A Technique to Irradiate Scar-like Line Target With Dynamic Electron Arc Radiation Therapy

Abstract

For superficial target such as surgical scar, standard radiotherapy technique is the static electron beam. Its use can become cumbersome when target area is long and/or the skin has large curvature, because the combination of multiple beams often produces hot and cold spots in the junction regions and surrounding healthy tissues can be over-irradiated. In addition, manual setup of each field is required which is often impractical clinically. In this study, we investigate a new technique to irradiate any line target with the dynamic electron arc radiotherapy (DEAR) where both the couch position and Linac gantry angle are synchronized with the radiation beam. The electron applicator is kept in DEAR to preserve beam penumbra. To avoid collision between Linac and patient, couch is moved simultaneously with gantry rotation to maintain a constant source to skin distance (SSD) and an en face beam direction at all times. The DEAR plan, in the form of XML, follows the MU-position trajectory model using control point (CP) similar to IMRT and VMAT, is currently deliverable on Truebeam Linac in research mode. The line target is represented by a series of connected points whose 3D coordinates are first extracted from treatment planning system. They are used to determine the gantry angle, couch position, collimator angle, and MU at each CP. The XML plan also includes some fixed parameters: energy, applicator, cutout, and dose rate. The plan and CT are fed to the Virtual Linac Monte Carlo simulation platform for the final dose calculation and prescription normalization. Finally the plan is verified dosimetrically on phantom with radiochromic film. Various target orientations and lengths were studied on flat, cylindrical, and anthropomorphic phantoms. Either 6 or 9 MeV electron beams were chosen depending on the depth of the target. The smallest applicator (6 × 6 cm2) is used for increased clearance. Cutout shape is in square (3 × 3 cm2), rectangular (3 × 1 cm2), or circular (dia = 3 cm). Dose rate is based on the prescription dose, speed of couch motion and gantry rotation, and delivery time. Plans are generated in both forward and reverse motion directions. Any number of setup beams during the motion trajectory can be created and mode up during the dry run. Gantry angle is chosen so that beam is always perpendicular to the target. For circular cutout, the collimator angle is fixed; for others, it is parallel to the scanning direction. Finally, the MU at each CP is proportional to the distance between the current and the neighboring target points. For a 20 cm long target with 200 cGy prescription dose, the delivery can be completed within 2 minutes at dose rate of 1000 MU/min. The film measurements agree well with the Monte Carlo dose calculations. We have developed a technique to irradiate any line target using DEAR. It improves dose distributions substantially over the standard electron beam. Further investigations are in progress to expand into two dimensional targets.