Purpose: To demonstrate a method to mitigate the step‐and‐shoot (S&S) IMRT overdose phenomenon for set of patients which initially failed IMRT QA process. Methods: Five S&S IMRT patients treated on ...Varian 2100C‐EX linacs with larger than +4.5% phantom IC point dose difference relative to planned dose were investigated. For every patient plan, five fractions (Fx) were delivered. Dynalog files were recorded and centiMU pulses from dose integrator board for every control point (CP) were counted using a BK1856D (B&K Precision Corp.) pulse counter (PC). The recorded MUs were imported to Pinnacle9.2 (Philips Healthcare), the 3D dose was recalculated and compared to the planned dose distribution. The initial plans were then modified by adding one starting segment to every beam. This segment consisted of an MLC aperture of 0.5x1cm2 hidden under the jaws with 1MU. For each patient, five Fx of the modified plan were delivered, recorded and the delivered 3D dose recalculated. Individual voxels' dose in high dose region, isocenter point dose, segment MUs, PTV D95 and OAR D2cc were compared between the original plan and the results of both deliveries. Results: The PC recorded total number showed the delivered beam MUs exceeded the plan by only ∼0.2%. For all initial plans, the first CP on average overshot by 0.6MU. It was determined that this on average translated to an increased dose to isocenter, PTV D95, bladder D2cc and high‐dose‐region voxels' dose by 2.66% (2.21%–3.12%), 2.06% (1.59%–2.48%), 2.57% (1.97%–3.11%), and 2.19% (1.75%–2.75%) respectively. All modified plans had inconsequential 0.05MU overshoot in first unmodified CP and 0.04%(−0.45% −0.55%), −0.07%(−0.44–‐0.48%), 0.08%(−0.28%–0.33%) and −0.07%(−0.31%–0.20%) differences for dosimetric parameters listed above. Conclusion: Our proposed method of attaching a special aperture 1MU CP to each beam was experimentally verified by using centiMU pulse counts to successfully eliminate the overdosage.
Purpose: To investigate the accuracy of carbon ion beam dose deposition simulated with the MCNPX Monte Carlo code. Method and Materials: The capability for heavy ion transport was added to the MCNPX ...code beginning in v2.6, but to date there has been no benchmarking for use in radiotherapy applications. We have simulated the interactions of 12C6+ ions in the energy range 130 – 430MeV/u using three versions of MCNPX, the 2.7b and 2.7c released versions, and a modified 2.7b_m version. The phantom consisted of a cubic PMMA container, 1.6mm thick, filled with water. The depth dose data obtained were normalized to their respective entrance doses and were compared to published experimental data. Results: We have evaluated the position and height of the Bragg peaks, curve shapes and exit doses. v2.7b did not generate projectile fragments while v2.7b_m did. While v2.7c calculates fragmentation in a manner similar to v2.7b_m, there are still substantial deviations from experimental data in the tail and in the region 6 mm proximal to Bragg peak. The positioning of the Bragg peaks and their heights were found to be acceptable. In the case of the 330MeV/u 12C6+ beam, for example, the calculated peak locations coincided with measurements within available experimental uncertainties, and relative heights differed by less than 10%. At 6mm beyond the peak the predicted dose was almost 50% lower than the measured value and the simulated curve's width was larger by 2mm at the FWHM. Conclusions: The discrepancy in the tail of the distributions indicates that the released transport codes insufficiently model target and/or projectile fragmentation and transport. The discrepancy in FWHM suggests slightly incorrect stopping powers which might be due to the water ionization potential employed in the code. However, this is contraindicated by the good positioning of the peaks and is being investigated.
Purpose: To compare volumetric modulated arc therapy (VMAT) treatment plans calculated with Eclipse 10 (RapidArc, Varian Medical Systems) and Pinnacle 9.2 (SmartArc, Philips Healthcare) to those ...computed with an independent verification system utilizing a DICOM‐RT framework Mobius3D (M3D 1.1, Mobius Medical Systems, LP). Methods: Mobius3D (M3D) utilizes standardized machine data, supplemented by custom scaled measured depth dose curves, output factors and off‐axis ratios, to create institution specific beam models as an independent dose validation check. A randomly selected set of clinical radiotherapy plans generated by two treatment planning systems (TPS) consisting of 26 RapidArc prostate plans and 29 SmartArc head and neck plans were exported to M3D. The requisite DICOM‐RT files sent to M3D included CT images, contoured structure sets, RT plan and RT dose. This data was used to recalculate 3D dose distributions using a collapsed cone algorithm in M3D. Dose distributions were compared to those calculated by the TPS 3D dose using a 3D gamma analysis with 3% global dose difference and 3 mm isodose point distance criteria. Additionally, TPS's and M3D's target coverage and regions of interests sparing were compared by calculating the mean dose percent difference for each structure. DVH from each system were also compared. Results: On average, M3D plans showed excellent 3D gamma passing rates agreement of (99.1%±1.1%) and (95.7%±2.2%) relative to RapidArc and SmartArc plans, respectively. Similarly, the M3D recalculated and RapidArc and SmartArc calculated mean target dose percent differences demonstrated small differences of (−0.3%±1.2%) and (− 1.2%±0.9%), respectively. Conclusion: Mobius3D enables a paradigm shift in clinical quality assurance practice by moving beyond single point dose and MU verification to a full 3D treatment plan QA check. The VMAT results obtained utilizing fast and fully automated M3D software showed that M3D is suitable for an independent check of RapidArc and SmartArc plans.
Purpose: The gamma index method, as currently implemented in all commercial QA software, calls for selection of a normalization point to evaluate agreement between two dose distributions. The ...implication of this is that there is an infinite number of possible solutions! Which one to pick? A unique and more relevant solution is obtained only if no normalization point is used.Methods and Materials: The set of test cases suggested by the AAPM TG1 19 were planned using Pinnacle 8.0m and delivered on a Varian 21EX linac for 6 and 18 MV photons. The recommended point and planar dose measurements were obtained using a Pinpoint ion chamber, EDR2 film and MatriXX. The gamma index method using typical 3%, 3 mm criteria with and without a normalization point was used to assess the agreement between calculated and delivered planar dose distributions. The analysis was extended to a set of data for clinically treated patients. Results: The comparison with the TG119 benchmark data showed that all point dose and planar measurements for 6 MV were within the published range. Similar results, although without published data to compare with, were obtained for 18 MV as well. For all complex tests, the percentage of points passing the gamma criteria of 3%, 3 mm was (95.8±1.6)% and (95.6±1.0)% for 6 MV and 18 MV, respectively. Without a normalization point, however, the same gamma analysis fell to (20.7±6.7)% and (13.9±4.0)% for 6 MV and 18 MV, respectively. The clinical data set showed the same trend, with the gamma passing rate declining from (98.9±0.7)% to (33.4±13.1)%. Conclusion: The gamma index method provides a unique answer for gamma passing rate only without normalizing dose distributions to any particular point. The common gamma criteria of 3%, 3 mm, however, is a very poor metric in that case.
Purpose: Accurate deformation vector field (DVF) acting on organ surface is vital for surface dose summation of deformable cavitary organs in adaptive radiotherapy. The aim of this study is to ...develop a contour‐guided deformable image registration (DIR) scheme to establish accurate DVF on an organ surface. Methods: A critical cavitary organ, for example bladder, is delineated by a clinician on fraction 1 and fraction 2 CT images used for cervical high‐dose rate (HDR) brachytherapy planning. Both CT sets are preprocessed by creating binary images by setting one inside of the organ under evaluation and zero outside. The organ surface is subsequently extracted and discretized with a triangular mesh. The DVF on designed vertices is estimated through an inverse‐consistent demons‐DIR between the paired binary images. Due to shortcomings in DIR algorithms, the DVF derived through DIR always has limited accuracy. To increase the DVF accuracy, an iterative closest point (ICP) algorithm is adopted in this study to match corresponding vertices of the paired surface meshes through an affine registration. The accuracy of the resulting registration is then evaluated. Results: Using contours drawn by a clinician as ground truth we evaluated DVF accuracy through organ surface‐to‐surface distance (SSD), defined as the shortest Euclidean distance between the vertices on the target and the deformed surfaces. In our evaluation case, the maximum SSD is 27.8mm before registration, decreases to 15.0mm following demons registration, and 8.33mm after further affine alignment. The 95 percentile SSD decreases from 23.9mm to 10.8mm and to 3.8mm, respectively. Conclusion: The proposed contour‐guided DIR scheme substantially improves the accuracy of DVF on organ surfaces. Results from the evaluation case demonstrate that the proposed contour‐guided DIR scheme can provide accurate DVF making it a useful approach to be applied in adaptive radiotherapy.
Purpose: To evaluate the Eclipse Acuros XB (AXB) dose calculation engine for Stereotactic Ablative Radiotherapy (SAbR) of the thoracic spine using step‐and‐shoot IMRT and RapidArc. Methods: The ...accuracy of the AXB dose calculation engine was first verified by comparing with film and ion chamber measurements for open beams with field sizes from 2×2cm2‐10×10cm2, delivered to a heterogeneous phantom of a slab‐geometry. Acquired images of an anthropomorphic thoracic phantom, composed of tissue equivalent lungs, vertebrae, and soft tissue, were then used to create step‐and‐shoot IMRT and RapidArc treatment plans. Point dose measurements in vertebral bone and spinal cord were compared to Acuros calculations. Finally, step‐and‐shoot IMRT and dual‐arc RapidArc plans were created using the Analytical Anisotropic Algorithm (AAA) for five patients with thoracic spinal metastases following our institutional protocol. All treatment plans were then recalculated with AXB while keeping beam parameters the same. Clinically significant dosimetric parameters, such as GTV coverage and spinal cord dose were compared. Results: Excellent agreement was achieved between point dose and planar dose measurements and the AXB calculations in the heterogeneous slab and anthropomorphic phantoms. Patient IMRT and RapidArc plans had GTV D90 and mean dose values 0.2%–2.6% and 0.2%–2.1% lower, respectively, for AXB calculations compared with AAA. The maximum cord dose was also lower for AXB compared with AAA (−2.7%±2.4% for IMRT; −2.6%±1.2% for RapidArc). AXB dose calculations were 4−6 times faster than AAA for RapidArc plans, but 7−9 times slower for IMRT plans. Conclusion: AXB calculations were shown to be dosimetrically accurate for heterogeneous media. This work demonstrates that AXB calculates a reduced dose compared with AAA in both the target and spinal cord for thoracic spine SAbR. The dosimetric differences should be considered when using AAA for thoracic spine treatment planning.
Purpose:
To put forth an innovative clinical paradigm for weekly chart checking so that treatment status is periodically checked accurately and efficiently. This study also aims to help optimize the ...chart checking clinical workflow in a busy radiation therapy clinic.
Methods:
It is mandated by the Texas Administrative code to check patient charts of radiation therapy once a week or every five fractions, however it varies drastically among institutions in terms of when and how it is done. Some do it every day, but a lot of efforts are wasted on opening ineligible charts; some do it on a fixed day but the distribution of intervals between subsequent checks is not optimal. To establish an optimal chart checking procedure, a new paradigm was developed to achieve 1) charts are checked more accurately and more efficiently; 2) charts are checked on optimal days without any miss; 3) workload is evened out throughout a week when multiple physicists are involved. All active charts will be accessed by querying the R&V system. Priority is assigned to each chart based on the number of days before the next due date followed by sorting and workload distribution steps. New charts are also taken into account when distributing the workload so it is reasonably even throughout the week.
Results:
Our clinical workflow became more streamlined and smooth. In addition, charts get checked in a more timely fashion so that errors would get caught earlier should they occur.
Conclusion:
We developed a new weekly chart checking diagram. It helps physicists check charts in a timely manner, saves their time in busy clinics, and consequently reduces possible errors.
