MV Unflattened Beams Research Articles

Mucosal dosimetry on unflattened photon beams: a Monte Carlo phantom study

Objective: This study investigated the dependences of mucosal dose and photon energy distribution on the flattened and unflattened photon beams using Monte Carlo simulation. Methods: A heterogeneous mucosa phantom with a variation of mucosal thickness (0.5–3 mm) was irradiated by the 6 and 10 MV flattened and unflattened photon beams generated by a Varian TrueBeam linac. The photon energy distributions at the mucosa and depth doses at the bones and mucosa were calculated using the EGSnrc-based Monte Carlo code. Results: It is found that the 6 MV unflattened photon beam contained more particle fluence in the low-energy range (0–100 keV) than the 10 MV. However, mucosal doses for the unflattened photon beams were found lower than the flattened. This is different from the skin dose having dose enhancement on the unflattened beam compared to the flattened, though both mucosal and skin structure involve an air-soft tissue interface. The particle fluence of the 6 and 10 MV unflattened photon beams increased with an increase of the mucosal thickness. This agreed well with the relative depth doses that increased in the ranges of 3.4%–3.6% and 2.5%–3.0% at the upper mucosa, and 5.1%–5.7% and 2.9%–3.2% at the lower mucosa, with their thickness increasing from 0.5–3 mm for the 6 and 10 MV unflattened photon beams. Conclusion: It is concluded that the unflattened photon beam results in a lower mucosal dose than the flattened, and the 6 MV unflattened beam has a larger dependence of mucosal dose on its thickness when compared to the 10 MV.

Comparison of patient megavoltage cone beam CT images acquired with an unflattened beam from a carbon target and a flattened treatment beama)

To use an imaging beam line (IBL) to obtain the first megavoltage cone-beam computed tomography (MV CBCT) images of patients with a low atomic number (Z) target, and to compare these images to those taken of the same patients with the 6 MV flattened beam from the treatment beam line (TBL). The IBL, which produces a 4.2 MV unflattened beam from a carbon target, was installed on a linear accelerator in use for radiotherapy. Provision was made for switching between the IBL and TBL for imaging the same patient with beams from the low-Z and high-Z targets. Dose was quoted as monitor units times the dose per monitor unit for the standard calibration geometry. Images were acquired with institutional approval and patient consent with both the IBL and TBL on a series of 23 patients undergoing radiotherapy. Patients were imaged daily to weekly and aligned to the planning CT using the images. Doses were reduced over the course of treatment to determine the minimum doses required for alignment. Images were assessed offline. IBL MV CBCT images of prostate, head and neck, lung, and abdomen showed improvement in soft tissue contrast for the same dose as the TBL images. Bony anatomy, air cavities, and fiducial markers were sharper. CBCT with a dose of 1 cGy was sufficient for alignment of prostate and head and neck patients based on bony anatomy or implanted gold seeds, 2-4 cGy for lung, abdomen, and pelvis. Photon scatter in the patient had minimal effect on image quality. The metallic hip prosthesis in one patient showed reduced artifacts compared to diagnostic CT. The IBL has the advantage of improved image quality at the same dose, or reduced dose for the same image quality, over the TBL.

On the existence of low-energy photons (

Low-energy photons (<150 keV) are essential for obtaining high quality x-ray radiographs. These photons are usually produced in the accelerator target, but are effectively absorbed by the flattening filter and, at least partially, by the target itself. Experimental proof is presented for the existence of low-energy photons in the unflattened x-ray beam produced by a 6 MeV electron beam normally incident on the thinner of the two existing ports of the all-Cu radiotherapeutic target of a Clinac 18 (Varian Associates) linear accelerator. A number of one-shot absorption measurements were carried out with 12 foils of Pb absorbers with thicknesses varying from 0.25 to 3 mm in steps of 0.25 mm arranged symmetrically around the central axis on a 7.2 cm radius circumference. A Kodak ECL film–screen–cassette combination was used as a detector in the absorption measurements, in which optical density was measured as a function of the thickness of the Pb absorbers. Two sets of absorption measurements were carried out: the first one with the Clinac 18 6 MV unflattened beam and the second one with the Clinac 600C 6 MV therapeutic counterpart beam. There is a striking difference between the two sets: the optical density versus Pb-absorber thickness curve shows a sharp increase in optical density at small absorber thicknesses in the case of the unflattened 6 MV x-ray beam as compared with a gently sloping dependence in the case of the 6 MV therapeutic beam. A semi-quantitative assessment of the low-energy photon contribution to the whole optical density/contrast is presented. A 0.85 mm thick Pb absorber intercepting the 6 MV unflattened x-ray beam eliminates almost totally the sharp peak in the optical density curve at small Pb-absorber thicknesses. This constitutes additional evidence for the existence of low-energy photons (<150 keV) in the unflattened 6 MV beam from the Cu therapeutic target.

Tumour dose enhancement using modified megavoltage photon beams and contrast media

This study examines the magnitude of tumour dose enhancement achieved by injection of gadolinium or iodine contrast media (CM) and treatment using modified x-ray photon spectra from linear accelerators. Monte Carlo modelling of the linear accelerator and patient geometry was used to explore the effect of removing the flattening filter for various beam qualities and the resultant effect on dose enhancement. In addition, ionization measurements were conducted to observe dose enhancement within a phantom containing CM. Simulation results indicate that for flattened 6–24 MV photon beams and realistic CM tumour concentrations, the dose enhancement remains below 5%. However, if the flattening filter is removed, dose enhancement is increased significantly. For a 30 mg ml−1 gadolinium CM tumour concentration, for example, 8.4%, 10.8%, 13.7% and 23.1% dose enhancements are achieved for 18 MV, 6 MV, 4 MV and 2 MV unflattened beams, respectively. In contrast to the phototherapy technique, which uses the orthovoltage beam from a modified CT scanner to achieve dose enhancement, all unflattened spectra preserve the dose build-up at the surface, and thus the skin and bone are spared.