May 29, 2012
Additional tissue sparing with MERT
A study has demonstrated that healthy tissue can be spared from unnecessary bremsstrahlung radiation dose during modulated electron radiotherapy (MERT). Sparing was achieved by removing the scattering foil from the treatment head of the linear accelerator (Phys. Med. Biol. 57 3259).
The research, led by Jan Seuntjens at McGill University in Montreal, Canada, is seen as further evidence of MERT's promise as a new treatment technique. "Sufficient work by multiple groups shows that MERT has dosimetric advantages over current techniques for certain treatment sites such as the breast, chest wall and head-and-neck cancers. Above all, the advantages of combining MERT with modulated photon therapies seem particularly promising. We believe that MERT will offer complementary advantages over pure photon treatments for specific sites," first author Tanner Connell told medicalphysicsweb.
Modulation of electron beam energy and intensity using MERT conforms the prescribed dose more closely to the distal edge of the tumour than is possible with conventional fixed energy, fixed aperture electron beam therapy. Consequently, tissue beyond the tumour is spared.
Eliminating unnecessary dose
The McGill researchers went one step further, sparing healthy tissue further still by removing the scattering foil from the beamline. "The primary motivation of this study was to further improve the main advantage of electron therapy, which is the already low dose to healthy tissue beyond the target. The scattering foils in their current form are not optimal for MERT as they unnecessarily add contamination photons," explained Connell.
The photons increase the dose received by healthy tissue, known as a bremsstrahlung tail, while providing no therapeutic advantage. The bremsstrahlung dose component increases with decreasing field size. "The unflattened beams that result from removing the scattering foil can be compensated for by the optimization algorithm in the treatment planning system, much as it does in conventional IMRT with flattening filter-free photon beams," explained Connell.
In-house electron beam modulation
The researchers used experimental measurements and a treatment planning study to compare the doses resulting from standard MERT irradiation versus those obtained when the scattering foil was removed. Modulation of electron beam intensity was achieved using a previously reported custom-built collimation device comprising four computer-controlled jaws. The device was attached to the electron applicator on the treatment head.
Using a water tank and an unshielded diode, dose measurements were made for a 2 x 2 cm2 field for the clinical range of beam energies. The measurements demonstrated the expected decrease in dose contribution from the bremsstrahlung tail upon removal of the scattering foil: reductions by up to a factor of 12.2 were observed. The measured data were used to commission two Monte Carlo-based beam calculation models, for foil and foil-free irradiation, for the treatment planning study that followed.
A planning target volume (PTV) and two organs-at-risk (OAR) were added to a CT scan of a solid water phantom that was imported into the treatment planning system. Dose calculations using the phantom assessed the relative abilities of foil and foil-free irradiation to provide target coverage and minimize OAR dose. Removing the scattering foil proved to have no marked effect on PTV coverage. Dose reductions in the two OARs was observed. In particular, the volume of phantom bathed in low-level dose was observable in the calculated dose distributions and dose volume histograms.
The researchers were pleased with their results. "Our findings showed that by making only minor changes to the treatment head design, the beam characteristics of MERT could be greatly improved. As a result of this work, we hope to renew interest by physicists and clinicians to incorporate this technique in their clinics and foster a climate in which commercial solutions can be developed," Connell commented.
The group are continuing with related research that will improve the efficacy of dosimetry and treatment planning of the technique. "We are working towards seeing our MERT solution in the clinic in the near future," said Connell.
• This article forms part of a special section in PMB. The Monte Carlo special section is a collection of articles selected from the Fourth International Workshop on Recent Advances in Monte Carlo Techniques for Radiation Therapy, (8–10 June 2011, Montreal, Quebec, Canada) and highlights some of the best research in the field. All articles included within the special section are free to read online until the end of 2012.
About the author
Jude Dineley is a freelance science writer and former medical physicist based in Sydney, Australia.