Useful island block geometries of a passive intensity modulator used for intensity-modulated bolus electron conformal therapy

Purpose This project determined the range of island block geometric configurations useful for the clinical utilization of intensity-modulated bolus electron conformal therapy (IM-BECT). Methods Multiple half-beam island block geometries were studied for seven electron energies 7-20 MeV at 100 and 103 cm source-to-surface distance (SSD). We studied relative fluence distributions at 0.5 cm and 2.0 cm depths in water, resulting in 28 unique beam conditions. For each beam condition, we studied intensity reduction factor (IRF) values of 0.70, 0.75, 0.80, 0.85, 0.90, and 0.95, and hexagonal packing separations for the island blocks of 0.50, 0.75, 1.00, 1.25, and 1.50 cm, that is, 30 unique IM configurations and 840 unique beam-IM combinations. A combination was deemed acceptable if the average intensity downstream of the intensity modulator agreed within 2% of that intended and the variation in fluence was less than +/- 2%. Results For 100 cm SSD, and for 0.5 cm depth, results showed that beam energies above 13 MeV did not exhibit sufficient scatter to produce clinically acceptable fluence (intensity) distributions for all IRF values (0.70-0.95). In particular, 20 MeV fluence distributions were unacceptable for any values, and acceptable 16 MeV fluence distributions were limited to a minimum IRF of 0.85. For the 2.0 cm depth, beam energies up to and including 20 MeV had acceptable fluence distributions. For 103 cm SSD and for 0.5 cm and 2.0 cm depths, results showed that all beam energies (7-20 MeV) had clinically acceptable fluence distributions for all IRF values (0.70-0.95). In general, the more clinically likely 103 cm SSD had acceptable fluence distributions with larger separations (r), which allow larger block diameters. Conclusion The geometric operating range of island block separations and IRF values (block diameters) producing clinically appropriate IM electron beams has been determined.