AbstractCurrent available secondary dose calculation software for Gamma Knife radiosurgery falls short in situations where the target is shallow in depth or when the patient is positioned with a gamma angle other than 90°. In this work, we evaluate a new secondary calculation software which utilizes an innovative method to handle nonstandard gamma angles and image thresholding to render the skull for dose calculation. 800 treatment targets previously treated with our GammaKnife Icon system were imported from our treatment planning system (GammaPlan 11.0.3) and a secondary dose calculation was conducted. The agreement between the new calculations and the TPS were recorded and compared to the original secondary dose calculation agreement with the TPS using a Wilcoxon Signed Rank Test. Further comparisons using a Mann‐Whitney test were made for targets treated at a 90° gamma angle against those treated with either a 70 or 110 gamma angle for both the new and commercial secondary dose calculation systems. Correlations between dose deviations from the treatment planning system against average target depth were evaluated using a Kendall’s Tau correlation test for both programs. The Wilcoxon Signed Rank Test indicated a significant difference in the agreement between the two secondary calculations and the TPS, with a P‐value < 0.0001. With respect to patients treated at nonstandard gamma angles, the new software was largely independent of patient setup, while the commercial software showed a significant dependence (P‐value < 0.0001). The new secondary dose calculation software showed a moderate correlation with calculation depth, while the commercial software showed a weak correlation (Tau = −.322 and Tau = −.217 respectively). Overall, the new secondary software has better agreement with the TPS than the commercially available secondary calculation software over a range of diverse treatment geometries.

The accuracy of MRI-based brachytherapy with titanium tandem and ovoid applicators for cervical cancer is affected by metal artifacts in imaging. We hypothesized that orthopedic metal artifact reduction (O-MAR) MRI may reduce artifact associated with the titanium tandem, and that variations in applicator reconstruction due to artifact can affect brachytherapy dosimetry. We performed a prospective clinical study to test these hypotheses. We enrolled 12 patients receiving brachytherapy for cervical cancer on a prospective institutional review board-approved imaging protocol. We intended for each patient to receive six MRI-based brachytherapy treatments interdigitated with intensity modulated radiation. After tandem and ovoid implantation, patients received MRI simulation on a 1.5 T scanner with T2-weighted (T2W), diffusion weighted imaging (DWI), proton density weighted imaging (PDW), and PDW with O-MAR. The O-MAR method incorporated slice-selective excitation pulses with large bandwidth, view angle tilting, and "weak" slice encoding for metal artifact correction with 7 z-phase encodes. The tumor was contoured on the DWI, bladder, and rectum on T2W, and the tandem was reconstructed based on the PDW. The primary endpoint was reduction in the dimensions of the "bloom" artifact associated with the tip of the titanium tandem as measured in the PDW and O-MAR images. We evaluated dosimetry effects of variations in reconstruction due to the artifact by creating mock brachytherapy plans in which the tandem and dwell positions were shifted distally by a distance equal to half the improvement in the artifact length in O-MAR compared to PDW. Values are reported as mean and range. Differences were evaluated using the paired t-test. We acquired 62 MRI simulation scans with O-MAR, out of 72 brachytherapy fractions. The tandem tip "bloom" artifact mean length was 6.7 mm (3.6-8.9) without O-MAR and 3.4 (1.4-6.7) with O-MAR (P < 0.01), and the artifact mean width was 7.5 mm (4.4-9.4) without O-MAR and 3.7 mm (1.8-5.9) with O-MAR (P < 0.01). The mock brachytherapy plan representing potential variation in tandem reconstruction was generated with a mean shift of 1.7 mm (0.6-3.0), and resulted in a mean decrease of 1.3% (-4.5 to 3.2) in point A dose (P < 0.01), 4.4% (-1.9 to 18) in mean tumor dose (P < 0.01), 1.5% (-8.9 to 7.4) in tumor D100% (P < 0.01), and 2.8% (-1.4 to 8.5) in tumor D90% (P < 0.01) compared to baseline plans. Additionally, the maximum bladder dose was not significantly changed (P = 0.08), but there was a mean decrease of 0.5% (-14 to 4.0) in mean bladder dose (P = 0.01), 1.2% (0.3 to 7.0) in maximum rectum dose (P < 0.01), and 1.1% (-2.9 to 2.9) in mean rectum dose (P < 0.01) compared to baseline plans. O-MAR decreases the artifact associated with the titanium tandem. Variability in reconstruction due to the artifact at the tip of a tandem may lead to a mean tumor dose of 4.4% less than expected, highlighting the importance of accurate applicator reconstruction in brachytherapy treatment planning.

Purpose:To implement image‐guided proton therapy (IGPT) based on daily proton dose distribution.Methods:Unlike x‐ray therapy, simple alignment based on anatomy cannot ensure proper dose coverage in proton therapy. Anatomy changes along the beam path may lead to underdosing the target, or overdosing the organ‐at‐risk (OAR). With an in‐room mobile computed tomography (CT) system, we are developing a dose‐based IGPT software tool that allows patient positioning and treatment adaption based on daily dose distributions. During an IGPT treatment, daily CT images are acquired in treatment position. After initial positioning based on rigid image registration, proton dose distribution is calculated on daily CT images. The target and OARs are automatically delineated via deformable image registration. Dose distributions are evaluated to decide if repositioning or plan adaptation is necessary in order to achieve proper coverage of the target and sparing of OARs. Besides online dose‐based image guidance, the software tool can also map daily treatment doses to the treatment planning CT images for offline adaptive treatment.Results:An in‐room helical CT system is commissioned for IGPT purposes. It produces accurate CT numbers that allow proton dose calculation. GPU‐based deformable image registration algorithms are developed and evaluated for automatic ROI‐delineation and dose mapping. The online and offline IGPT functionalities are evaluated with daily CT images of the proton patients.Conclusion:The online and offline IGPT software tool may improve the safety and quality of proton treatment by allowing dose‐based IGPT and adaptive proton treatments.Research is partially supported by Mevion Medical Systems.

Purpose: To describe the clinical use of a Linear Accelerator (Linac) DailyQA system with only EPID and OBI. To assess the reliability over an 18-month period and improve the robustness of this system based on QA failure analysis. Methods: A DailyQA solution utilizing an in-house designed phantom, combined EPID and OBI image acquisitions, and a web-based data analysis and reporting system was commissioned and used in our clinic to measure geometric, dosimetry and imaging components of a Varian Truebeam Linac. During an 18-month period (335 working days), the Daily QA results, including the output constancy, beam flatness and symmetry, uniformity, TPR20/10, MV and KV imaging quality, were collected and analyzed. For output constancy measurement, an independent monthly QA system with an ionization chamber (IC) and annual/incidental TG51 measurements with ADCL IC were performed and cross-compared to Daily QA system. Thorough analyses were performed on the recorded QA failures to evaluate the machine performance, optimize the data analysis algorithm, adjust the tolerance setting and improve the training procedure to prevent future failures. Results: A clinical workflow including beam delivery, data analysis, QA report generation and physics approval was established and optimized to suit daily clinical operation. The output tests over the 335 working day period cross-correlated with the monthly QA system within 1.3% and TG51 results within 1%. QA passed with one attempt on 236 days out of 335 days. Based on the QA failures analysis, the Gamma criteria is revised from (1%, 1mm) to (2%, 1mm) considering both QA accuracy and efficiency. Data analysis algorithm is improved to handle multiple entries for a repeating test. Conclusion: We described our 18-month clinical experience on a novel DailyQA system using only EPID and OBI. The long term data presented demonstrated the system is suitable and reliable for Linac daily QA.

Purpose: We retrospectively evaluate the dosimetric impact of a 3.5% range uncertainty on CTV coverage and normal organ toxicity for a cohort of brain patients. Methods: Twenty treatment plans involving 20 brain cancer patients treated with Mevions S250 were reviewed. Forty uncertain plans were made by changing the ranges in original plans by ±3.5% while keeping all devices unchanged. Fidelity to the original plans was evaluated with gamma index. Changes in generalized equivalent uniform dose (gEUD) were reported for the following structures: CTV coverage, brainstem, optic chiasm, and optic nerves. Comparisons were made by plotting the relevant endpoints from the uncertain plans as a function of the same endpoints from the original clinical plan. Results: Gamma‐index analysis resulted in a 50% pass rate of the uncertain plans using a 90% passing rate and 3%/3mm criterion. A 9.5% decrease in the slope of gEUD plot for the CTV was observed for the 3.5% downward range shift. However, the change in slope did not result in a gEUD change greater than 1.1% for the CTV. The slopes of the gEUD plots for normal structures increased by 3.1% 3.9% 2.4% and 0.2% for the chiasm, brainstem, left optic nerve and right optic nerve respectively. The maximum deviation from the gEUD of the clinical plan for normal structures was: 64% in the chiasm, 31% for the brainstem, and 19% for both optic nerves. Conclusion: A retrospective review shows moderate radiobiological impact of range uncertainty in passively scattered proton therapy with sporadic catastrophe. The linear regression analysis on the statistical data indicates a systematic deviation of gEUD from treatment planning in the light of range uncertainty.