Off-axis dose distribution with stand-in and stand-off configurations for superficial radiotherapy treatments.

Current practice when delivering dose for superficial skin radiotherapy is to adjust the monitor units so that the prescribed dose is delivered to the central axis of the superficial unit applicator. Variations of source-to-surface distance due to patient's anatomy protruding into the applicator or extending away from the applicator require adjustments to the monitor units using the inverse square law. Off-axis dose distribution varies significantly from the central axis dose and is not currently being quantified. The dose falloff at the periphery of the field is not symmetrical in the anode-cathode axis due to the heel effect. This study was conducted to quantify the variation of dose across the surface being treated and model a simple geometric shape to estimate a patient's surface with stand-in and stand-off. Isodose plots and color-coded dose distribution maps were produced from scans of GAFChromic EBT-3 film irradiated by a Gulmay D3300 orthovoltage x-ray therapy system. It was clear that larger applicators show a greater dose falloff toward the periphery than smaller applicators. Larger applicators were found to have a lower percentage of points above 90% of central axis dose (SA90). Current clinical practice does not take this field variation into account. Stand-in can result in significant dose falloff off-axis depending on the depth and width of the protrusion, while stand-off can result in a flatter field due to the high-dose region near the central axis being further from the source than the peripheral regions. The central axis also received a 7% increased or decreased dose for stand-in or stand-off, respectively.