Homogeneity study of proton and carbon ion scanning beams using combinations of different spot sizes and grid sizes

Different scanning ion beam delivery systems have different delivery accuracies, and the resulting delivery errors will affect field homogeneity. This study was performed to determine an appropriate combination of spot size (FWHM) and spot grid size (GS), which can provide homogenous dose distributions for both proton and carbon ion scanning beam radiotherapy. The combination of the two parameters is represented by a combination factor named n, which is the quotient of FWHM divided by GS.

Delivery uncertainties of our beam delivery system were analyzed using log files from the treatment of 28 patients. Square fields for different n values were simulated with and without considering the delivery uncertainties, and the homogeneity of these square fields was analyzed. All spots were located on a rectilinear grid with equal spacing in the x and y directions. In addition to the simulations, we performed experimental measurements using both protons and carbon ions. We selected six energy levels for both proton and carbon ions. For each energy level, we created six square field plans with different n values (1, 1.5, 2, 2.5, 3, 3.5). These plans were delivered and the field homogeneity was determined using a film measurement.

The simulations demonstrated that under ideal condition (i.e., the delivery system has no delivery errors), the homogeneity is within 3% when n ≥ 1.1. When delivery uncertainties were included in the simulation, the homogeneity is within 3% when n ≥ 2.3. For film measurements, homogeneity under 3% was achieved when n ≥ 2.5.

A practical method to determine the appropriate combination of spot size and grid size is here presented. Considering the uncertainties of the beam delivery system, an n value of 2.5 is good enough to meet the lateral homogeneity requests in our center. The methods used here can be easily repeated in other particle therapy centers.