Efficient double-scattering proton therapy with a patient-specific bolus

Abstract

Purpose: Passive scattering proton radiotherapy utilizes beam-specific compensators to shape the dose to the distal end of the tumor target. These compensators typically require therapists to enter the treatment room to mount between beams. This study investigates a novel approach that utilizes a single patient-specific bolus to accomplish the role of multi-field compensators to improve the efficiency of the treatment delivery.

Methods: Ray-tracing from the proton virtual source was used to convert the beam-specific compensators (mounted on the gantry nozzle) into an equivalent bolus thickness on the patient surface. The field bolus contours were combined to create a single bolus. A 3D acrylic bolus was milled for a head phantom. The dose distribution of the compensator plan was compared to the bolus plan using 3D Gamma analysis and film measurements. Boluses for two clinical patients were also designed.

Results: The calculated phantom dose distribution of the original proton compensator plan was shown to be equivalent to the plan with the surface bolus. Film irradiations with the proton bolus also confirmed the dosimetric equivalence of the two techniques. The dose distribution equivalency of the bolus plans for the clinical patients were demonstrated.

Conclusions: We presented a novel approach that uses a single patient-specific bolus to replace patient compensators during passive scattering proton delivery. This approach has the potential to reduce the treatment time, the compensator manufacturing costs, the risk of potential collision between the compensator and the patient/couch, and the waste of compensator material.

Keywords: Bolus; Compensator; Passive scattering; Proton.

Fig. 1.

A traditional field-specific compensator for double scattering proton therapy.

Fig. 2.

Projection of traditional compensator (solid outline) onto patient surface to generate a patient specific surface bolus (dotted outline): along each ray, the WET in the compensator is the same as in the bolus that conforms to the patient contour.

Fig. 3.

Flowchart of custom bolus design and treatment planning.

Fig. 4.

(a) Virtual bolus designed for each individual field (pink, magenta, and red). (b) 3D rendering of the stitched bolus with shoulder (c) Patient-specific bolus manufactured by .decimal^® and (d) patient-specific bolus placed on head phantom.

Fig. 5.

The dose distribution of a) original plan with traditional compensators; b) plan with virtual bolus; c) plan with physical bolus.

Fig. 6.

(a) The irradiated film that was embedded in a sagittal plane of the head phantom. The top and bottom holes were used for alignment and (b) calibrated dose map with isodose lines for comparison with the virtual bolus plan.

Fig. 7.

The bolus plans demonstrated equivalent dose distribution as the original plans for a) brain and b) liver patients. The DVHs for the target and OARs of the compensator plans (−) and bolus plans (–) showed good agreement in c) brain and d) liver patients.

Fig. 8.

The bolus for a liver patient with the weight support structure.