Room: Exhibit Hall | Forum 9
Purpose: (1) To introduce and demonstrate passive intensity modulators using island blocks for bolus electron conformal therapy (ECT). (2) To determine how to select optimal island block configuration for intensity modulators. (3) To demonstrate a patient example where such intensity modulators would improve bolus ECT treatment plans.
Methods: Intensity modulated electron therapy (IMET) is delivered passively through the use of island blocks (6-mm thick tungsten) placed within the aperture of the electron treatment beam(s). The intensity reduction is proportional to the local area of the beam covered by these island blocks. One possible geometric configuration for these island blocks is a hexagonal grid. In this configuration the resulting intensity reduction is determined by the diameters of the island blocks and block spacing. A pencil-beam algorithm (PBA) was used to simulate a wide range of block diameter and separation combinations to determine those most clinically desirable for specific intensity reductions (0.70-0.99). Based on these findings, a block optimization method was applied to select island block configurations necessary to achieve desired intensity distributions for a set of previously treated radiotherapy patients. Using a research version of p.d software (.decimal, Sanford, FL) for bolus ECT, the benefit of intensity modulators is evaluated.
Results: PBA calculations and measurements verified that passive intensity modulators delivered the desired intensity modulation. PBA calculations provided data that determined the largest block separation that yields a clinically acceptable beam when designing modulators, which is desirable due to fewer island blocks and reduction in effects of electron scatter from the island blocks. The benefit of intensity modulators for a bolus ECT head and neck plan is illustrated.
Conclusion: Variable diameter island blocks in a hexagonal array can provide passive intensity modulation for bolus electron conformal therapy in the radiotherapy clinic.