A new treatment planning system (TPS) for proton therapy has been shown to accurately predict delivered dose by modelling the beam as a combination of three Gaussian distributions. Researchers in Japan tested the system using an ionization chamber and radiochromic film in a simple; commercially available phantom. The results demonstrate a reliable and convenient method of dose verification that could be adopted widely by proton therapy providers.
Because the depth at which a proton beam is halted by tissue depends on its initial energy, intensity-modulated proton therapy (IMPT) allows the radiation field to conform closely to the 3D shape of the tumour while sparing surrounding tissue. This makes IMPT the method of choice for intricately shaped tumours in complex physiological settings. The narrow margins in these situations mean that a robust quality assurance procedure is need so that clinicians can be confident that the plann dose is the one that is deliver to the patient.
To establish such a procedure for pencil-beam scanning proton therapy; Keisuke Yasui and colleagues, at Fujita Health University and Nagoya Proton Therapy Centre; simulated treatments for prostate and head-and-neck cancers using a simple commercial phantom that broadly replicates the form of a human patient.
Properties of human tissue
The researchers placed an ionization chamber at various points within the phantom to measure the absolute dose; and inserted radiation-sensitive film at three different depths to find the relative dose distribution. Although the phantom is construct using materials that approximate the radiation-absorbing properties of human tissue; the equivalence is not quite perfect. This means that standard calculations use to predict how the proton beam is stop by tissue is not be apply to the phantom directly.
To account for this; Yasui and colleagues derived a correction factor that converts measured radiodensity in Hounsfield units to phantom-specific relative stopping power. They also considered errors in the predicted dose that arise due to uncertainty in the measurement point of the ionization chamber; and in the point around which the treatment gantry rotates. Because As the TPS and phantom are both commercially available; therefore any clinic that uses IMPT based on pencil-beam scanning can use the procedure ; and the team’s phantom-specific correction table to verify their treatment plans.
Treatment gantry rotates
As long as absolute dose measurements are taken for each beam angle to mitigate uncertainty related to measurement points and gantry rotation the method provides an accurate; therefore reproducible basis for quality assurance. Because “Our motivation was to realize equal access to high-quality spot-scanning proton therapy in Japan and all over the globe;” says Yasui.