MR-linac MLC positioning QA by digitally stitching dual double-exposed films.

PURPOSE: The picket fence (PF) test is highly recommended for multi-leaf collimator (MLC) quality assurance. However, since the electronic portal imaging device (EPID) on the Elekta Unity only covers a small area, it is not feasible to perform the PF test for the entire MLC. Here, we propose a technique for the PF test by stitching two double-exposed films. METHODS: Two EBT3 films were used to encompass the entire MLC, with each one covering one half of the area. Two fields were employed to apply double exposure: a PF pattern consisting of 11 2 mm wide pickets and a 2.84 cm x 22 cm open field. The edges of the open field defined by the diaphragms were used to correct film rotation as well as align them horizontally. The PF pattern was also measured with the EPID where the pickets were used to align the films vertically. Individual leaf positions were detected on the merged film for quantitative analysis. Various MLC positioning errors were introduced to evaluate the technique's sensitivity. RESULTS: The merged films covered 72 leaf pairs properly (four leaf pairs on both sides were outside the treatment couch). With the EPID, the leaf positioning accuracy was -0.02 ± 0.07 mm (maximum: 0.29 mm) and the picket width variation was 0.00 ± 0.03 mm (maximum: 0.11 mm); with the films, the position accuracy and width variation were -0.03 ± 0.13 mm (maximum: 0.80 mm) and 0.00 ± 0.13 mm (maximum: 0.74 mm), respectively. The EPID was able to detect errors of 0.5 mm or above with submillimeter accuracy; the films were only able to detect errors > 1.0 mm. CONCLUSION: We developed a quantitative technique for the PF test on the Elekta Unity. The merged films covered nearly the entire MLC leaf banks. The technique exhibited clinically acceptable accuracy and sensitivity to MLC positioning errors.

Comparison of skin dose calculated by the dose tracking system (DTS) with a beam angular correction factor and that calculated by Monte-Carlo.

Skin dose is dependent on the incident beam angle and corrections are needed for accurate estimation of the risk of deterministic effects of the skin. Angular-correction factors (ACF) were calculated and incorporated into our skin-dose-tracking system (DTS) and the results compared to Monte-Carlo simulations for a neuro-interventional procedure. To obtain the ACF's, EGSnrc Monte-Carlo (MC) software was used to calculate the dose averaged over 0.5, 1, 2, 3, 4 and 5 mm depth into the entrance surface of a water phantom at the center of the field as a function of incident beam to skin angle from 90-10 degrees for beam field sizes from 5-15 cm and for beam energies from 60-120 kVp. These values were normalized to the incident primary dose to obtain the ACF. The angle of incidence at each mesh vertex in the beam on the surface of the DTS patient graphic was calculated as the complement of the angle between the normal vector and the vector of the intersecting ray from the tube focal spot; skin dose at that vertex was calculated using the corresponding ACF. The skin-dose values with angular correction were compared to those calculated using MC with a matching voxelized phantom. The results show the ACF decreases with decreasing incident angle and skin thickness, and increases with increasing field size and kVp. Good agreement was obtained between the skin dose calculated by the angular-corrected DTS and MC, while use of the ACF allows the real-time performance of the DTS to be maintained.

The effect of underlying bone on the beam angular correction in calculating the skin dose of the head in neuro-interventional imaging.

Skin dose depends on the surface shape, underlying tissue, beam energy, field size, and incident beam angle. These dependencies were determined in order to apply corrections in the skin-dose-tracking system (DTS) for accurate estimation of the risk of deterministic skin effects during fluoroscopically-guided neuro-interventional procedures. The primary-plus-scatter dose was calculated averaged over the skin thickness with underlying subcutaneous fat, and various thicknesses of skull bone on the surface of a cylindrical water phantom to simulate the head. The skin dose was calculated using EGSnrc Monte-Carlo (MC) software with 2×1010 incident photons and was normalized to the incident primary dose. Simulations were done for beam incident angles from 90 to 10 degrees with the skin surface, field sizes from 5 to 15 cm, bone thicknesses of 0, 1, 5, and 9 mm, and beam energies from 60 to 120 kVp. The results show the scatter-plus-primary to incident-primary dose ratio decreases with decreasing incident angle to the skin and with increasing thickness of underlying bone, while it increases with increasing field size and with increasing beam energy. The correction factor reduces the skin dose for angled rays and the reduction can be substantial for small angles of incidence, especially for angles below 50 degrees. For neuro-interventional procedures, the skin dose-area product (SDAP) with angular and bone correction is shown to be less than that without correction. The results of this study can be used to increase the accuracy of patient-skin-dose estimation for the head during fluoroscopic procedures.

A parametric fitting technique for rapid determination of a skin-dose correction factor for angle of beam incidence during image-guided endovascular procedures.

Skin dose is dependent on the incident beam angle and corrections are needed for accurate estimation of the risk of deterministic effects of the skin. To obtain the angular correction factors (ACF's), EGSnrc Monte Carlo (MC) software was used to calculate the skin dose as a function of incident x-ray beam angle at the center of the field for beam energies from 60 to 120 kVp, field sizes from 5 to 15 cm, and thicknesses of Cu beam filters from 0.2 to 0.5 mm. All MC simulations used 3×1010 incident photons. The dose was averaged over a 1 mm depth on the entrance surface of a 40×40 cm by 20 cm thick water phantom and was then normalized to the incident primary dose which was calculated using NIST mass energy absorption coefficients and by integrating over the beam energy spectrum. The Matlab tool, 'cftool', was used to fit these normalized dose values to power law equations as a function of incident beam angle, with coefficients that were fit to polynomials as a function of kVp. Separate fitting was done for different beam sizes and beam filters. The skin dose values calculated using the ACF determined from the fitted functional formulas agreed with that calculated by MC with a mean absolute percentage error (MAPE) less than 3% over the entire range of incident angles and kVp values. This fitting technique allows an ACF to be quickly determined for accurate skin dose calculation.