Out of field scatter from electron applicator in modern linear accelerators.

BACKGROUND: Electron out-of-field scatter is generally not given importance mainly in electron fields. However, this is important when applicator down and boost treatments are given usually at an angle from the central axis. The electron scatter dose is found to be far away from the central axis which could be easily ignored. PURPOSE: This study aims to investigate the out-of-field radiation doses from electron applicators and their effects on clinical treatment. By identifying the parameters that contribute to out-of-field doses and to explore potential strategies for reducing these doses in order to improve patient outcomes from modern machines. METHODS: Measurements were performed in water phantom using electron diode for modern Elekta and Varian machines. Dose profiles were acquired at surface and dmax with 0° and 90° collimation angle. Various gantry angles were also studied for some data with IC Profiler. The profiles were normalized with respect to the central axis dose. RESULTS: The scatter dose peaks were found at a distance between 11 and 28 cm from the central axis on all machines. However, the peak shifts to 15 cm at 90° collimator when beam is tilted. The position and intensity of the dose varies with depth, collimator, and gantry angles for both Elekta and Varian machines. Due to clearance issues more gantry angles were studied for Elekta applicator compared to Varian. In general, Varian TrueBeam has a lower scatter that Elekta Infinity. The 90° collimator angle has a higher scatter compared to zero degree for both machines. CONCLUSIONS: There are clinically significant peripheral doses around 3% of the central axis dose from the electron applicator. Elekta has a slightly higher scatter (3%) than Varian (2%) that peaks at 25 cm which is clinically important but often overlooked.

A comparison of proton stopping power measured with proton CT and x-ray CT in fresh postmortem porcine structures.

PURPOSE: Currently, calculations of proton range in proton therapy patients are based on a conversion of CT Hounsfield units of patient tissues into proton relative stopping power. Uncertainties in this conversion necessitate larger proximal and distal planned target volume margins. Proton CT can potentially reduce these uncertainties by directly measuring proton stopping power. We aim to demonstrate proton CT imaging with complex porcine samples, to analyze in detail three-dimensional regions of interest, and to compare proton stopping powers directly measured by proton CT to those determined from x-ray CT scans. METHODS: We have used a prototype proton imaging system with single proton tracking to acquire proton radiography and proton CT images of a sample of porcine pectoral girdle and ribs, and a pig's head. We also acquired close in time x-ray CT scans of the same samples and compared proton stopping power measurements from the two modalities. In the case of the pig's head, we obtained x-ray CT scans from two different scanners and compared results from high-dose and low-dose settings. RESULTS: Comparing our reconstructed proton CT images with images derived from x-ray CT scans, we find agreement within 1% to 2% for soft tissues and discrepancies of up to 6% for compact bone. We also observed large discrepancies, up to 40%, for cavitated regions with mixed content of air, soft tissue, and bone, such as sinus cavities or tympanic bullae. CONCLUSIONS: Our images and findings from a clinically realistic proton CT scanner demonstrate the potential for proton CT to be used for low-dose treatment planning with reduced margins.