A Perez-Andujar

Assessment of image quality and dose calculation accuracy on kV CBCT, MV CBCT, and MV CT images for urgent palliative radiotherapy treatments.

A clinical workflow was developed for urgent palliative radiotherapy treatments that integrates patient simulation, planning, quality assurance, and treatment in one 30‐minute session. This has been successfully tested and implemented clinically on a linac with MV CBCT capabilities. To make this approach available to all clinics equipped with common imaging systems, dose calculation accuracy based on treatment sites was assessed for other imaging units. We evaluated the feasibility of palliative treatment planning using on‐board imaging with respect to image quality and technical challenges. The purpose was to test multiple systems using their commercial setup, disregarding any additional in‐house development. kV CT, kV CBCT, MV CBCT, and MV CT images of water and anthropomorphic phantoms were acquired on five different imaging units (Philips MX8000 CT Scanner, and Varian TrueBeam, Elekta VersaHD, Siemens Artiste, and Accuray Tomotherapy linacs). Image quality (noise, contrast, uniformity, spatial resolution) was evaluated and compared across all machines. Using individual image value to density calibrations, dose calculation accuracies for simple treatment plans were assessed for the same phantom images. Finally, image artifacts on clinical patient images were evaluated and compared among the machines. Image contrast to visualize bony anatomy was sufficient on all machines. Despite a high noise level and low contrast, MV CT images provided the most accurate treatment plans relative to kV CT‐based planning. Spatial resolution was poorest for MV CBCT, but did not limit the visualization of small anatomical structures. A comparison of treatment plans showed that monitor units calculated based on a prescription point were within 5% difference relative to kV CT‐based plans for all machines and all studied treatment sites (brain, neck, and pelvis). Local dose differences >5% were found near the phantom edges. The gamma index for 3%/3 mm criteria was ≥95% in most cases. Best dose calculation results were obtained when the treatment isocenter was near the image isocenter for all machines. A large field of view and immediate image export to the treatment planning system were essential for a smooth workflow and were not provided on all devices. Based on this phantom study, image quality of the studied kV CBCT, MV CBCT, and MV CT on‐board imaging devices was sufficient for treatment planning in all tested cases. Treatment plans provided dose calculation accuracies within an acceptable range for simple, urgently planned palliative treatments. However, dose calculation accuracy was compromised towards the edges of an image. Feasibility for clinical implementation should be assessed separately and may be complicated by machine specific features. Image artifacts in patient images and the effect on dose calculation accuracy should be assessed in a separate, machine‐specific study. PACS number(s): 87.55.D‐, 87.57.C‐, 87.57.Q‐A clinical workflow was developed for urgent palliative radiotherapy treatments that integrates patient simulation, planning, quality assurance, and treatment in one 30-minute session. This has been successfully tested and implemented clinically on a linac with MV CBCT capabilities. To make this approach available to all clinics equipped with common imaging systems, dose calculation accuracy based on treatment sites was assessed for other imaging units. We evaluated the feasibility of palliative treatment planning using on-board imaging with respect to image quality and technical challenges. The purpose was to test multiple systems using their commercial setup, disregarding any additional in-house development. kV CT, kV CBCT, MV CBCT, and MV CT images of water and anthropomorphic phantoms were acquired on five different imaging units (Philips MX8000 CT Scanner, and Varian TrueBeam, Elekta VersaHD, Siemens Artiste, and Accuray Tomotherapy linacs). Image quality (noise, contrast, uniformity, spatial resolution) was evaluated and compared across all machines. Using individual image value to density calibrations, dose calculation accuracies for simple treatment plans were assessed for the same phantom images. Finally, image artifacts on clinical patient images were evaluated and compared among the machines. Image contrast to visualize bony anatomy was sufficient on all machines. Despite a high noise level and low contrast, MV CT images provided the most accurate treatment plans relative to kV CT-based planning. Spatial resolution was poorest for MV CBCT, but did not limit the visualization of small anatomical structures. A comparison of treatment plans showed that monitor units calculated based on a prescription point were within 5% difference relative to kV CT-based plans for all machines and all studied treatment sites (brain, neck, and pelvis). Local dose differences >5% were found near the phantom edges. The gamma index for 3%/3 mm criteria was ≥95% in most cases. Best dose calculation results were obtained when the treatment isocenter was near the image isocenter for all machines. A large field of view and immediate image export to the treatment planning system were essential for a smooth workflow and were not provided on all devices. Based on this phantom study, image quality of the studied kV CBCT, MV CBCT, and MV CT on-board imaging devices was sufficient for treatment planning in all tested cases. Treatment plans provided dose calculation accuracies within an acceptable range for simple, urgently planned palliative treatments. However, dose calculation accuracy was compromised towards the edges of an image. Feasibility for clinical implementation should be assessed separately and may be complicated by machine specific features. Image artifacts in patient images and the effect on dose calculation accuracy should be assessed in a separate, machine-specific study. PACS number(s): 87.55.D-, 87.57.C-, 87.57.Q.

Estimating the probability of underdosing microscopic brain metastases with hippocampal-sparing whole-brain radiation

PURPOSE/OBJECTIVES Whole-brain radiation for brain metastases can result in cognitive side effects. Hippocampal-sparing techniques have been developed to decrease morbidity, but they carry the risk of underdosing lesions near the hippocampus due to the unavoidable dose gradient from the hippocampal surface to the prescription isodose surface. This study examines the impact of variable levels of hippocampal sparing on the underdosing of potential brain metastases. MATERIALS/METHODS Helical intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans were developed for hippocampal-sparing whole-brain treatment. For all plans, 30Gy was prescribed in 10 fractions to result in mean hippocampal doses of 6-12Gy. From a series of expanded shells, we determined the distance from the hippocampus at which the parenchyma would receive less than specified doses. Then, using published data, a mathematical model was constructed to predict the incident probability of potential brain metastases receiving different doses for different levels of hippocampal sparing. RESULTS Whole-brain radiation plans were able to spare the hippocampi to mean doses of 7-12Gy under our planning constraints; more stringent constraints compromised brain coverage. The dose gradients were ∼4% per mm, regardless of the hippocampal constraint, and they decreased sharply by a factor of almost 4 at approximately 15mm from the hippocampi. A mathematical model was constructed and combined the plan information with published data on the distribution of brain metastases, to determine the percentage of potential brain metastases receiving specified doses, as a function of technique and level of hippocampal sparing. CONCLUSIONS Our results describe the characteristics of an array of hippocampal-sparing whole-brain radiation dose distributions. These can be used as a decision-making guideline for weighing the benefit of decreased dose to the hippocampi against the cost of decreased dose to potential brain metastases when deciding on a hippocampal-sparing whole-brain irradiation treatment approach.

Characterization of the effect of a new commercial transmission detector on radiation therapy beams

PURPOSE To evaluate the influence of a new commercial transmission detector on radiation therapy beams. METHODS AND MATERIALS A transmission detector designed for online treatment monitoring was characterized on a TrueBeam STx linear accelerator with 6-MV, 6-flattening filter free, 10-MV, and 10-flattening filter free beams. Measurements of percentage depth doses, in-plane and cross-plane off-axis profiles at different depths, transmission factors, and skin dose were acquired with 3 × 3, 5 × 5, 10 × 10, 20 × 20, and 40 × 40 cm2 field sizes at 100 cm and 80 cm source-to-surface distance (SSD). A CC04 chamber was used for all profile and transmission factor measurements. Skin dose was assessed at 100, 90, and 80 cm SSD using a variety of detectors (Roos and Markus parallel-plate chambers and optically stimulated luminescent dosimeters [OSLDs]). Skin dose was also assessed for various patient sample plans with OSLDs. RESULTS The percentage depth doses showed small differences between the unperturbed and perturbed beams for 100 cm SSD (≤4 mm depth of maximum dose difference, <1.2% average profile difference) for all field sizes. At 80 cm SSD, the differences were larger (≤8 mm depth of maximum dose difference, <3% average profile difference). The differences were larger for the flattened beams and larger field sizes. The off-axis profiles showed similar trends. Field penumbras looked similar with and without the transmission detector. Comparisons in the profile central 80% showed a maximum average (maximum) profile difference between all field sizes of 1.0% (2.6%) and 1.4% (6.3%) for 100 and 80 cm SSD, respectively. The average measured skin dose increase at 100 cm (80 cm) SSD for a 10 × 10 cm2 field size was <4% (<35%) for all energies. For a 40 × 40 cm2 field size, this increased to <31% (≤63%). For the sample patient plans, the average skin dose difference was 0.53% (range, -6.6% to 10.4%). CONCLUSIONS The transmission detector has minimal effect on clinically relevant radiation therapy beams for intensity modulated radiation therapy and volumetric arc therapy (field sizes 10 × 10 cm2 and less). For larger field sizes, some perturbations are observable that would need to be assessed for clinical impact.

SU‐F‐T‐307: Peripheral Dose Comparison Between Static and Dynamic Jaw Tracking On a High Definition MLC System

PURPOSE To investigate the effect of dynamic and static jaw tracking on patient peripheral doses. MATERIALS AND METHODS A patient plan with a large sacral metastasis (volume 800cm3, prescription 600cGyx5) was selected for this study. The plan was created using 2-field RapidArc with jaw tracking enabled (Eclipse, V11.0.31). These fields were then exported and edited in MATLAB with static jaw positions using the control point with the largest field size for each respective arc, but preserving the optimized leaf sequences for delivery. These fields were imported back into Eclipse for dose calculation and comparison and copied to a Rando phantom for delivery analysis. Points were chosen in the phantom at depth and on the phantom surface at locations outside the primary radiation field, at distances of 12cm, 20cm, and 30cm from the isocenter. Measurements were acquired with OSLDs placed at these positions in the phantom with both the dynamic and static jaw deliveries for comparison. Surface measurements included an additional 1cm bolus over the OSLDs to ensure electron equilibrium. RESULTS The static jaw deliveries resulted in cumulative jaw-defined field sizes of 17.3% and 17.4% greater area than the dynamic jaw deliveries for each arc. The static jaw plan resulted in very small differences in calculated dose in the treatment planning system ranging from 0-16cGy. The measured dose differences were larger than calculated, but the differences in absolute dose were small. The measured dose differences at depth (surface) between the two deliveries showed an increase for the static jaw delivery of 2.2%(11.4%), 15.6%(20.0%), and 12.7%(12.7%) for distances of 12cm, 20cm, and 30cm, respectively. Eclipse calculates a difference of 0-3.1% for all of these points. The largest absolute dose difference between all points was 6.2cGy. CONCLUSION While we demonstrated larger than expected differences in peripheral dose, the absolute dose differences were small.

Development and clinical evaluation of an ionization chamber array with 3.5 mm pixel pitch for quality assurance in advanced radiotherapy techniques.

PURPOSE To characterize a new air vented ionization chamber technology, suitable to build detector arrays with small pixel pitch and independence of sensitivity on dose per pulse. METHODS The prototype under test is a linear array of air vented ionization chambers, consisting of 80 pixels with 3.5 mm pixel pitch distance and a sensitive volume of about 4 mm(3). The detector has been characterized with (60)Co radiation and MV x rays from different linear accelerators (with flattened and unflattened beam qualities). Sensitivity dependence on dose per pulse has been evaluated under MV x rays by changing both the source to detector distance and the beam quality. Bias voltage has been varied in order to evaluate the charge collection efficiency in the most critical conditions. Relative dose profiles have been measured for both flattened and unflattened distributions with different field sizes. The reference detectors were a commercial array of ionization chambers and an amorphous silicon flat panel in direct conversion configuration. Profiles of dose distribution have been measured also with intensity modulated radiation therapy (IMRT), stereotactic radiosurgery (SRS), and volumetric modulated arc therapy (VMAT) patient plans. Comparison has been done with a commercial diode array and with Gafchromic EBT3 films. RESULTS Repeatability and stability under continuous gamma irradiation are within 0.3%, in spite of low active volume and sensitivity (∼200 pC/Gy). Deviation from linearity is in the range [0.3%, -0.9%] for a dose of at least 20 cGy, while a worsening of linearity is observed below 10 cGy. Charge collection efficiency with 2.67 mGy/pulse is higher than 99%, leading to a ±0.9% sensitivity change in the range 0.09-2.67 mGy/pulse (covering all flattened and unflattened beam qualities). Tissue to phantom ratios show an agreement within 0.6% with the reference detector up to 34 cm depth. For field sizes in the range 2 × 2 to 15 × 15 cm(2), the output factors are in agreement with a thimble chamber within 2%, while with 25 × 25 cm(2) field size, an underestimation of 4.0% was found. Agreement of field and penumbra width measurements with the flat panel is of the order of 1 mm down to 1 × 1 cm(2) field size. Flatness and symmetry values measured with the 1D array and the reference detectors are comparable, and differences are always smaller than 1%. Angular dependence of the detector, when compared to measurements taken with a cylindrical chamber in the same phantom, is as large as 16%. This includes inhomogeneity and asymmetry of the design, which during plan verification are accounted for by the treatment planning system (TPS). The detector is capable to reproduce the dose distributions of IMRT and VMAT plans with a maximum deviation from TPS of 3.0% in the target region. In the case of VMAT and SRS plans, an average (maximum) deviation of the order of 1% (4%) from films has been measured. CONCLUSIONS The investigated technology appears to be useful both for Linac QA and patient plan verification, especially in treatments with steep dose gradients and nonuniform dose rates such as VMAT and SRS. Major limitations of the present prototype are the linearity at low dose, which can be solved by optimizing the readout electronics, and the underestimation of output factors with large field sizes. The latter problem is presently not completely understood and will require further investigations.

Physics of Stereotactic Radiosurgery and Stereotactic Body Radiotherapy

Stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) are delivered by complicated systems with advanced treatment and imaging capabilities. Therefore, clinical validation of treatment planning, linac delivery, and imaging systems must be validated rigorously, both by performing end-to-end measurements and also by using third-party phantoms, such as that provided by IROC (Imaging and Radiation Oncology Corte), before using the system clinically. Routine quality assurance (QA) and quality control (QC) must be strictly maintained to ensure delivery accuracy and patient safety.

SU-F-T-435: Helical Tomotherapy for Craniospinal Irradiation: What We Have Learned from a Multi-Institutional Study

PURPOSE To report cranio-spinal irradiation (CSI) planning experience, compare dosimetric quality and delivery efficiency with Tomotherapy from different institutions, and to investigate effect of planning parameters on plan quality and treatment time. METHODS Clinical helical tomotherapy IMRT plans for thirty-nine CSI cases from three academic institutions were retrospectively evaluated. The planning parameters: field width (FW), pitch, modulation factor (MF), and achieved dosimetric endpoints were cross-compared. A fraction-dose-delivery-timing index (FDTI), defined as treatment time per fraction dose per PTV length, was utilized to evaluate plan delivery efficiency. A lower FDTI indicates higher delivery efficiency. We studied the correlation between planning quality, treatment time and planning parameters by grouping the plans under specific planning parameters. Additionally, we created new plans using 5cm jaw for a subset of plans that used 2.5cm jaw to exam if treatment efficiency can be improved without sacrificing plan quality. RESULTS There were significant dosimetric differences for organ at risks (OARs) among different institutions (A,B,C). Using the lowest average MF (1.9±0.4) and 5cm field width, C had the highest lung, heart, kidney, liver mean doses and maximum doses for lens. Using the same field width of 5cm, but higher MF (2.6±0.6), B had lower doses to the OARs in the thorax and abdomen area. Most of As plans were planned with 2.5cm jaw, the plans yielded better PTV coverage, higher OAR doses and slightly shorter FDTI compared to institution B. The replanned 5cm jaw plans achieved comparable PTV coverage and OARs sparing, while saving up to 44.7% treatment time. CONCLUSION Plan quality and delivery efficiency could vary significantly in CSI planning on Tomotheapy due to choice of different planning parameters. CSI plans using a 5cm jaw, with proper selection of pitch and MF, can achieve comparable/ better plan quality with shorter delivery time compared to 2.5cm jaw plans.

SU‐F‐T‐311: Comparison of Measurements From Two Diode Array QA Devices to Deep Point Dose Measurements with Two Treatment Planning Model Settings for Brain VMAT SRT Patient‐Specific QA

PURPOSE To investigate the sensitivity of traditional gamma-index-based analysis using diode arrays to detect significant dosimetric errors in patientspecific measurements for VMAT brain SRT plans. METHODS Quality assurance measurements were performed using an ion chamber and two different diode array devices for four brain SRT treatment plans. All cases were planned with Eclipse AAA RapidArc using 6MV VMAT for a TrueBeam STx. Ion chambers measurements were performed with a PinPoint 3D micro-ion chamber in both the Standard Imaging Stereotactic Dose Verification (SI) Phantom and in the ArcCheck (AC) cavity plug. Point dose measurements were taken at isocenter within the target. Array measurements were acquired with the ArcCheck and gantry-mounted MapCheck (MC) and assessed using the SNC patient software with a 3%/3mm absolute dose gamma-index analysis with global normalization and a 10% dose threshold. Calculations and their corresponding measurements were performed and compared before and after a dynamic leaf gap (DLG) adjustment. RESULTS The measured doses at isocenter dropped from 6.3%-8.7% in the SI phantom and 4.0%-9.3% for AC down to -0.1%-1.1% in the SI phantom and 0.8%-2.9% in the AC after adjusting the DLG. However, the cumulative dose differences to MCs central diode ranged -2.0%-1.9% before the DLG adjustment and were -7.7%-0.2% afterwards. Similarly, the gamma-index analysis results for MC were found to be 94.1%-100% before the DLG adjustment and 94.4%-100% afterwards. The gamma-index results from AC were 72.9%-98.7% before and 95.9%-99.7% after. CONCLUSION Our data suggest that traditional passing criteria using gantry-mounted MapCheck may not have the sensitivity to catch significant dose errors introduced by DLG settings. ArcCheck measurements did flag some dose differences, however there appears to be a possibility that dose discrepancies may not always be observed for VMAT SRT treatments, and point dose measurements are recommended for verification.

SU‐F‐T‐475: An Evaluation of the Overlap Between the Acceptance Testing and Commissioning Processes for Conventional Medical Linear Accelerators

PURPOSE This works objective is to determine the overlap of processes, in terms of sub-processes and time, between acceptance testing and commissioning of a conventional medical linear accelerator and to evaluate the time saved by consolidating the two processes. METHOD A process map for acceptance testing for medical linear accelerators was created from vendor documentation (Varian and Elekta). Using AAPM TG-106 and inhouse commissioning procedures, a process map was created for commissioning of said accelerators. The time to complete each sub-process in each process map was evaluated. Redundancies in the processes were found and the time spent on each were calculated. RESULTS Mechanical testing significantly overlaps between the two processes - redundant work here amounts to 9.5 hours. Many beam non-scanning dosimetry tests overlap resulting in another 6 hours of overlap. Beam scanning overlaps somewhat - acceptance tests include evaluating PDDs and multiple profiles but for only one field size while commissioning beam scanning includes multiple field sizes and depths of profiles. This overlap results in another 6 hours of rework. Absolute dosimetry, field outputs, and end to end tests are not done at all in acceptance testing. Finally, all imaging tests done in acceptance are repeated in commissioning, resulting in about 8 hours of rework. The total time overlap between the two processes is about 30 hours. CONCLUSION The process mapping done in this study shows that there are no tests done in acceptance testing that are not also recommended to do for commissioning. This results in about 30 hours of redundant work when preparing a conventional linear accelerator for clinical use. Considering these findings in the context of the 5000 linacs in the United states, consolidating acceptance testing and commissioning would have allowed for the treatment of an additional 25000 patients using no additional resources.

SU‐E‐J‐89: GammaPlan MR to CT Image Registration Errors: Implications for Extend and Preplanned Treatments

Purpose: To compare the spatial errors in coordinate localization created by MR‐based stereotaxia in GammaPlan for two different Methods: direct stereotactic MR localization and MR co‐registration with a stereotactic CT. Methods: Seven patients underwent Gamma Knife SRS and were scanned with our clinical stereotactic MR and stereotactic CT protocols. In GammaPlan, the stereotactic coordinate system for each MR and CT was defined using the fiducials of the localizer. Separately, each MR was co‐registered (by the patients anatomy) to the stereotactic CT using GammaPlans automated co‐registration algorithm. A rigid transformation relationship was determined between the direct stereotactic MR (stereoMR) and the MR co‐registered to the stereotactic CT (coregMR). Spatial errors in coordinate definition between these two methods were calculated at the center of stereotactic space and at the centroid of the target. A total of seventeen MR scans were analyzed by this method including both axial and coronal acquisitions. Results: Mean errors in fiducial definition were found to be 0.6±0.3mm and 0.7±0.3mm for CT and MR, respectively. At the center of stereotactic space, the mean magnitude error in coordinate localization between stereoMR and coregMR was found to be 1.6±0.6mm. At the centroid of the target, the mean magnitude error was found to be 1.5±0.7mm. These results were statistically significant compared to the errors in fiducial definition (p<0.002 for both). No statistically significant systematic errors along any axes were found in comparing stereoMR and coregMR coordinate localizations. Conclusion: Statistically significant differences were found in coordinate localizations between the two methods. These errors may primarily be due to sub‐optimal co‐registrations when using GammaPlans automated registration algorithm. The magnitude of these errors may be clinically significant as they are on average greater than 1.5mm. Errors in co‐registration are important for preplanned SRS and Extend SRT treatments where direct stereotactic MR localization is not utilized.