributes included volume, depth (ie, beam range), range-shifter air gap, and the projected area of the target volume in the beam's eye view (BEV). The proposed model approximates the PTV as a simplified cuboid region of interest as defined by its volume and BEV projected area. Output factors (cGy/MU) were then tabulated for the idealized geometry through TPS simulations using region of interests with a range of dimensions expected to be seen clinically. Correction factors were applied that account for differences between the PTV and the idealized conditions, and MUs for each beam were then scaled according to the measured spread out Bragg peak (SOBP) dose in water.Results:Our model was applied to various treatment sites, including pelvis, brain, lung, and head and neck. Monitor units prescribed by the TPS were compared to those predicted by using the model for 78 treatment beams. The total mean percentage difference for all beams was −0.2% ± 3.8%.Conclusion:This work demonstrates the potential for reasonably accurate secondary verification of MUs in pencil beam scanning proton therapy for SFUD treatment plans with the proposed method. Required inputs are few, and are readily accessible, facilitating automation and clinical application. Further investigation will expand the current model to accommodate a broader range of potential optimization problems, and intensity-modulated proton therapy treatment plans.

Highlights

Pencil beam scanning (PBS) proton therapy does not lend itself to traditional monitor unit (MU) calculation methods used for passive scattering systems
For passive scattering proton therapy systems, calculations of MUs are based on factors derived from the dosimetric characteristics of various spread out Bragg peak (SOBP) beams in conjunction with the compensator and snout required to deliver the prescribed dose to a single point [5]
In PBS proton therapy, treatments are delivered with thousands of individual magnetically steered pencil beams that are delivered in a polyenergetic spot pattern such that the target volume receives a uniform dose, and are almost exclusively created by using inverse optimization techniques

Summary

Introduction

Pencil beam scanning (PBS) proton therapy does not lend itself to traditional monitor unit (MU) calculation methods used for passive scattering systems. Pencil beam scanning systems are not capable of delivering uniform volumetric reference fields without the use of inverse optimization, which complicates cGy/MU output characterization for the composite beam, even when the desired dose distribution is uniform For this reason, it is common to first create a beam model, and subsequently validate absolute and relative dose distributions with deliverable treatment plans from the treatment planning system (TPS) [4, 6]. While inherently dependent upon the volume, takes into account the shape and depth of the target, allowing it to be applied more broadly to other treatment sites This approach differs from typical second-check calculations in that it does not require detailed beam delivery and spot information from the TPS, but rather determines MUs on the basis solely of the defined structures and beam orientation. It can serve to verify or to predict prescribed MUs for SFUD treatment beams

Materials and Methods

Results

Discussion