Planning and delivery of intensity modulated bolus electron conformal therapy

Abstract

Purpose Bolus electron conformal therapy (BECT) is a clinically useful, well‐documented, and available technology. The addition of intensity modulation (IM) to BECT reduces volumes of high dose and dose spread in the planning target volume (PTV). This paper demonstrates new techniques for a process that should be suitable for planning and delivering IM‐BECT using passive radiotherapy intensity modulation for electrons (PRIME) devices. Methods The IM‐BECT planning and delivery process is an addition to the BECT process that includes intensity modulator design, fabrication, and quality assurance. The intensity modulator (PRIME device) is a hexagonal matrix of small island blocks (tungsten pins of varying diameter) placed inside the patient beam‐defining collimator (cutout). Its design process determines a desirable intensity‐modulated electron beam during the planning process, then determines the island block configuration to deliver that intensity distribution (segmentation). The intensity modulator is fabricated and quality assurance performed at the factory (.decimal, LLC, Sanford, FL). Clinical quality assurance consists of measuring a fluence distribution in a plane perpendicular to the beam in a water or water‐equivalent phantom. This IM‐BECT process is described and demonstrated for two sites, postmastectomy chest wall and temple. Dose plans, intensity distributions, fabricated intensity modulators, and quality assurance results are presented. Results IM‐BECT plans showed improved D90‐10 over BECT plans, 6.4% versus 7.3% and 8.4% versus 11.0% for the postmastectomy chest wall and temple, respectively. Their intensity modulators utilized 61 (single diameter) and 246 (five diameters) tungsten pins, respectively. Dose comparisons for clinical quality assurance showed that for doses greater than 10%, measured agreed with calculated dose within 3% or 0.3 cm distance‐to‐agreement (DTA) for 99.9% and 100% of points, respectively. Conclusion These results demonstrated the feasibility of translating IM‐BECT to the clinic using the techniques presented for treatment planning, intensity modulator design and fabrication, and quality assurance processes.

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

“…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. The landscape may change in the future with higher energy electrons (often referred to as very high-energy electrons [VHEEs]) delivered with beam scanning.…”

“…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.…”

FLASH radiotherapy treatment planning and models for electron beams

“…As the HEE delivery machines currently deliver beam pulses every few milliseconds, and the pulses are typically on the order of one or more Gray-per-pulse, this would require a very fast moving MLC to achieve the desired field modulation and such a technology is not currently yet available [36]. Alternatively, there has been development of intensity modulated passive scattering applicator devices to achieve conformal and homogeneous dose, without compromising substantially maximum depths that can be treated [37,38]. Rahman et al [39] demonstrated the feasibility of plan and dose calculation with the use of this passive delivery method for an electron UHDR beam produced from a modified medical linac.…”

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

“…BECT has the benefit of providing adequate dose coverage to the PTV, while reducing dose to distal critical structures and normal tissues and ensuring low whole‐body dose. BECT has been shown useful for treatment of post‐mastectomy chest wall 3 , 4 , 5 , 6 , 7 ; shallow sarcomas 3 , 4 , 8 ; ear, temple, 9 parotid, and buccal mucosa 4 , 10 ; nose 11 ; eye canthi 12 , 13 ; and extremities. 8 Currently, BECT is commercially available from two companies (.decimal, LLC, Sanford, FL and Adaptiiv Medical Technologies, Inc., Halifax, NS, Canada) that provide software based on bolus design algorithms, 8 , 14 integrate that software with the customer's treatment planning system, and allow for bolus fabrication through either milling of machinable wax 14 , 15 or 3D printing.…”

“…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.…”