Clinical assessment of a novel machine-learning automated contouring tool for radiotherapy planning.

Contouring has become an increasingly important aspect of radiotherapy due to inverse planning. Several studies have suggested that the clinical implementation of automated contouring tools can reduce inter-observer variation while increasing contouring efficiency, thereby improving the quality of radiotherapy treatment and reducing the time between simulation and treatment. In this study, a novel, commercial automated contouring tool based on machine learning, the AI-Rad Companion Organs RT™ (AI-Rad) software (Version VA31) (Siemens Healthineers, Munich, Germany), was assessed against both manually delineated contours and another commercially available automated contouring software, Varian Smart Segmentation™ (SS) (Version 16.0) (Varian, Palo Alto, CA, United States). The quality of contours generated by AI-Rad in Head and Neck (H&N), Thorax, Breast, Male Pelvis (Pelvis_M), and Female Pelvis (Pevis_F) anatomical areas was evaluated both quantitatively and qualitatively using several metrics. A timing analysis was subsequently performed to explore potential time savings achieved by AI-Rad. Results showed that most automated contours generated by AI-Rad were not only clinically acceptable and required minimal editing, but also superior in quality to contours generated by SS in multiple structures. In addition, timing analysis favored AI-Rad over manual contouring, indicating the largest time saving (753s per patient) in the Thorax area. AI-Rad was concluded to be a promising automated contouring solution that generated clinically acceptable contours and achieved time savings, thereby greatly benefiting the radiotherapy process.

Varian ethos online adaptive radiotherapy for prostate cancer: Early results of contouring accuracy, treatment plan quality, and treatment time.

The Varian Ethos system allows for online adaptive treatments through the utilization of artificial intelligence (AI) and deformable image registration which automates large parts of the anatomical contouring and plan optimization process. In this study, treatments of intact prostate and prostate bed, with and without nodes, were simulated for 182 online adaptive fractions, and then a further 184 clinical fractions were delivered on the Ethos system. Frequency and magnitude of contour edits were recorded, as well as a range of plan quality metrics. From the fractions analyzed, 11% of AI generated contours, known as influencer contours, required no change, and 81% required minor edits in any given fraction. The frequency of target and noninfluencer organs at risk (OAR) contour editing varied substantially between different targets and noninfluencer OARs, although across all targets 72% of cases required no edits. The adaptive plan was the preference in 95% of fractions. The adaptive plan met more goals than the scheduled plan in 78% of fractions, while in 15% of fractions the number of goals met was the same. The online adaptive recontouring and replanning process was carried out in 19 min on average. Significant improvements in dosimetry are possible with the Ethos online adaptive system in prostate radiotherapy.

Validation of the preconfigured Varian Ethos Acuros XB Beam Model for treatment planning dose calculations: A dosimetric study.

Varian (Palo Alto, California, United States) recently released an online adaptation treatment platform, Ethos, which has introduced a new Dose Preview and Automated Plan Generation module despite sharing identical beam data with the existing Halcyon linac. The module incorporates a preconfigured beam model and the Acuros XB algorithm (Ethos AXB model) to generate final dose calculations from an initial fluence optimization. In this study, we comprehensively validated the accuracy of the Ethos AXB model by comparing it against the Halcyon AXB model, the Halcyon Anisotropic Analytical Algorithm (AAA) model, and measurements acquired on an Ethos linac. Results indicated that the Ethos AXB model demonstrated a comparable if not superior dosimetric accuracy to the Halcyon AXB model in basic and complex calculations, and at the same time its dosimetric accuracy in modulated and heterogeneous plans was better than that of the Halcyon AAA model. Despite the fact that the same algorithm was utilized, the Ethos AXB model and the Halcyon AXB model still exhibited variations across a range of tests, although these variations were predominantly insignificant in the clinical environment. The accuracy of the Ethos AXB model has been successfully verified in this study and is considered appropriate for the current clinical scope. On the basis of this study, clinical physicists can perform a data validation instead of a full data commissioning when implementing the Ethos system, thereby adopting a more efficient approach for Ethos installation.

A multi-institutional evaluation of machine performance check system on treatment beam output and symmetry using statistical process control.

BACKGROUND: The automated and integrated machine performance check (MPC) tool was verified against independent detectors to evaluate its beam uniformity and output detection abilities to consider it suitable for daily quality assurance (QA). METHODS: Measurements were carried out on six linear accelerators (each located at six individual sites) using clinically available photon and electron energies for a period up to 12 months (n = 350). Daily constancy checks on beam symmetry and output were compared against independent devices such as the SNC Daily QA 3, PTW Farmer ionization chamber, and SNC field size QA phantom. MPC uniformity detection of beam symmetry adjustments was also assessed. Sensitivity of symmetry and output measurements were assessed using statistical process control (SPC) methods to derive tolerances for daily machine QA and baseline resets to account for drifts in output readings. I-charts were used to evaluate systematic and nonsystematic trends to improve error detection capabilities based on calculated upper and lower control levels (UCL/LCL) derived using standard deviations from the mean dataset. RESULTS: This study investigated the vendor's method of uniformity detection. Calculated mean uniformity variations were within ± 0.5% of Daily QA 3 vertical symmetry measurements. Mean MPC output variations were within ± 1.5% of Daily QA 3 and ±0.5% of Farmer ionization chamber detected variations. SPC calculated UCL values were a measure of change observed in the output detected for both MPC and Daily QA 3. CONCLUSIONS: Machine performance check was verified as a daily quality assurance tool to check machine output and symmetry while assessing against an independent detector on a weekly basis. MPC output detection can be improved by regular SPC-based trend analysis to measure drifts in the inherent device and control systematic and random variations thereby increasing confidence in its capabilities as a QA device. A 3-monthly MPC calibration assessment was recommended based on SPC capability and acceptability calculations.

Safety and Efficiency Analysis of Operational Decision-Making During Cone Beam Computed Tomography-Based Online Adaptive Radiation Therapy.

PURPOSE: Cone beam computed tomography (CBCT)-based online adaptive radiation therapy (ART) is especially beneficial for patients with large interfractional anatomic changes. However, treatment planning and review decisions need to be made at the treatment console in real-time and may be delegated to clinical staff whose conventional scope of practice does not include making such decisions. Therefore, implementation can create new safety risks and inefficiencies. The objective of this work is to systematically analyze the safety and efficiency implications of human decision-making during the treatment session for CBCT-based online ART. METHODS AND MATERIALS: The analysis was performed by applying the Systems-Theoretical Process Analysis technique and its extension for human decision-making. Four centers of different CBCT-based online ART practice models comprised the analysis team. RESULTS: The general radiation therapy control structure was refined to model the interactions between routine treatment delivery staff and in-person or remote support staff. The treatment delivery staff perform 6 key control actions. Eighteen undesirable states of those control actions were identified as affecting safety and/or efficiency. In turn, 97 hazardous clinical scenarios were identified, with the control action "prepare and position patient" having the least number of scenarios and "delineate/edit influencer and target structures" having the most. Five of these are specific to either in-person or remote support during the treatment session, and 12 arise from staff support in general. CONCLUSIONS: An optimally safe and efficient online ART program should require little to no support staff at the treatment console to reduce staff coordination. Uptraining of the staff already at the treatment console is needed to achieve this goal. Beyond the essential knowledge and skills such as contour editing and the selection of an optimal plan, uptraining should also target the specific cognitive biases identified in this work and the cognitive strategies to overcome these biases. Additionally, technological and organizational changes are necessary.

Intrafraction Motion and Margin Assessment for Ethos Online Adaptive Radiotherapy Treatments of the Prostate and Seminal Vesicles.

Purpose: Online adaptive radiation therapy (OART) uses daily imaging to identify changes in the patient's anatomy and generate a new treatment plan adapted to these changes for each fraction. The aim of this study was to determine the intrafraction motion and planning target volume (PTV) margins required for an OART workflow on the Varian Ethos system. Methods and Materials: Sixty-five fractions from 13 previously treated OART patients were analyzed for this retrospective study. The prostate and seminal vesicles were contoured by a radiation oncologist on 2 cone beam computed tomography scans (CBCT) for each fraction, the initial CBCT at the start of the treatment session, and the verification CBCT immediately before beam-on. In part 1 of the study, PTVs of different sizes were defined on the initial CBCT, and the geometric overlap with the clinical target volume (CTV) on the verification CBCT was used to determine the optimal OART margin. This was performed with and without a patient realignment shift by registering the verification CBCT to the initial CBCT. In part 2 of the study, the margins determined in part 1 were used for simulated Ethos OART treatments on all 65 fractions. The resultant coverage to the CTV on the verification CBCT, was compared with an image guided radiation therapy (IGRT) workflow with 7-mm margins. Results: Part 1 of the study found, if a verification CBCT and shift is performed, a 4-mm margin on the prostate and 5 mm on the seminal vesicles resulted in 95% of the CTV covered by the PTV in >90% of fractions, and 98% of the CTV covered by the PTV in >80% of fractions. Part 2 of the study found when these margins were used in an Ethos OART workflow, they resulted in CTV coverage that was superior to an IGRT workflow with 7-mm margins. Conclusions: A 4mm prostate margin and 5-mm seminal vesicles margin in an OART workflow with verification imaging are adequate to ensure coverage on the Varian Ethos system. Larger margins may be required if using an OART workflow without verification imaging.

Dosimetry implications of VMAT delivery angles and intra-fraction motion for SBRT prostate treatments.

For a cohort of retrospectively treated patients with prostate carcinoma using a Stereotactic body radiation therapy (SBRT) technique and with implanted localization fiducials, this study determined whether there were any correlations between theoretical fiducial visibility using intra-fraction megavoltage imaging and intra-fraction motion induced dosimetric effects. Treatment planning data for 20 retrospective patients that used a SBRT prostate technique were investigated in this study. An in-house script was created that split each of the 360-degree volumetric modulated arc therapy arcs into 12 sectors, with each sector being 30 degrees. The script created a total of 24 sectors for each SBRT plan ranging from 180 to 210 degrees to 180 to 150 degrees. Resulting data was assessed to determine whether there was dosimetric impact from intra-fractional prostate motion and if it correlated with the theoretical fiducial visibility. Forty percent of the 4 highest CTV D98% mean dose differences occurred within the angles of 240 and 270 degrees and 25% occurred between the angles of 90 and 120 degrees. The 4 highest average percentage differences in PTV D98% coverage during angular sectors of 270 to 240 degrees, 90 to 120 degrees, 240 to 270 degrees and 60 to 90 degrees were -1.19%, -1.14%, -1.10%, and 1.01% respectively. It was also the case for the PTV D95%in sectors 90 to 120 degrees, 240 to 270 degrees, 270 to 240 degrees and 270 to 300 degrees, which reduced by -0.97%, -0.93%, -0.92%, and -0.82% respectively. From the 4 highest rectal dose differences in V32Gy and V18Gy, it was found that 50% of the highest rectum V32Gyand V18Gy dose increases occurred between the angles of 90 and 120 degrees and 37.5% occurred between the angles of 240 and 270 degrees. The MU for each sector showed that 240_270, 240_210, 270_240, and 120_90 had the highest average MU with 150.8, 134.6, 129.2, and 124.3 respectively. This study demonstrated that there is a high correlation between dosimetric impact of intra-fractional motion and the theoretical fiducial visibility. As a result, modification to treatment plans to enable fiducial visibility at all angular sectors throughout a treatment may not be required. Further sector analysis tests are required to develop patient specific megavoltage imaging gantry angles for SBRT prostate patients.

Measured vs simulated portal images for low MU fields on three accelerator types: possible consequences for 2D portal dosimetry.

PURPOSE: As external beam treatment plans become more dynamic and the dose to normal tissue is further constrained, treatments may consist of a larger number of beams, each delivering smaller doses (or monitor units, MU), in, e.g., volumetric modulated arc therapy (VMAT). Electronic portal imaging devices (EPID) may be used to verify external beam treatments on integrated fractions as well as in a more time dependent manner such as field by field. For treatment verification performed during a fraction (e.g., individual fields or VMAT control points), the lower limit of EPID measurement capability becomes important. The authors quantified the signal and timing accuracy of EPID images for low MU intensity modulated radiotherapy (IMRT) and conformal fields. METHODS: EPID images were collected from three different vendor's accelerators for low MU fields and compared to expected images. Simulations were performed to replicate the EPID acquisition pattern and to enhance the understanding of EPID readout schemes. RESULTS: Large discrepancies between observed and predicted images were noted due to an under-response to single low MU fields. It is shown that a variability of up to 37% can be observed for low MU fields in clinically used EPID acquisition modes and that the majority of this variability can be accounted for by the readout scheme, integration, and timing of EPID acquisitions. Simulations have confirmed the causes of the discrepancies. The occurrence and extent of the variation has been estimated for clinical settings. CONCLUSIONS: Incorrect absolute EPID signals collected for low MU fields in external beam treatments will negatively affect quantitative applications such as individual field based EPID dosimetry, typically appearing as an underdose, unless corrections to currently employed EPID readout schemes are made.

Characterization of an advanced cone beam CT (CBCT) reconstruction algorithm used for dose calculation on Varian Halcyon linear accelerators.

In this study, the performance of a new iterative reconstruction algorithm, the pre-clinical AcurosXB iCBCT algorithm, has been characterized on Varian Halcyon linear accelerators with respect to the potential of radiotherapy dose calculations on CBCT images. The study utilized various phantom setups to verify the accuracy of the pre-clinical algorithm under different scatter conditions and compared dose calculations performed on CBCT images reconstructed with the pre-clinical algorithm to those performed on typical planning CT images. The results indicated that despite showing improvements compared to the existing iCBCT protocol, certain restrictions should be introduced when the pre-clinical AcurosXB iCBCT algorithm was used for dose calculations. Changes in the scatter condition exhibited a larger effect on CBCTs than on planning CTs. Therefore, users should be careful in offsetting the patient and positioning the patient's arms if the resultant images will be used for dose calculations. In addition, protocols with different kV settings should be approached with caution, where 100 kV protocols should only be used to scan the head and neck area, while the rest of the body should be scanned with the 125 kV and 140 kV protocols. When the patient is set up properly and the appropriate energy is selected for the anatomical area, the uncertainty of using the novel AcurosXB iCBCT algorithm for treatment planning dose calculation is within ±2.0%.

Assessment of semi-automated stereotactic treatment planning for online adaptive radiotherapy in ethos.

INTRODUCTION: The Ethos treatment planning system allows for the rapid generation of online adaptive treatment plans while the patient is on the treatment couch. One promising application of online adaptive radiotherapy is its use in stereotactic radiotherapy. The purpose of this study was to ensure the Ethos treatment planning system (TPS) can produce clinically acceptable stereotactic plans, that are non-inferior to those from the Eclipse TPS. METHOD: Forty patients that received previous stereotactic radiotherapy treatment on a Halcyon, 20 of which were lung cases, and 20 that were brain cases, were replanned using the Ethos TPS. The generated IMRT and VMAT plans were compared to the clinical Eclipse VMAT plan. RESULTS: This study found that the Ethos TPS can produce VMAT plans of equivalent quality (target coverage, conformity and OAR doses) to those from the Eclipse TPS for lung SBRT and brain SRT. The IMRT plans produced by the Ethos planning system were marginally inferior to Eclipse VMAT plans, with the differences likely primarily due to beam geometry rather than the optimization system. Ethos plans were generally more modulated than Eclipse plans. With careful selection of optimization structures and reduction in the body contour, VMAT plan generation time could be reduced by 87%. CONCLUSION: Ethos can generate stereotactic VMAT plans that are equivalent to those from Eclipse in the timeframe required for online adaptive radiotherapy.

Modelling of a novel technique to improve the visualisation of implanted fiducial markers for intra-fraction MV imaging of prostate VMAT targets.

Purpose. This study explored a novel technique to improve the MV imaging based fiducial visibility for a cohort of prostate radiotherapy patients, without compromising the original treatment plan. The study also compared these results to visibility using single MLC control points, as well as short arcs.Methods. Geometric data from 68 prostate radiotherapy treatments, each with implanted gold fiducials, was retrospectively analysed. Fiducials were contoured for each patient, and conventional and SBRT treatment plans were generated using a VMAT technique. Using an in-house script, fiducial contours were projected onto the VMAT MLC control points. Resulting data was assessed to determine whether the fiducial contours were theoretically visible for single MLC control points and groups of MLC control points (short arcs), both being surrogates for intra-fraction MV imaging. Using this data, a theoretical quadrant technique was investigated that assessed the region surrounding each fiducial to determine if visualisation would theoretically improve.Results. Using a conventional treatment type, mean fiducial visibility for single MLC control points across the patient cohort ranged from 2.5% up to 17.8%. For SBRT, fiducial visibility ranged from 1.8% up to 19.7%. For short arcs, fiducial visibility for conventional treatment types ranged from 5.9% up to 20.7%. For SBRT, fiducial visibility ranged from 4.6% up to 23.1%. When the novel fiducial quadrant technique was used, theoretical visibility improved two-fold, from 22.7% up to 52.5% and from 24.7% up to 55.3% for conventional and SBRT treatment types respectively.Conclusions. Fiducial visibility was assessed for a cohort of VMAT prostate patients. Using the novel quadrant technique, it was demonstrated that theoretical visualisation and localisation of the implanted fiducials could be improved two-fold, without sacrificing treatment plan quality.

Use ofin vivotransit portal images to detect gross inter-fraction patient geometry changes on an O-ring type linear accelerator for pelvis and head/neck patients.

The purpose of this work was to investigate the use of the Varian Portal Dosimetry application in conjunction within vivomegavoltage portal images on a Varian Halcyon O-ring type linear accelerator as anin vivodosimetry constancy (IVDc) tool for pelvis and head/neck patients receiving VMAT treatments. Sensitivity testing was conducted on phantoms with varying thicknesses (0.2 cm-1.0 cm) using static and modulated fields. A cohort of 96 portal dose images across eight patients was then compared with PTV metrics derived from daily CBCT image based treatment plan re-calculations to determine whether the IVDc tool could detect gross inter-fraction anatomical changes. A final cohort of 315 portal dose images across 22 patients was then assessed to demonstrate the application of IVDc tool. The IVDc tool, using 2%/2 mm criteria, detected all phantom thickness changes of 1.0 cm, some phantom thickness changes of 0.5 cm, and no changes of 0.2 cm. For the cohort of 96 results, a IVDc passing criteria of 95% (2%, 2 mm) was able to identify all cases that had PTV metric changes of 2% or more. Using the IVDc tool on the cohort of 315 results, and the IVDc passing criteria of 95%, resulted in 74 IVDc failures. A simple, easy to implement, methodology has been presented that is capable of detecting gross inter-fraction changes in patient geometry on the Varian Halcyon O-ring linac linear accelerator.

Report of the ACPSEM radiation oncology medical physics workforce modelling project task group.

The ACPSEM radiation oncology medical physics workforce modelling project task group was formed to acquire a snapshot of practices in Australia and New Zealand and to develop an activity-based workforce model. To achieve this, two surveys were carried out, capturing the work practices of 98 radiation oncology departments and 182 college members. The member survey provided a snapshot of the current workforce: their demographics, work conditions, professional recognition, and future plans. The facility survey provided an Australian and New Zealand contextualisation of the volume-based activities defined in the International Atomic Energy Agency activity-based radiation oncology staffing model at a granular level. An ACPSEM ROMP workforce model was developed to be a modelling tool applicable at both the facility and sector levels.

Evaluation of a new hybrid VMAT-IMRT multi-criteria optimization plan generation algorithm.

To evaluate the Varian 'Fast hybrid multi-criteria optimization (MCO) volumetric modulated arc therapy (VMAT)' (H-VMAT) tool for both its dosimetric accuracy and calculation time. This is a new function within V15.6 of the Varian Eclipse treatment planning system that allows VMAT optimization and dose calculation using the graphical processing unit (GPU). In versions prior to V15.6 VMAT MCO calculations were only possible using central processing unit (CPU) not GPU. We termed this approach as native VMAT (N-VMAT). The study consisted of a cohort of 53 patients representing a range of anatomical treatment sites; bladder (5), brain (6), gynaecological (5), head & neck (5), lung (7), mediastinum (7) prostate (6), oesophagus (7), and rectum (5). Each case was planned to that of a clinical standard (Base) which was compared to a H-VMAT and N-VMAT approach. The study analyzed plan calculation time data, dose to organ at risk (OAR) and target coverage for each approach. Negligible dosimetric differences were found between the H-VMAT and N-VMAT approach for the cohort of patients evaluated. The largest dosimetric changes where observed in the optic chiasm and lacrimal gland where the H-VMAT achieved a max dose of 50.9 ± 7.7 Gy and 8.0 ± 0.5 Gy in comparison to the N-VMAT 53.1 ± 6.3 Gy and 10.2 ± 2.9 Gy, respectively. Several OAR's provided indistinguishable dose outcomes, namely; brainstem, heart, kidney's, lens, parotid, and spinal cord. Large time savings were found using the H-VMAT technique when compared to N-VMAT, being 5 to 40 times faster or up to 75 minutes time saving (average of 25 minutes). Negligible dosimetric change between the 2 techniques and large time savings were observed with the GPU enabled H-VMAT approach. We have shown that the H-VMAT technique has been safely implemented and is ready for clinical use.

Evaluation of a new GPU-enabled VMAT multi-criteria optimisation plan generation algorithm.

To evaluate the new Varian, graphical processing unit (GPU)-enabled, volumetric-modulated arc therapy (VMAT) multi-criteria optimisation (MCO) tool for both its dosimetric accuracy and calculation time. This is a new capability within V16.0 and greater of the Varian Eclipse treatment planning system that allows VMAT optimisation and dose calculation using the GPU (termed GPU-VMAT). In versions prior to V16.0 VMAT multi-criteria optimisation calculations were only possible using central processing unit (CPU) (termed CPU-VMAT) and Hybrid-VMAT (H-VMAT). The H-VMAT method breaks down the VMAT plan into IMRT fields which utilised GPU calculations. The study consisted of a cohort of 50 patients representing a range of anatomical treatment sites; bladder (5), brain (5), gynae (5), head & neck (5), lung (7), mediastinum (7) prostate (4), oesophagus (7) and rectum (5). Each case was planned to that of a clinical standard (Base) which was compared to a CPU-VMAT, GPU-VMAT and H-VMAT approaches. The study analysed dose to organ at risk (OAR) and target coverage, plan calculation time data and plan complexity through monitor unit (MU) for each approach. Negligible dosimetric differences were found between the CPU-VMAT, GPU-VMAT and H-VMAT approaches for the cohort of patients evaluated. The largest dosimetric change were observed in the lacrimal gland for a head and neck case, where the GPU-VMAT and H-VMAT achieved a max dose of +2.8 ± 0.0 Gy and -4.6 ± 0.0 Gy, respectively, when compared to CPU-VMAT. The majority of organ at risk's (OAR) provided indistinguishable dosimetric outcomes, namely: heart, kidneys, femur, lens, oral cavity and oesophagus. Large time savings were found using the GPU-VMAT technique compared to CPU-VMAT, a mean decrease in calculation time across all sites of 60.2% ± 15.6%. Negligible dosimetric change between the 2 techniques and large time saving were observed with the GPU-VMAT and H-VMAT approaches when compared to the CPU-VMAT. We have shown that the GPU-VMAT technique has been safely implemented with minimal differences from CPU-VMAT, but with significant optimisation and calculation times savings.

Quality and access - Early experience of implementing a virtual stereotactic chart round across a national network.

INTRODUCTION: Stereotactic radiation therapy is a highly specialised technique which requires careful and structured implementation. As part of a national stereotactic programme implementation, protocols were developed and a national stereotactic chart round was formed, which strongly recommended attendance and presentation of all cases before treatment. Herein, we describe our experiences launching a national chart round and its importance in a stereotactic programme. METHOD: Stereotactic chart rounds were held via videoconference between July 2018 and July 2019. Data collected included attendances, patient-related information including, diagnosis, clinical background, treatment intent, prescribed dose and fractionation and technical approach. Consensus recommendations regarding changes to treatment approaches were also recorded. RESULTS: For the 12 months recorded, there were 1126 attendances, from 144 individual attendees, across 21 locations. In total, 285 cases (237 new cases, and 48 re-presentations) were presented by 27 radiation oncologists (ROs) from 13 different locations. From the cases presented, 65 changes were recommended from 53 patients (22.3%), including 27 (11.4%) changes to contours, 18 (7.6%) changes to dose prescription/fractionation, 9 (3.8%) changes to plan dosimetry, 1 (0.4%) changes to treatment technique and 10 (4.2%) recommendations for which stereotactic radiation therapy was not advised. A significant inverse relationship was found between frequency of recommended changes and the individual RO's stereotactic case load (P < 0.002). CONCLUSION: The implementation of a national stereotactic chart held via videoconference has ensured national protocol compliance across the network of locations. Furthermore, the chart rounds have allowed the entire multidisciplinary team to be provided with mentorship and guidance. Increasing number of cases presented was associated with lower rates of recommended changes highlighting the impact of experience and the need for continued mentorship.

Using the iterative kV CBCT reconstruction on the Varian Halcyon linear accelerator for radiation therapy planning for pelvis patients.

PURPOSE: The ability to utilise a linear accelerators kV cone beam CT, for treatment planning and dosimetry calculation, has a number of potential uses including adaptive planning and daily dose analysis. This work validates the use of the iterative reconstruction of the Varian Halcyon cone beam CT (iCBCT) as datasets for radiation therapy planning of pelvis patients. METHODS: A CT to electron density (ED) curve was created in the Varian Eclipse treatment planning system (TPS) using scans of a CIRS ED phantom under a variety of scattering conditions. A pelvis phantom was imaged using a diagnostic CT scanner and Halcyon iCBCT. Ten pelvis patient plans were copied onto each dataset and compared using a global 3D gamma analysis. Each patient being treated on Halcyon has a daily iCBCT, and each plan was also recalculated on their respective day 1 iCBCT dataset. RESULTS: 3D Gamma analysis results for the patients planned on the pelvis phantom were analysed using a 1%/1mm and 2%/2mm, 10% low dose threshold criteria. The average pass rate was 99.4% ± 0.2%. The same metrics were used to analyse the patient plans recalculated on day 1 iCBCTs. The average result for the 1%/1mm gamma analysis was 94.4% ± 6.1%, and 98.6% ± 2.0% for 2%/2mm. A comparison of typical patient volumes showed average mean dose volume differences of 0.0% ± 1.4%, 0.0% ± 0.9% and 1.0% ± 4.0% for the PTV, Bladder and Rectum volumes respectively. CONCLUSIONS: Halcyon iCBCT datasets can be used for dose calculation in radiation therapy treatment planning for pelvis patients.

Accuracy and efficiency of graphics processing unit (GPU) based Acuros XB dose calculation within the Varian Eclipse treatment planning system.

To evaluate, in terms of dosimetric accuracy and calculation efficiency, the implementation of a graphic processing unit (GPU)-based Acuros XB dose calculation engine within version 15.5 of the Varian Eclipse treatment planning system. Initial phantom based calculations and a range of 101 clinical cases were analyzed on a dedicated test system. Dosimetric differences, based on dose-volume histrogram parameters and plan comparison, were compared between central processing unit (CPU) and GPU based calculation. Calculation times were also compared between CPU and GPU, as well as PLAN and FIELD modes. No dosimetric differences were found between CPU and GPU. CPU based calculations ranged from 25 to 533 seconds per plan, reducing to 13 to 70 seconds for GPU. GPU was 4.4 times more efficient than CPU. FIELD mode was up to 1.3 times more efficient than PLAN mode. For the clinical cases and version of Eclipse used, no dosimetric differences were found between CPU and GPU. Based on this, GPU architecture has been safely implemented and is ready for clinical use. GPU based calculation times were superior to CPU, being on average, 4.4 times faster.

Film dosimetry using a smart device camera: a feasibility study for point dose measurements.

In this work, a methodology for using a smartphone camera, in conjunction with a light-tight box operating in reflective transmission mode, is investigated as a proof of concept for use as a film dosimetry system. An imaging system was designed to allow the camera of a smartphone to be used as a pseudo densitometer. Ten pieces of Gafchromic EBT3 film were irradiated to doses up to 16.89 Gy and used to evaluate the effects of reproducibility and orientation, as well as the ability to create an accurate dose response curve for the smartphone based dosimetry system, using all three colour channels. Results were compared to a flatbed scanner system. Overall uncertainty was found to be best for the red channel with an uncertainty of 2.4% identified for film irradiated to 2.5 Gy and digitised using the smartphone system. This proof of concept exercise showed that although uncertainties still exceed a flatbed scanner system, the smartphone system may be useful for providing point dose measurements in situations where conventional flatbed scanners (or other dosimetry systems) are unavailable or unaffordable.

Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film.

In this work, the apparent treatment dose that kV planar or CBCT imaging contributes to Gafchromic EBT3 film used for in vivo dosimetry, was investigated. Gafchromic EBT3 film pieces were attached to a variety of phantoms and irradiated using the linear accelerator's built-in kV imaging system, in both kV planar mode and CBCT mode. To evaluate the sensitivity of the film in the clinical scenario where dose contributions are received from both imaging and treatment, additional pieces of film were irradiated using base doses of 50cGy and then irradiated using selected kV planar and CBCT techniques. For kV planar imaging, apparent treatment doses of up to 3.4cGy per image pair were seen. For CBCT, apparent treatment doses ranged from 0.22cGy to 3.78cGy. These apparent doses were reproducible with and without the inclusion of the 50cGy base dose. The contribution of apparent treatment dose from both planar kV as well as CBCT imaging can be detected, even in conjunction with an actual treatment dose. The magnitude of the apparent dose was found to be dependent on patient geometry, scanning protocol, and measurement location. It was found that the apparent treatment dose from the imaging could add up to 8% of additional uncertainty to the in vivo dosimetry result, if not taken into account. It is possible for this apparent treatment dose to be accounted for by subtraction of the experimentally determined apparent doses from in vivo measurements, as demonstrated in this work.