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

Abstract

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 AE2%. 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.

Planning and delivery of intensity modulated bolus electron conformal therapy

“…The segmentation process was based on an algorithm developed by Chambers. 19 It takes the initial island block pattern, determined using the method described below, then calculates the underlying intensity pattern using a pencil beam algorithm. 23 After comparing the calculated with the desired intensity pattern, island block diameters are modified.…”

“… 17 , 18 The objective of this paper is to describe a process that should be suitable for planning and delivery of IM‐BECT. The process, detailed and demonstrated for two patient sites, includes adding techniques for design, fabrication, and quality assurance of the intensity modulator 19 , 20 to the current BECT process. 6 …”

Treatment planning for Flash radiotherapy: General aspects and applications to proton beams

“…14 Such systems are, however, not routinely available in clinical practice. An alternative approach is the use of passive modulators, 15,16 which can achieve superior dose distributions with respect to bolus electron conformal therapy. 17 In principle, one could therefore use existing equipment to deliver electron treatments on shallow targets with a conformal dose distribution achieving FLASH regime.…”

Modeling scatter through sides of island blocks used for intensity‐modulated bolus electron conformal therapy

“…They are oriented to align with the divergence of the beam such that their axes project back to the nominal source position (100 cm upstream of isocenter). 9 , 17 , 19 , 20 Incident electrons are absorbed by the island blocks so that the underlying fluence in the patient is reduced locally by approximately the fraction of the cross‐sectional area of the field covered by the overlying nearby island blocks. This is possible because lateral scattering of the electrons, primarily due to air upstream of the intensity modulator and the machinable foam slab in which it is embedded, restores locally the downstream fluence uniformity, but to a reduced value.…”

“…Therefore, all electrons incident on the upstream surface of the island block were assumed to be absorbed and all others transmitted to the patient plane. 9 , 17 , 19 To model perfect collimation, the planar fluence distribution for the n th energy bin resulting from a small square pencil beam (side = ) of area equal to the circular cross section of each i th island block was transported from the collimator plane to points on the patient surface. Then, the planar fluence for each small square pencil beam was subtracted from the primary electron fluence of the n th energy bin resulting from the open beam (without island blocks), that is, where is the open field planar fluence (intensity) distribution in the n th energy bin at position ( x,y,z ) on the patient surface, is the planar fluence distribution in the n th energy bin from the i th pencil beam simulating the island block located at ( x i ,y i ) in the PRIME device, and is the planar fluence distribution of the n th energy bin after incorporating the island blocks into the calculation.…”