Evaluation of the effectiveness of steel for shielding photoneutrons produced in medical linear accelerators: A Monte Carlo particle transport study.

PURPOSE: To confirm the effectiveness of steel as a material for shielding photoneutrons produced in medical linear accelerators and report values for neutron first and second tenth value layer to be used as radiation protection quantities. METHODS: Monte Carlo code Geant4 was used to simulate transport of photoneutrons through primary barriers containing layers of concrete and steel. The photoneutron spectrum generated in high energy modern accelerator head components was simulated and projected to barriers of different thicknesses of steel including a control case of no barrier. To derive the neutron tenth value layer (TVLn), absorbed dose was evaluated at 1 cm depth in a water phantom outside the barrier. The fluence and energy spectrum of neutrons leaking outside the barrier were also calculated. The neutron source model was used to quantitatively assess the neutron shielding for a newly designed bunker. RESULTS: For an 18 MV photon beam produced by a linear accelerator, the first and second TVLn, for steel as a neutron shielding material were determined to be 15.7-16.1 cm and 28.5-32.4 cm, respectively, depending on the vendor. The lateral distribution of neutrons outside steel barriers is investigated. Also, we show the relative efficiency of steel for fast and epithermal neutrons and determine the neutron energy spectrum outside a steel barrier. CONCLUSION: This paper agrees with previous studies for Pu-Be sources, and we would advocate using these values for the purposes of high energy linear accelerator bunker design.

Reducing ExacTrac intrafraction imaging uncertainty for prostate stereotactic body radiotherapy using a pre-treatment CBCT.

This study evaluated the intrafractional auto-matching uncertainties of prostate-implanted fiducial markers when using the ExacTrac v6.5 (Brainlab, Feldkirchen, Germany) X-ray stereoscopic system. A customised phantom with 3 implanted gold seeds was initially positioned at the isocentre using a cone beam CT (CBCT) prior to intrafractional imaging. Progressive offsets were applied to the phantom in all six directions (3 translational, 3 rotational) of 0 mm, 1 mm, 2 mm, 0°, 1° and 2°. Subsequently, the ability of the ExacTrac image-matching functions to detect and correct these offsets was tested. For comparison, this procedure was repeated, but without a CBCT for pre-treatment positioning. The auto-matching uncertainties when a CBCT was introduced into the workflow were significantly reduced, and overall, the auto-matching statistics using the implanted marker (seeds) matching function was found to be more precise than the bony anatomy function in-phantom. The total standard deviations for the translational shifts using the implanted marker and bony anatomy functions respectively were 0.1 mm and 0.3 mm vertically, 0.1 mm and 0.3 mm longitudinally, and 0.1 mm and 0.4 mm laterally. The standard deviations for the rotational shifts using the implanted marker and bony anatomy matching functions respectively were 0.2° and 1.2° for the yaw (angle vert), 0.3° and 1.1° for the pitch (angle long), and 0.2° and 1.2° for the roll (angle lat) directions. The reduced uncertainties from introducing a CBCT for initial localisation resulted in decreased probability of inhibits due to false positives during treatment.

Dose uncertainties associated with a set density override of unknown hip prosthetic composition.

The dosimetric uncertainties associated with radiotherapy through hip prostheses while overriding the implant to a set density within the TPS has not yet been reported. In this study, the uncertainty in dose within a PTV resulting from this planning choice was investigated. A set of metallic hip prosthetics (stainless steel, titanium, and two different Co-Cr-Mo alloys) were CT scanned in a water bath. Within the TPS, the prosthetic pieces were overridden to densities between 3 and 10 g/cm3 and irradiated on a linear accelerator. Measured dose maps were compared to the TPS to determine which density was most appropriate to override each metal. This was shown to be in disagreement with the reported literature values of density which was attributed to the TPS dose calculation algorithm and total mass attenuation coefficient differences in water and metal. The dose difference was then calculated for a set density override of 6 g/cm3 in the TPS and used to estimate the dose uncertainty beyond the prosthesis. For beams passing through an implant, the dosimetric uncertainty in regions of the PTV may be as high as 10% if the implant composition remains unknown and a set density override is used. These results highlight limitations of such assumptions and the need for careful consideration by radiation oncologist, therapist, and physics staff.

Post-upgrade testing on a radiotherapy oncology information system with an embedded record and verify system following the IAEA Human Health Report No. 7 recommendations.

Record and verify (R&V) systems have proven that their application in radiotherapy clinics leads to a significant reduction in mis-treatments of patients. The purpose of this technical note is to share our experience of acceptance testing, commissioning and setting up a quality assurance programme for the MOSAIQ® oncology information system and R&V system after upgrading from software version 2.41 to 2.6 in a multi-vendor, multi-site environment. Testing was guided primarily by the IAEA Human Report No. 7 recommendations, but complemented by other departmental workflow specific tests. To the best of our knowledge, this is the first time successful implementation of the IAEA Human Health Report Series No. 7 recommendations have been reported in the literature.