Commissioning a passive-scattering proton therapy nozzle for accurate SOBP delivery

Abstract

Proton radiotherapy centers that currently use passively scattered proton beams do field specific calibrations for a non-negligible fraction of treatment fields, which is time and resource consuming. Our improved understanding of the passive scattering mode of the IBA universal nozzle, especially of the current modulation function, allowed us to re-commission our treatment control system for accurate delivery of SOBPs of any range and modulation, and to predict the output for each of these fields. We moved away from individual field calibrations to a state where continued quality assurance of SOBP field delivery is ensured by limited system-wide measurements that only require one hour per week. This manuscript reports on a protocol for generation of desired SOBPs and prediction of dose output.

Figure 1

Schematic, not to scale, of our IBA universal nozzle in double-scattering mode. Components are a binary fixed scatterer system (FS), a range modulator track (RM), magnets (not used for double scattering), a contoured second scatterer (SS), collimator jaws, monitor unit chamber (IC), and snout.

Figure 2

Pristine Bragg peak and SOBP depth dose distributions for a range of 14.7 cm. The pristine Bragg peak was the deepest peak in all SOBPs shown. The curve labeled “TCS-incorrect” is the result of CMF optimization aimed at the flattest possible SOBP. The curve labeled “TCS-correct” is optimized toward, first, the steepest distal dose gradient and, second, a flat SOBP.

Figure 3

Modulation as a function of stop digit for the three suboptions (high, medium, and low) of option A5. The solid line indicates the modulation transfer function as implemented in our TCS for this option.

Figure 4

Output for option A5 as a function of r_small. (Top) Logarithmic scale. (Bottom) Linear scale. The output is measured at the maximum range in the option (solid) and at the minimum range (open).

Figure 5

Effect of jaw position on the SOBP for both the 12 and 25 cm snouts using maximum sized apertures. The SOBP had a range of 15.7 cm and full modulation. (Top) Depth dose curves, normalized at 12 cm depth with, e.g., “sn12_j18” denoting use of the 12 cm diameter snout and jaws opened to 18 cm. (Bottom) Output as a function of jaw position, as measured at a depth of 12 cm.

Figure 6

Effect of the range within the option on the output for (top row) option A2 (range interval: 5.82–7.49 cm) and (bottom row) option A5 (range interval: 11.65–15.54 cm). Left hand graphs: Output as a function of range. Data are corrected for the first two terms in Eq. 1. Right hand graphs: The black columns show the accuracy of the output model, as given in Eq. 1, for historical verification data acquired since recommissioning the gantry. The white bars are for a best fit to the data when forcing the range correction term in Eq. 1 to unity.

Figure 7

Output for all options as a function of r_small. (a) Output for r_small≤1; (b) output for r_small≥1.

Figure 8

Results of the weekly TCS verification. The figure shows the difference between expected and measured (a) ranges (R), (b) modulations (M₉₈), and (c) outputs (Ψ). Negative values mean that the value as measured was lower than expected. All fields for which the measured modulation differed by more than 3 mm from the requested modulation passed the 3% dose criterion, i.e., at least 95% dose at the expected location of the proximal 98% isodose level.