Reduce Patient Treatment wait time in a Proton Beam Facility - A Gatekeeper Approach.

Patient wait time can negatively impact treatment quality in a proton therapy center, where multiple treatment rooms share one proton beam. Wait time increases patient discomfort that can lead to patient motion, dissatisfaction, and longer treatment delay. This study was to develop a patient call-back model that reduced patient wait while efficiently utilizing the proton beam. A "Gatekeeper" logic allowing therapists to adjust the time of a patient's call-back to the treatment room was developed. It uses a two-pronged approach to minimize overlap of long treatment and the possibility of excessive wait in the queue to receive the proton beam. The goal was to reduce the maximum wait time to less than eight minutes per field for a four-room facility. The effectiveness of this logic was evaluated through simulation, and five scenarios were compared. Four scenarios implementing various levels of gatekeeper logic were compared with the original scenario without the logic. The best performing model provided a reduction of the maximum field wait by 26% and met the predefined goal. Adjusting call-back extended the treatment day length by an average of 6 min and a maximum of 12 min in total. The use of this gatekeeper logic significantly reduces patient field wait with minimal impact on treatment day length for a four-room proton facility. A sample interface that adopts this logic for therapists to make informed decision on patient call-back time is demonstrated.

AAPM medical physics practice guideline 7.a.: Supervision of medical physicist assistants.

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances. Approved by AAPM's Executive Committee May 28, 2019.

Optimization of motion management parameters in a synchrotron-based spot scanning system.

PURPOSE: To quantify the effects of combining layer-based repainting and respiratory gating as a strategy to mitigate the dosimetric degradation caused by the interplay effect between a moving target and dynamic spot-scanning proton delivery. METHODS: An analytic routine modeled three-dimensional dose distributions of pencil-beam proton plans delivered to a moving target. Spot positions and weights were established for a single field to deliver 100 cGy to a static, 15-cm deep, 3-cm radius spherical clinical target volume with a 1-cm isotropic internal target volume expansion. The interplay effect was studied by modeling proton delivery from a clinical synchrotron-based spot scanning system and respiratory target motion, patterned from surrogate patient breathing traces. Motion both parallel and orthogonal to the beam scanning direction was investigated. Repainting was modeled using a layer-based technique. For each of 13 patient breathing traces, the dose from 20 distinct delivery schemes (combinations of four gate window amplitudes and five repainting techniques) was computed. Delivery strategies were inter-compared based on target coverage, dose homogeneity, high dose spillage, and delivery time. RESULTS: Notable degradation and variability in plan quality were observed for ungated delivery. Decreasing the gate window reduced this variability and improved plan quality at the expense of longer delivery times. Dose deviations were substantially greater for motion orthogonal to the scan direction when compared with parallel motion. Repainting coupled with gating was effective at partially restoring dosimetric coverage at only a fraction of the delivery time increase associated with very small gate windows alone. Trends for orthogonal motion were similar, but more complicated, due to the increased severity of the interplay. CONCLUSIONS: Layer-based repainting helps suppress the interplay effect from intra-gate motion, with only a modest penalty in delivery time. The magnitude of the improvement in target coverage is strongly influenced by individual patient breathing patterns and the tumor motion trajectory.

Clinical implementation of respiratory-gated spot-scanning proton therapy: An efficiency analysis of active motion management.

PURPOSE: The aim of this work is to describe the clinical implementation of respiratory-gated spot-scanning proton therapy (SSPT) for the treatment of thoracic and abdominal moving targets. The experience of our institution is summarized, from initial acceptance and commissioning tests to the development of standard clinical operating procedures for simulation, motion assessment, motion mitigation, treatment planning, and gated SSPT treatment delivery. MATERIALS AND METHODS: A custom respiratory gating interface incorporating the Real-Time Position Management System (RPM, Varian Medical Systems, Inc., Palo Alto, CA, USA) was developed in-house for our synchrotron-based delivery system. To assess gating performance, a motion phantom and radiochromic films were used to compare gated vs nongated delivery. Site-specific treatment planning protocols and conservative motion cutoffs were developed, allowing for free-breathing (FB), breath-holding (BH), or phase-gating (Ph-G). Room usage efficiency of BH and Ph-G treatments was retrospectively evaluated using beam delivery data retrieved from our record and verify system and DICOM files from patient-specific quality assurance (QA) procedures. RESULTS: More than 70 patients were treated using active motion management between the launch of our motion mitigation program in October 2015 and the end date of data collection of this study in January 2018. During acceptance procedures, we found that overall system latency is clinically-suitable for Ph-G. Regarding room usage efficiency, the average number of energy layers delivered per minute was <10 for Ph-G, 10-15 for BH and ≥15 for FB, making Ph-G the slowest treatment modality. When comparing to continuous delivery measured during pretreatment QA procedures, the median values of BH treatment time were extended from 6.6 to 9.3 min (+48%). Ph-G treatments were extended from 7.3 to 13.0 min (+82%). CONCLUSIONS: Active motion management has been crucial to the overall success of our SSPT program. Nevertheless, our conservative approach has come with an efficiency cost that is more noticeable in Ph-G treatments and should be considered in decision-making.

Treatment planning for metals using an extended CT number scale.

Metal implants which saturate the CT number scale may require dosimetrist and physicist involvement to manually contour and assign an appropriate value to the metal for accurate dose calculation. This study investigated dose calculation based directly on extended CT scale images for different metals and geometries. The aim was to evaluate extended CT accuracy as a suitable alternative to standard CT methods in the presence of high-Z materials and artifacts, despite the reduced HU resolution of extended CT. Gafchromic film measurements were made for comparison to calculated doses. The method of direct dose calculation on extended CT scale was compared to our institution's standard method of manually contouring and assigning metal values on saturated CT images for each of the metal samples. Clinical patient plans with metal implants were investigated and DVHs were compared between standard CT and extended CT dose calculations. Dose calculations showed agreement within 2% between the two methods of metal characterization and the film measurement in the case of the strongest metal attenuator, cobalt-chromium. In the clinical treatment plans, the greatest dose discrepancy between the two methods was 1.2%. This study suggests that direct dose calculation on an extended scale CT image in the presence of metal implants can produce accurate clinically viable treatment plans, thereby improving efficiency of clinical workflow and eliminating a potential source of human error by manual CT number assignment.

AAPM Medical Physics Practice Guideline 3.a: Levels of supervision for medical physicists in clinical training.

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States.The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner.Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized.The following terms are used in the AAPM practice guidelines:Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline.Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Rotating Gantries Provide Individualized Beam Arrangements for Charged Particle Therapy.

PURPOSE: This study evaluates beam angles used to generate highly individualized proton therapy treatment plans for patients eligible for carbon ion radiotherapy (CIRT). METHODS AND MATERIALS: We retrospectively evaluated patients treated with pencil beam scanning intensity modulated proton therapy from 2015 to 2020 who had indications for CIRT. Patients were treated with a 190° rotating gantry with a robotic patient positioning system. Treatment plans were individualized to provide maximal prescription dose delivery to the tumor target volume while sparing organs at risk. The utilized beam angles were grouped, and anatomic sites with at least 10 different beam angles were sorted into histograms. RESULTS: A total of 467 patients with 484 plans and 1196 unique beam angles were evaluated and characterized by anatomic treatment site and the number of beam angles utilized. The most common beam angles used were 0° and 180°. A wide range of beam angles were used in treating almost all anatomic sites. Only esophageal cancers had a predominantly unimodal grouping of beam angles. Pancreas cancers showed a modest grouping of beam angles. CONCLUSIONS: The wide distribution of beam angles used to treat CIRT-eligible patients suggests that a rotating gantry is optimal to provide highly individualized beam arrangements.

A Radiomics-Based Classifier for the Progression of Oropharyngeal Cancer Treated with Definitive Radiotherapy.

In this study, we investigated whether radiomics features from pre-treatment positron emission tomography (PET) images could be used to predict disease progression in patients with HPV-positive oropharyngeal cancer treated with definitive proton or x-ray radiotherapy. Machine learning models were built using a dataset from Mayo Clinic, Rochester, Minnesota (n = 72) and tested on a dataset from Mayo Clinic, Phoenix, Arizona (n = 22). A total of 71 clinical and radiomics features were considered. The Mann-Whitney U test was used to identify the top 2 clinical and top 20 radiomics features that were significantly different between progression and progression-free patients. Two dimensionality reduction methods were used to define two feature sets (manually filtered or machine-driven). A forward feature selection scheme was conducted on each feature set to build models of increased complexity (number of input features from 1 to 6) and evaluate model robustness and overfitting. The machine-driven features had superior performance and were less prone to overfitting compared to the manually filtered features. The four-variable Gaussian Naïve Bayes model using the 'Radiation Type' clinical feature and three machine-driven features achieved a training accuracy of 79% and testing accuracy of 77%. These results demonstrate that radiomics features can provide risk stratification beyond HPV-status to formulate individualized treatment and follow-up strategies.

Early Impact of Proton Beam Therapy on Electrophysiological Characteristics in a Porcine Model.

BACKGROUND: Particle therapy is a noninvasive, catheter-free modality for cardiac ablation. We previously demonstrated the efficacy for creating ablation lesions in the porcine heart. Despite several earlier studies, the exact mechanism of early biophysical effects of proton and photon beam delivery on the myocardium remain incompletely resolved. METHODS: Ten normal and 9 infarcted in situ porcine hearts received proton beam irradiation (40 Gy) delivered to the left ventricular myocardium with follow-up for 8 weeks. High-resolution electroanatomical mapping of the left ventricular was performed at baseline and follow-up. Bipolar voltage amplitude, conduction velocity, and connexin-43 were determined within the irradiated and nonirradiated areas. RESULTS: The irradiated area in normal hearts showed a significant reduction of bipolar voltage amplitude (10.1±4.9 mV versus 5.7±3.2, P<0.0001) and conduction velocity (85±26 versus 55±13 cm/s, P=0.03) beginning at 4 weeks after irradiation. In infarcted myocardium after irradiation, bipolar voltage amplitude of the infarct scar (2.0±2.9 versus 0.8±0.7 mV, P=0.008) was significantly reduced as well as the conduction velocity in the infarcted heart (43.7±15.7 versus 26.3±11.4 cm/s, P=0.02). There were no significant changes in bipolar voltage amplitude and conduction velocity in nonirradiated myocardium. Myocytolysis, capillary hyperplasia, and dilation were seen in the irradiated myocardium 8 weeks after irradiation. Active caspase-3 and reduction of connexin-43 expression began in irradiated myocardium 1 week after irradiation and decreased over 8 weeks. CONCLUSIONS: Irradiation of the myocardium with proton beams reduce connexin-43 expression, conduction velocity, and bipolar conducted electrogram amplitude in a large porcine model. The changes in biomarkers preceded electrophysiological changes after proton beam therapy.

Nuclear Fragmentation Imaging for Carbon-Ion Radiation Therapy Monitoring: an In Silico Study.

Purpose: This article presents an in vivo imaging technique based on nuclear fragmentation of carbon ions in irradiated tissues for potential real-time monitoring of carbon-ion radiation therapy (CIRT) treatment delivery and quality assurance purposes in clinical settings. Materials and Methods: A proof-of-concept imaging and monitoring system (IMS) was devised to implement the technique. Monte Carlo simulations were performed for a prospective pencil-beam scanning CIRT nozzle. The development IMS benchmark considered a 5×5-cm2 pixelated charged-particle detector stack positioned downstream from a target phantom and list-mode data acquisition. The abundance and production origins, that is, vertices, of the detected fragments were studied. Fragment trajectories were approximated by straight lines and a beam back-projection algorithm was built to reconstruct the vertices. The spatial distribution of the vertices was then used to determine plan relevant markers. Results: The IMS technique was applied for a simulated CIRT case, a primary brain tumor. Four treatment plan monitoring markers were conclusively recovered: a depth dose distribution correlated profile, ion beam range, treatment target boundaries, and the beam spot position. Promising millimeter-scale (3-mm, ≤10% uncertainty) beam range and submillimeter (≤0.6-mm precision for shifts <3 cm) beam spot position verification accuracies were obtained for typical therapeutic energies between 150 and 290 MeV/u. Conclusions: This work demonstrated a viable online monitoring technique for CIRT treatment delivery. The method's strong advantage is that it requires few signal inputs (position and timing), which can be simultaneously acquired with readily available technology. Future investigations will probe the technique's applicability to motion-sensitive organ sites and patient tissue heterogeneities. In-beam measurements with candidate detector-acquisition systems are ultimately essential to validate the IMS benchmark performance and subsequent deployment in the clinic.

An accelerated threshold-based back-projection algorithm for compton camera image reconstruction.

PURPOSE: Compton camera imaging (CCI) systems are currently under investigation for radiotherapy dose reconstruction and verification. The ability of such a system to provide real-time images during dose delivery will be limited by the computational speed of the image reconstruction algorithm. In this work, the authors present a fast and simple method by which to generate an initial back-projected image from acquired CCI data, suitable for use in a filtered back-projection algorithm or as a starting point for iterative reconstruction algorithms, and compare its performance to the current state of the art. METHODS: Each detector event in a CCI system describes a conical surface that includes the true point of origin of the detected photon. Numerical image reconstruction algorithms require, as a first step, the back-projection of each of these conical surfaces into an image space. The algorithm presented here first generates a solution matrix for each slice of the image space by solving the intersection of the conical surface with the image plane. Each element of the solution matrix is proportional to the distance of the corresponding voxel from the true intersection curve. A threshold function was developed to extract those pixels sufficiently close to the true intersection to generate a binary intersection curve. This process is repeated for each image plane for each CCI detector event, resulting in a three-dimensional back-projection image. The performance of this algorithm was tested against a marching algorithm known for speed and accuracy. RESULTS: The threshold-based algorithm was found to be approximately four times faster than the current state of the art with minimal deficit to image quality, arising from the fact that a generically applicable threshold function cannot provide perfect results in all situations. The algorithm fails to extract a complete intersection curve in image slices near the detector surface for detector event cones having axes nearly parallel to the image plane. This effect decreases the sum of the image, thereby also affecting the mean, standard deviation, and SNR of the image. All back-projected events associated with a simulated point source intersected the voxel containing the source and the FWHM of the back-projected image was similar to that obtained from the marching method. CONCLUSIONS: The slight deficit to image quality observed with the threshold-based back-projection algorithm described here is outweighed by the 75% reduction in computation time. The implementation of this method requires the development of an optimum threshold function, which determines the overall accuracy of the method. This makes the algorithm well-suited to applications involving the reconstruction of many large images, where the time invested in threshold development is offset by the decreased image reconstruction time. Implemented in a parallel-computing environment, the threshold-based algorithm has the potential to provide real-time dose verification for radiation therapy.

Improving patient safety in radiation oncology.

Beginning in the 1990s, and emphasized in 2000 with the release of an Institute of Medicine report, healthcare providers and institutions have dedicated time and resources to reducing errors that impact the safety and well-being of patients. But in January 2010 the first of a series of articles appeared in the New York Times that described errors in radiation oncology that grievously impacted patients. In response, the American Association of Physicists in Medicine and the American Society of Radiation Oncology sponsored a working meeting entitled "Safety in Radiation Therapy: A Call to Action." The meeting attracted 400 attendees, including medical physicists, radiation oncologists, medical dosimetrists, radiation therapists, hospital administrators, regulators, and representatives of equipment manufacturers. The meeting was cohosted by 14 organizations in the United States and Canada. The meeting yielded 20 recommendations that provide a pathway to reducing errors and improving patient safety in radiation therapy facilities everywhere.

Using Novel Statistical Techniques to Accurately Determine the Predictive Dose Range in a Study of Overall Survival after Definitive Radiotherapy for Stage III Non-Small Cell Lung Cancer in Association with Heart Dose.

PURPOSE: Recent studies of radiotherapy (RT) for stage III non-small-cell lung cancer (NSCLC) have associated high dose to the heart with cardiac toxicity and decreased overall survival (OS). We used advanced statistical techniques to account for correlations between dosimetric variables and more accurately determine the range of heart doses which are associated with reduced OS in patients receiving RT for stage III NSCLC. METHODS: From 2006 to 2013, 119 patients with stage III NSCLC received definitive RT at our institution. OS data was obtained from institutional tumor registry. We used multivariate Cox model to determine patient specific covariates predictive for reduced overall survival. We examined age, prescription dose, mean lung dose, lung V20, RT technique, stage, chemotherapy, tumor laterality, tumor volume, and tumor site as candidate covariates. We subsequently used novel statistical techniques within multivariate Cox model to systematically search the whole heart dose-volume histogram (DVH) for dose parameters associated with OS. RESULTS: Patients were followed until death or 2.5 to 81.2 months (median 30.4 months) in those alive at last follow up. On multivariate analysis of whole heart DVH, the dose of 51 Gy was identified as a threshold dose above which the dose volume relationship becomes predictive for OS. We identified V55Gy (percentage of the whole heart volume receiving at least 55 Gy) as the best single DVH index which can be used to set treatment optimization constraints (Hazard Ratio = 1.044 per 1% increase in heart volume exposed to at least 55 Gy, P = 0.03). Additional characteristics correlated with OS on multivariate analysis were age, stage (IIIA/IIIB), and administration of chemotherapy. CONCLUSION: Doses above 51 Gy, applied to small volumes of the heart, are associated with worse OS in stage III NSCLC patients treated with definitive RT. Higher stage, older age and lack of chemotherapy were also associated with reduced OS.

Dosimetric Assessment of a High Precision System for Mouse Proton Irradiation to Assess Spinal Cord Toxicity.

The uncertainty associated with the relative biological effectiveness (RBE) in proton therapy, particularly near the Bragg peak (BP), has led to the shift towards biological-based treatment planning. Proton RBE uncertainty has recently been reported as a possible cause for brainstem necrosis in pediatric patients treated with proton therapy. Despite this, in vivo studies have been limited due to the complexity of accurate delivery and absolute dosimetry. The purpose of this investigation was to create a precise and efficient method of treating the mouse spinal cord with various portions of the proton Bragg curve and to quantify associated uncertainties for the characterization of proton RBE. Mice were restrained in 3D printed acrylic boxes, shaped to their external contour, with a silicone insert extending down to mold around the mouse. Brass collimators were designed for parallel opposed beams to treat the spinal cord while shielding the brain and upper extremities of the animal. Up to six animals may be accommodated for simultaneous treatment within the restraint system. Two plans were generated targeting the cervical spinal cord, with either the entrance (ENT) or the BP portion of the beam. Dosimetric uncertainty was measured using EBT3 radiochromic film with a dose-averaged linear energy transfer (LETd) correction. Positional uncertainty was assessed by collecting a library of live mouse scans (n = 6 mice, two independent scans per mouse) and comparing the following dosimetric statistics from the mouse cervical spinal cord: Volume receiving 90% of the prescription dose (V90); mean dose to the spinal cord; and LETd. Film analysis results showed the dosimetric uncertainty to be ±1.2% and ±5.4% for the ENT and BP plans, respectively. Preliminary results from the mouse library showed the V90 to be 96.3 ± 4.8% for the BP plan. Positional uncertainty of the ENT plan was not measured due to the inherent robustness of that treatment plan. The proposed high-throughput mouse proton irradiation setup resulted in accurate dose delivery to mouse spinal cords positioned along the ENT and BP. Future directions include adapting the setup to account for weight fluctuations in mice undergoing fractionated irradiation.

Evaluation of Pretreatment Magnetic Resonance Elastography for the Prediction of Radiation-Induced Liver Disease.

PURPOSE: Magnetic resonance (MR) elastography (E) is a noninvasive technique for quantifying liver stiffness (LS) for fibrosis. This study evaluates whether LS is associated with risk of developing radiation-induced liver disease (RILD) in patients receiving liver-directed radiation therapy (RT). METHODS AND MATERIALS: Based on prior studies, LS ≤3 kPa was considered normal and LS >3.0 kPa as representing fibrosis. RILD was defined as an increase in Child-Pugh (CP) score of ≥2 from baseline within 1 year of RT. Univariate and multivariate Cox models were used to assess correlation. RESULTS: One hundred two patients, 51 with primary liver tumors and 51 with liver metastases, were identified with sufficient follow-up. In univariate models, pre-RT LS >3.0 kPa (hazard ratio [HR] 4.9; 95% confidence interval [CI], 1.6-14; P = .004), body mass index (BMI), clinical cirrhosis, CP score, albumin-bilirubin (ALBI) grade 2, primary liver tumor, and mean liver dose were significantly associated with risk of post-RT RILD. In a multivariate analysis, LS >3.0 and mean liver dose both were significantly associated with RILD risk. CONCLUSIONS: Elevated pre-RT LS is associated with an increased risk of RILD in patients receiving liver-directed RT.

Inhibition of ATM Induces Hypersensitivity to Proton Irradiation by Upregulating Toxic End Joining.

Proton Bragg peak irradiation has a higher ionizing density than conventional photon irradiation or the entrance of the proton beam profile. Whether targeting the DNA damage response (DDR) could enhance vulnerability to the distinct pattern of damage induced by proton Bragg peak irradiation is currently unknown. Here, we performed genetic or pharmacologic manipulation of key DDR elements and evaluated DNA damage signaling, DNA repair, and tumor control in cell lines and xenografts treated with the same physical dose across a radiotherapy linear energy transfer spectrum. Radiotherapy consisted of 6 MV photons and the entrance beam or Bragg peak of a 76.8 MeV spot scanning proton beam. More complex DNA double-strand breaks (DSB) induced by Bragg peak proton irradiation preferentially underwent resection and engaged homologous recombination (HR) machinery. Unexpectedly, the ataxia-telangiectasia mutated (ATM) inhibitor, AZD0156, but not an inhibitor of ATM and Rad3-related, rendered cells hypersensitive to more densely ionizing proton Bragg peak irradiation. ATM inhibition blocked resection and shunted more DSBs to processing by toxic ligation through nonhomologous end-joining, whereas loss of DNA ligation via XRCC4 or Lig4 knockdown rescued resection and abolished the enhanced Bragg peak cell killing. Proton Bragg peak monotherapy selectively sensitized cell lines and tumor xenografts with inherent HR defects, and the repair defect induced by ATM inhibitor coadministration showed enhanced efficacy in HR-proficient models. In summary, inherent defects in HR or administration of an ATM inhibitor in HR-proficient tumors selectively enhances the relative biological effectiveness of proton Bragg peak irradiation. SIGNIFICANCE: Coadministration of an ATM inhibitor rewires DNA repair machinery to render cancer cells uniquely hypersensitive to DNA damage induced by the proton Bragg peak, which is characterized by higher density ionization.See related commentary by Nickoloff, p. 3156.

Uncertainty analysis of a Compton camera imaging system for radiation therapy dose reconstruction.

PURPOSE: Improved radiotherapy dose delivery techniques over the past decade have increased the necessity for accurate, independent verification of delivered dose. Compton camera imaging (CCI) systems may have the potential to quantitatively reconstruct three-dimensional dose delivered to the patient with little or no a priori information. METHODS: In this work, the adequacy of a Compton camera imaging system for application to radiotherapy dose reconstruction is explored using analytical models of system spatial and dosimetric resolution. The effects of scatter and absorption detector energy resolution, initial photon energy, and detector separation distance on system performance were calculated with the goal of determining whether current detector technology is adequate for such an application. RESULTS: Results indicate that the energy and spatial resolutions associated with current Si and Ge double-sided strip detectors in a planar configuration is sufficient to determine dose deposition to within an average of 1.9 mm and 2.5%. Minimum values of less than 0.5 mm and 1% are achievable under certain conditions. As the energy of the photon incident on the patient increases from 1.0 to 10 MeV, system performance improves at the expense of the range of patient and detector scattering angles over which the system is capable of reconstructing dose deposition to within the acceptable upper limits of 5 mm and 5%. System performance also improves with increasing distance between the scatter and absorption detectors, but is acceptable throughout the range of values likely to be associated with a gantry-mounted system (2-20 cm). CONCLUSIONS: The results indicate that Compton camera imaging systems based on current solid-state detector technology have the potential to provide independent verification of dose delivered to a patient during radiation therapy. Further consideration must be given to detector efficiency and image reconstruction algorithms for this application of CCI systems.

Systematic offset of kV and MV localization systems as a function of gantry angle.

Mechanical flex of the gantry and mounted imaging panels leads to systematic offsets in localization image isocenter as a function of gantry angle for linear accelerator-mounted image guidance systems. Subsequently, object positions obtained from localization radiographs may be offset, resulting in greater target positioning uncertainty. While current QA procedures measure kV/MV image agreement, these measurements do not provide insight to apparent isocenter position for either single imaging system as a function of gantry rotation. This study measures offset as a function of gantry angle in kV and MV imaging systems on four treatment machines to investigate the magnitude of systematic offsets and their reproducibility between systems and machines, as well as over time. It is shown that each machine and energy has a reproducible pattern of offset as a function of gantry angle that is independent of kV/MV agreement, and it varies by machine. kV and MV offset ranges are on the order of 1.5 mm in the R/L and A/P directions, and 0.5 mm in the S/I direction. Variability of kV-MV agreement is on the order of 0.7 mm. At certain angles, combinations of localization images could show a compounded offset of over 2 mm, exceeding the desired certainty threshold. Since these trends are persistent over time for each machine, online correction for image offsets as a function of gantry angle could improve the margin of positioning uncertainty.

Methodology for radiochromic film analysis using FilmQA Pro and ImageJ.

Radiochromic film (RCF) has several advantageous characteristics which make it an attractive dosimeter for many clinical tasks in radiation oncology. However, knowledge of and strict adherence to complicated protocols in order to produce accurate measurements can prohibit RCF from being widely adopted in the clinic. The purpose of this study was to outline some simple and straightforward RCF fundamentals in order to help clinical medical physicists perform accurate RCF measurements. We describe a process and methodology successfully used in our practice with the hope that it saves time and effort for others when implementing RCF in their clinics. Two RCF analysis software programs which differ in cost and complexity, the commercially available FilmQA Pro package and the freely available ImageJ software, were used to show the accuracy, consistency and limitations of each. The process described resulted in a majority of the measurements across a wide dose range to be accurate within ± 2% of the intended dose using either FilmQA Pro or ImageJ.