Modulated photon radiotherapy (XMRT): an algorithm for the simultaneous optimization of photon beamlet energy and intensity in external beam radiotherapy (EBRT) planning

Abstract

This is a proof of principle study on an algorithm for optimizing external beam radiotherapy in terms of both photon beamlet energy and fluence. This simultaneous beamlet energy and fluence optimization is denoted modulated photon radiotherapy (XMRT). XMRT is compared with single-energy intensity modulated radiotherapy (IMRT) for five clinically relevant test geometries to determine whether treating beamlet energy as a decision variable improves the dose distributions. All test geometries were modelled in a cylindrical water phantom. XMRT optimized the fluence for 6 and 18 MV beamlets while IMRT optimized with only 6 MV and only 18 MV. CERR (computational environment for radiotherapy research) was used to calculate the dose deposition matrices and the resulting dose for XMRT and IMRT solutions. Solutions were compared via their dose volume histograms and dose metrics, such as the mean, maximum, and minimum doses for each structure. The homogeneity index (HI) and conformity number (CN) were calculated to assess the quality of the target dose coverage. Complexity of the resulting fluence maps was minimized using the sum of positive gradients technique. The results showed XMRT's ability to improve healthy-organ dose reduction while yielding comparable coverage of the target relative to IMRT for all geometries. All three energy-optimization approaches yielded similar HI and CNs for all geometries, as well as a similar degree of fluence map complexity. The dose reduction provided by XMRT was demonstrated by the relative decrease in the dose metrics for the majority of the organs at risk (OARs) in all geometries. Largest reductions ranged between 5% to 10% in the mean dose to OARs for two of the geometries when compared with both single-energy IMRT schemes. XMRT has shown potential dosimetric benefits through improved OAR sparing by allowing beam energy to act as a degree of freedom in the EBRT optimization process.

Part 2: Dynamic mixed beam radiotherapy (DYMBER): Photon dynamic trajectories combined with modulated electron beams

“…For this purpose, a technique similar to dynamic electron arc radiotherapy (DEAR) could be utilized, but with pMLC collimation instead of applicator based collimation to maintain clinical workflow. Finally, energy modulation could also be used for photon beams as conventional treatment units usually support multiple photon beam energies . Such possible improvements will be investigated in future work.…”

“…Finally, energy modulation could also be used for photon beams as conventional treatment units usually support multiple photon beam energies. 53 Such possible improvements will be investigated in future work.…”

Simultaneous optimization of mixed photon energy beams in volumetric modulated arc therapy

“…Finally, the output of the MILP dispersion model, MP – VMAT intensity apertures, was used for final dose computation. The dose calculation was performed by using a pencil beam algorithm known as quadrant infinite beam (QIB) model, which has been a common approach in several optimization studies . CERR uses CT voxels density to perform first‐order heterogeneity correction along the path of the pencil beam.…”

“…Major benefits such as beam penetration and forward scattering of higher energy photon can be accrued by allowing higher energies in optimization. For IMRT, mixed photon energy beams have been used clinically for numerous treatment sites (i.e., prostate, pelvis, breast) as it has been shown to produce superior dosimetric influence compared to a single energy IMRT treatments . Only a few studies have investigated dosimetric outcome of mixed photon energies for VMAT.…”

“…For IMRT, mixed photon energy beams have been used clinically for numerous treatment sites (i.e., prostate, pelvis, breast) as it has been shown to produce superior dosimetric influence compared to a single energy IMRT treatments. 13,14 Only a few studies have investigated dosimetric outcome of mixed photon energies for VMAT. Both studies indicated better sparing of OARs in comparison to the single energy VMAT.…”