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Radiochromic film (RCF) dosimetry is usually based on densitometric methods which use an analyzing light source of a fixed or a broad spectrum of wavelengths. These methods have not exploited the sensitivity of the dose response of the RCF otherwise attainable by using a light source with wavelengths peaked at the two absorption peaks in the absorption spectrum of the RCF. A new algorithm of dual-peak dose measurement for the RCF has been proposed in this paper to make use of these dual absorption peaks to achieve the maximum attainable sensitivity. This technique relies on the measurement of the transmittance of the RCF at the wavelength of the major and minor absorption peaks, respectively. The dual-peak dose measurement is accomplished with the aid of a novel spectral microdensitometer developed in our Institute. The microdensitometer utilizes a monochromator to provide a light source of which the wavelength can be matched precisely to the wavelength of the absorption peaks of the RCF. The doses obtained at these wavelengths are fed into a weighted objective function and an optimum dose is searched by minimizing the objective function to give the best estimate of the dose deposited on the film. An initial test shows that there is a good agreement between the estimated and actual dose deposited; and the maximum discrepancy was found to be less than 1%.

Purpose: To develop a new algorithm for multiple channel radiochromic film calibration and to evaluate the accuracy of the algorithm with virtual simulated film images. Methods: The proposed calibration method was based on least-square optimization of discrepancies between measured and estimated optical density of the three colour channels, with the inclusion of the weighted dose gradient as penalty to this minimization problem. The regularization parameter λ was selected using the L-curve method to reduce the L2-norm of the dose gradient while preserving the data consistency. The situation of λ = 0 represented no regularization and the least-square solution would be obtained. The algorithm was tested with 4 virtual film images which were produced from dose planes exported from the planning system. Zero-mean Gaussian noise were added to simulate the real situation. The resultant dose maps calculated by both least-square method and regularization method were then compared with the original dose planes. Results: Comprehensive study of the 4 simulated cases with the L-curve method yielded an optimal regularization parameters of 2×10−8. The dose planes generated by least-square method showed significant noise retained when compared with that from the regularization method. In performing Gamma Index analysis with tolerance of 1% and 1 mm, it was observed that in the least-square solution, failed points exist everywhere on the dose plane, while only a small amount in the sharp dose gradient region of the regularized solution. It was also noted that the number of failed point in the least-square solution can be 20 to 150 times more than that in the regularized solution. Conclusion: The calibration algorithm proposed in this study successfully recovered the original dose plane from the noise contaminated film image with the regularization method. Regularization should be a better option to the least-square one in solving the radiochromic film calibration problem.

Purpose: To develop a formulation for 4D treatment planning for a tumour tracking volumetric modulated arc therapy treatment (VMAT) plan for lung cancer. Methods: A VMAT plan was optimized based on a reference phase of the 4DCT of a lung cancer patient. The PTV was generated from the GTV of the reference phase. The collimator angle was set to 90 degrees such that the MLC travels along superior-inferior direction which is the main component of movement of a lung tumour. Then, each control point of the VMAT plan was assigned to a particular phase of the 4DCT in chronological order. The MLC positions of each control point were shifted according to the position of the tumour centroid of its assigned phase to form a tumour tracking VMAT plan. The control points of the same phase were grouped to form a pseudo VMAT plan for that particular phase. Dose calculation was performed for each pseudo VMAT plan on the corresponding phase of the 4DCT. The CTs of all phases were registered to the reference phase CT according to the displacement of the tumour centroid. The individual dose distributions of the pseudo VMAT plans were summed up and displayed on the reference phase of the 4DCT. A control VMAT plan was optimized based on a PTV generated from the ITV of all phases and compared with the tumour tracking VMAT plan. Results: Both plans achieved >95% volume coverage at the prescription dose level (96% for the tumour tracking plan and 97% for the control plan). But the normal lung volume irradiated at the prescription dose level was 39% less for the tumour tracking plan than the control plan. Conclusion: A formulation of 4D treatment planning for tumour tracking VMAT plans for lung cancer was developed.

Purpose: To examine the feasibility of using radiochromic film with advanced calibration techniques for absolute and relative dose measurement in patient-specific SBRT QA. Methods: Ten SBRT cases were randomly selected. The treatment plans were mapped to a cylindrical Perspex phantom for QA measurement. A radiochromic film of 8”x10” was cut into a large piece of 20×21cm2 and 2 small pieces of 4×4cm2 each. The large piece was inserted in the mid-plane of the phantom which was irradiated with the treatment beam. One of the small pieces was irradiated with known dose. All irradiated films were allowed to stabilize for 24 hours before scanning. The protocol of single-scan with recalibration was followed and the 2 small films were used to adjust the calibration curve. The treatment film was calibrated with the dose gradient regularization method by an in-house developed MATLab program. The calculated dose planes were then exported to RIT113V4.4. Both treatment films and planned dose planes were normalized to the corresponding prescribed fractional dose for analysis. Results: The analysis was performed by normalizing both the treatment film and the planned dose planes to the prescribed fractional dose. Isodose line comparison was performed to visually check the alignment of the isodose level of steep dose gradient regions. Good agreements were observed in all 10 cases. Gamma index analysis with tolerance of 3% and 2mm was also carried out for quantitative analysis. All 10 cases were reported with a gamma index passing percentage of at least 98.9%, which were higher than the institute’s passing criteria of over 95%. Conclusion: By employing recalibration and regularization-based calibration techniques, radiochromic film can measure both the absolute and relative dose of SBRT treatment plans. This study suggested that radiochromic film, with proper application of advanced calibration methods, can be a promising tool for patient-specific SBRT QA.

Purpose: To develop a method using continuously triggered kV images acquired during gated RapidArc to reconstruct a gated CBCT for geometric accuracy verification of a moving target. Methods: A gated RapidArc treatment was planned to be delivered to a commercial breathing simulation phantom embedded with metallic markers using a Varian TrueBeam™ linear accelerator. The phantom was setup to the treatment position using 2D/2D match followed by marker match IGRT procedure. Continuously triggered on‐board‐imager kV images were taken immediately before each MV beam on cycle at various gantry angles during the gated RapidArc delivery. The kV images obtained were imported into an in‐house developed software to reconstruct a gated CBCT corresponding to the beam on triggering phase. The gated CBCT was then imported into a commercial treatment planning system. The geometric displacement of the target in the gated CBCT from its counterpart in the planning CT was determined through image registration based on marker alignment. The entire process was repeated 10 times using a variety of target movement magnitudes in the range of 1cm to 3cm and periods in the range of 4s to 8s. Results: The magnitude of the translational shifts of the target at the beam on triggering phase was 0.18+/−0.11mm in left/right direction, 0.38+/−0.15mm in anterior/posterior direction and 0.56+/−0.19mm in the superior/inferior direction. The composite 3D translational shift was 0.74+/−0.12mm. Conclusion: Gated CBCT reconstruction could be a practical and efficient way of utilizing continuously triggered kV images acquired during gated RapidArc treatments to verify geometric accuracy of a moving target.

Purpose: To compare the machine performance including the MU delivery, Gantry Angle trajectory and Multi-Leaf Collimator (MLC) leaf motion of RapidArc delivery based on trajectory log analysis. Method: Five RapidArc cases were selected for the study. Each case was delivered at Varian Unique, iX23 and TrueBeam Linear Accelerator (LINAC) system. The system log files were obtained after their corresponding field measurements. During measurements, an independent inclinometer was mounted on the machine to verify the gantry rotation dynamically. In-house software was developed to extract and reconstruct the data into trajectory plots for comparison. Another in-house software was used to compare the Integrated Transient Fluence Map (ITFM) to check the accuracy of MLC leaf position and dose index. Result: For all selected cases delivered in the Unique, iX23 and TrueBeam system, the typical deviations of gantry angle and MU were found to be (0.6+/−0.1°, 0.0+/−0.2 MU), (0.4+/−0.1°, 0.0+/−0.1 MU) and (0.0+/−0.1°, 0.0+/−0.0 MU) respectively. Their angular trajectories of the independent inclinometer agreed well with the digital readout of gantry angle. The trajectory plots of dose index against gantry angle were in a good agreement with their plans. The reconstructed ITFM showed no observable difference when compared with the planned one, and their typical correlation coefficients were very close to unity. This indicated accurate MU delivery and MLC leaf position during the delivery. The higher accuracy of RapidArc delivery was achieved by the digital control system (TrueBeam). Conclusion: The analysis of trajectory log is presented in the form of trajectory plot and ITFM, which are the powerful tools to check the machine performance of RapidArc delivery. For the inter-machine comparison, the integrated and digital control LINAC (TrueBeam) delivers the RapidArc treatment more precisely compared to its counterparts (iX23 and Unique).

Purpose: To evaluate the surface dose characteristics of the flattened and flattening filter free (FFF) megavoltage photon beam with radiochromic film and compare the Result: with that from ion chamber. Methods: A Varian TrueBeam linear accelerator capable to produce both flattened and FFF photon at 6 MV and 10 MV was used in this study. Surface dose measurement was performed with both Gafchromic EBT3 radiochromic film and PTW23342 parallel plate chamber. Build‐up dose was measured by adding different thickness of solid water slabs on the top of the measuring devices, at 100 cm SSD with field size of 2×2, 5×5, 10×10 and 20×20 cm2. The measurement data were normalized to PDD at 5 cm depth which was obtained from commissioning data. Percentage surface dose was calculated by extrapolating the PDD from 2 mm build‐up to the surface. Results: Surface doses of the FFF photon were 5 – 10% higher than that of the flattened photon at field size smaller than 10×10 cm2. In film measurement, the surface doses for 6X flattened and FFF photon increased from 12.0% to 30.6% and 20.5% to 36.1% respectively, when field size increased from 2×2 to 20×20 cm2; for 10X flattened and FFF photon, they were increased from 6.1% to 24.2% and 11.2% to 23.5% respectively. Surface dose obtained in chamber measurement showed 2 – 6% higher than the corresponding measurement with film, but with a similar trend with that in the film measurement. Conclusion: FFF photon has a higher surface dose than flattened photon at field size smaller than 10×10 cm2. Surface dose measured from chamber is found to be higher than that from film, which is probably due to the chamber design. Nonetheless, the trend of surface dose increases with field size is consistent between the two measuring devices.