Enhancement of Surface Dose In Electron Beams Using a Partial Shielding Technique with Water-Equivalent Material and Oil-Based Modeling Clay
Abstract
Purpose
Electron beam therapy is widely used for the treatment of superficial tumors; however, low-energy electron beams typically deliver a surface dose that is 20% lower than the maximum dose. Although the application of a bolus can increase the surface dose, it also shifts the percentage depth dose (PDD) curve toward the surface, thereby reducing the effective treatment depth. This study investigates a partial shielding technique using different bolus materials to optimize the dose weighting between non-bolus and bolus fields, aiming to improve insufficient superficial dose while maintaining treatment depth without increasing beam energy.
Methods
PDD curves of a 6 MeV electron beam were measured using a 3D water scanning system. Partial shielding technique was first simulated with water as bolus, and the optimal monitor unit (MU) weighting for the composite field was determined to be 8:2 for the non-bolus and bolus fields. Experimental measurements were subsequently performed using a solid water phantom with either a 2 cm water-equivalent bolus or a 1 cm modeling clay bolus placed on the surface. Percentage depth ionization (PDI) measurements were acquired using a Markus chamber (PTW Freiburg), and PDD curves were analyzed to evaluate surface dose.
Results
The results showed that a weighting of 0.8 (non-bolus) and 0.2 (with water-equivalent bolus) effectively compensated for surface dose deficiency, achieving more than 90% PDD from the surface to the depth of maximum dose. Furthermore, a custom-made 1 cm high-density modeling clay bolus demonstrated improved performance. With an MU weighting of 7:3, the partial shielding technique achieved a PDI exceeding 97% within the first 10 mm depth.
Conclusion
These findings indicate that the partial shielding technique can enhance surface dose while preserving sufficient treatment depth without increasing electron beam energy and bremsstrahlung production.