DrasCLR: A self-supervised framework of learning disease-related and anatomy-specific representation for 3D lung CT images

Large-scale volumetric medical images with annotation are rare, costly, and time prohibitive to acquire. Self-supervised learning (SSL) offers a promising pre-training and feature extraction solution for many downstream tasks, as it only uses unlabeled data. Recently, SSL methods based on instance discrimination have gained popularity in the medical imaging domain. However, SSL pre-trained encoders may use many clues in the image to discriminate an instance that are not necessarily disease-related. Moreover, pathological patterns are often subtle and heterogeneous, requiring the ability of the desired method to represent anatomy-specific features that are sensitive to abnormal changes in different body parts. In this work, we present a novel SSL framework, named DrasCLR, for 3D lung CT images to overcome these challenges. We propose two domain-specific contrastive learning strategies: one aims to capture subtle disease patterns inside a local anatomical region, and the other aims to represent severe disease patterns that span larger regions. We formulate the encoder using conditional hyper-parameterized network, in which the parameters are dependant on the anatomical location, to extract anatomically sensitive features. Extensive experiments on large-scale datasets of lung CT scans show that our method improves the performance of many downstream prediction and segmentation tasks. The patient-level representation improves the performance of the patient survival prediction task. We show how our method can detect emphysema subtypes via dense prediction. We demonstrate that fine-tuning the pre-trained model can significantly reduce annotation efforts without sacrificing emphysema detection accuracy. Our ablation study highlights the importance of incorporating anatomical context into the SSL framework. Our codes are available at https://github.com/batmanlab/DrasCLR.

ComBat Harmonization: Empirical Bayes versus fully Bayes approaches

Studying small effects or subtle neuroanatomical variation requires large-scale sample size data. As a result, combining neuroimaging data from multiple datasets is necessary. Variation in acquisition protocols, magnetic field strength, scanner build, and many other non-biologically related factors can introduce undesirable bias into studies. Hence, harmonization is required to remove the bias-inducing factors from the data. ComBat is one of the most common methods applied to features from structural images. ComBat models the data using a hierarchical Bayesian model and uses the empirical Bayes approach to infer the distribution of the unknown factors. The empirical Bayes harmonization method is computationally efficient and provides valid point estimates. However, it tends to underestimate uncertainty. This paper investigates a new approach, fully Bayesian ComBat, where Monte Carlo sampling is used for statistical inference. When comparing fully Bayesian and empirical Bayesian ComBat, we found Empirical Bayesian ComBat more effectively removed scanner strength information and was much more computationally efficient. Conversely, fully Bayesian ComBat better preserved biological disease and age-related information while performing more accurate harmonization on traveling subjects. The fully Bayesian approach generates a rich posterior distribution, which is useful for generating simulated imaging features for improving classifier performance in a limited data setting. We show the generative capacity of our model for augmenting and improving the detection of patients with Alzheimer's disease. Posterior distributions for harmonized imaging measures can also be used for brain-wide uncertainty comparison and more principled downstream statistical analysis.Code for our new fully Bayesian ComBat extension is available at https://github.com/batmanlab/BayesComBat.

Left Atrial Mechanics and Diastolic Function Among People Living With Human Immunodeficiency Virus (from the Veterans Aging Cohort Study)

Human immunodeficiency virus (HIV) infection is associated with subclinical cardiomyopathy, diastolic dysfunction, and increased risk of cardiovascular death. However, the relationship between left atrial (LA) mechanics and left ventricular (LV) diastolic function has not been evaluated in people living with HIV (PLWH) relative to HIV-uninfected (HIV-) controls. This is a multicenter, cross-sectional cohort analysis using the HIV Cardiovascular Disease substudy of the Veterans Aging Cohort Study database, which aimed to examine a cohort of PLWH and HIV- veterans without known cardiovascular disease. A total of 277 subjects (180 PLWH, 97 HIV-) with echocardiograms were identified. LV and LA phasic strain were derived and diastolic function was evaluated. Relationship between LA strain, LV strain, and the degree of diastolic dysfunction were assessed using analysis of variance and ordinal logistic regression with propensity weighting. In the PLWH cohort, 91.7% were on antiretroviral therapy and 86.1% had HIV viral loads <500 copies/ml. The mean (± SD) duration of infection was 9.7 ± 4.9 years. Relative to HIV- veterans, PLWH did not differ in LA mechanics and proportion of diastolic dysfunction (p = 0.31). Using logistic regression with propensity weighting, we found no association between HIV status and degree of diastolic dysfunction. In both cohorts, LA reservoir strain and LA conduit strain were inversely and independently associated with the degree of diastolic dysfunction. Compared with HIV- veterans, PLWH who are primarily virally suppressed and antiretroviral-treated did not differ in LA strain or LV diastolic dysfunction. If confirmed in other cohorts, HIV viral suppression may curtail adverse alterations in cardiac structure and function.

1602 A Quality Improvement Project on The Management of Patients with Traumatic Head Injury Presenting to A District General Hospital

Introduction

Traumatic Brain Injury (TBI) is associated with morbidity and mortality. All District General Hospitals (DGH) in West Midlands liaise with Neurosurgery centres for management of these patients through a referral system (NORSe). The aim of this study was to assess outcomes in TBI following the implementation of vital interventions.

Method

A retrospective data was collected between 1/1/2019 and 1/12/2020. We included all patients admitted with traumatic brain injury over the age of 16. We accessed the data from the hospital database, clinical notes, and NORSe. A re-audit was performed following implementations of recommendations (proforma and care of elderly input). Further data were collected prospectively.

Results

Our cohort included n1=61 patients (first cycle) and n2= 29 (second cycle). The demographic age and gender were comparable in both cycles. The median time for Neurosurgery advice was 229 minutes (IQR 60-690) in the first cycle as compared to 80 minutes (IQR 40-120, P &lt; 0.05) and the main cause for delay was inadequate information transfer. The mortality rate was 24% (5 were attributed to sepsis), as compared to 14% in the second cycle. The median length of stay (LOS) was 4 days (IQR 3-13.5) in the first cycle, which translates to a cost of £26,815 per quarter. In comparison, LOS was 3 days (IQR 2-4), which translates to the cost of bed occupancy of £12,771 and saving of £14,043 per quarter, in the second cycle.

Conclusions

The introduction of simple interventions in a DGH can help to significantly improve patient outcomes in those admitted with Traumatic brain injury.

Long-term cancer surveillance: results from the Forteo Patient Registry Surveillance Study

The Forteo Patient Registry estimated the incidence of osteosarcoma in US patients treated with teriparatide and enrolled in the study between 2009 and 2019. No incident cases of osteosarcoma were identified among patients registered, and the crude incidence rate was 0 (95% confidence interval [CI], 0-10.2) cases per million person-years.

PURPOSE

The prospective, voluntary Forteo Patient Registry was established to estimate the incidence of osteosarcoma in patients who have received treatment with teriparatide (Forteo).

METHODS

Information on US adults prescribed teriparatide and enrolled in the Forteo Patient Registry 2009-2019 was linked with data from participating state cancer registries annually (2010-2019) to identify incident osteosarcoma cases using a standardized linkage algorithm. Teriparatide exposure was ascertained from self-reported data that included teriparatide initiation and demographics necessary to complete linkage. Osteosarcoma cases diagnosed on or after January 1, 2009, were identified by participating state cancer registries. The crude incidence rate (IR) and standardized incidence ratio (SIR) of observed cases to the expected number of cases adjusted to the background rate (3 per million person-years) and corresponding 95% CIs for the occurrence of osteosarcoma were calculated whereby the cumulative amount of person-time observed was adjusted for mortality.

RESULTS

Data for 75,247 enrolled patients (representing 361,763 cumulative person-years) were linked to each of 42 participating state cancer registries (covering 93% of the US population), which included information on 6180 cases of osteosarcoma. No matches with incident cases of osteosarcoma following registry enrollment were found. The crude IR was 0 (95% CI, 0-10.2) cases per million person-years and the SIR was 0 (95% CI, 0-3.0).

CONCLUSIONS

The ability to draw conclusions about the incidence of osteosarcoma among patients participating in the registry was limited due to the smaller than expected amount of patient follow-up time and the fact that no cases were identified.

Efficacy and safety of catheter ablation for atrial fibrillation in patients with cancer

Background

Atrial fibrillation (AF) is the most common arrhythmia in patients with cancer. Management of AF in patients with cancer poses unique challenges. Long-term use of antiarrhythmic drug (AAD) therapy lacks evidence of efficacy in this population and poses risk of drug interactions. Catheter ablation is a well-established treatment modality for AAD resistant symptomatic AF and in patients with heart failure. Nevertheless, the effectiveness and safety of catheter ablation in patients with cancer is not well established.

Method

We retrospectively analyzed consecutive patients who underwent catheter ablation for AF, with either history of cancer (other than non-melanoma skin cancer) within 5-years prior or exposure to systemic anthracycline and/or thoracic radiation therapy at any time.

Results

The study included 162 patients. The mean age was 65.5 (26–84 years) years and 50% were female. Overall 133 (82%) patients had freedom from AF at 12 months following ablation. Of these 74 (54%) required post-ablation AAD, 18 (13.5%) required another ablation within the first 12 months and 9 (6.7%) required both AAD and a second ablation to maintain sinus rhythm. There were 14 adverse events (8.6%); 5 access site and 4 non-access site bleeding, 2 strokes, 2 cardiac tamponade and 1 pulmonary vein stenosis with ≈1% serious complications.

Conclusion

The success of catheter ablation for AF and the incidence of procedure related complications in patients with a history of recent cancer or prior exposure to cardiotoxic therapies are similar to that reported in patients without a history of cancer and hence if needed, it should be considered in select patients.

Funding Acknowledgement

Type of funding source: Private hospital(s). Main funding source(s): Dr. S Ganatra is supported by Lahey Physician Research Stipend Program.

A-35 On-Field Dizziness and Timing of Referral to Vestibular Therapy Following Adolescent Sports-Related Concussion: Initial Findings

Objective

To examine if adolescent patients who experienced on-field dizziness immediately following sports-related concussion (SRC) and were referred to vestibular therapy (VT) in 7–9 days returned to play faster when compared to referrals made &gt; 10 days.

Method

Registry data for an outpatient clinic specializing in adolescent SRC was analyzed. Of the 85 adolescent patients included, 67 (79%) experienced dizziness immediately following injury; 36 (54%) of which were referred to VT. Mean age at time of injury was 15.3 years; 61.1% were male (n = 22); most were injured while participating in football (38.9%), soccer (16.7%), or wrestling (13.9%). Days to initial VT evaluation from time of injury and days from VT to clearance from concussion protocol were analyzed using a Poisson regression model; age and sex were controlled.

Results

Patients referred to VT 7–9 days post-injury returned to play 16 ± 13.4 days earlier (20.7 ± 18.6 days; p &lt; .0001) than patients who received VT 10–20 days post SRC (36.7 days±32). While non-significant, individuals referred to VT &gt; 21 days post-injury returned to play 6 days later than those referred &lt; 10 days (26.3 ± 32.9; p = .14).

Conclusions

Appropriate and timely referrals to VT following the presence of on-field dizziness after SRC may play an integral role in recovery, although more research in this area is needed. Initial findings suggest that when determining optimal time frame for referral to VT, 7–9 days post-injury may be most beneficial for adolescents following SRC.

Rapid, complete and sustained tumour response to the TRK inhibitor larotrectinib in an infant with recurrent, chemotherapy-refractory infantile fibrosarcoma carrying the characteristic ETV6-NTRK3 gene fusion

BACKGROUND

The ETV6-NTRK3 gene fusion is present in the majority of cases of infantile fibrosarcoma (IFS) and acts as a potent oncogenic driver. We report the very rapid, complete, and sustained response of an advanced, chemotherapy-refractory, recurrent IFS to targeted treatment with the oral tropomyosin receptor kinase (TRK) inhibitor larotrectinib.

PATIENT AND METHODS

A male infant born with a large congenital IFS of the tongue had the tumour surgically resected at age 4 days. Within 2 months, he developed extensive lymph node recurrence that progressed during two cycles of vincristine-doxorubicin-cyclophosphamide chemotherapy. At screening, a large right cervical mass was clinically visible. Magnetic resonance imaging (MRI) revealed bilateral cervical and axillary lymph node involvement as well as infiltration of the floor of the mouth. The largest lesion measured 5.5×4.5×4.4 cm (ca. 55 cm3). The patient started outpatient oral larotrectinib at 20 mg/kg twice daily at age 3.5 months.

RESULTS

After 4 days on treatment, the parents noted that the index tumour was visibly smaller and softer. The rapid tumour regression continued over the following weeks. On day 56 of treatment, the first scheduled control MRI showed the target lesion had shrunk to 1.2×1.2×0.8 cm (ca. 0.6 cm3), corresponding to a complete response according to the Response Evaluation Criteria In Solid Tumors version 1.1. This response was maintained over subsequent follow-up visits, and on day 112 at the second control MRI the target lymph node was completely normal. At last follow-up, the disease remained in complete remission after 16 months on larotrectinib, with negligible toxicity and no safety concerns.

CONCLUSION(S)

Selective TRK inhibition by larotrectinib offers a novel, highly specific and highly effective therapeutic option for IFS carrying the characteristic ETV6-NTRK3 gene fusion. Its use should be considered when surgery is not feasible. (NCT02637687).

Skull fractures in forensic putrefied/skeletonised cases: The challenge of estimating the post-traumatic interval

The objective of our study was to assess the reliability of the estimation of posttraumatic survival time (PTST) in forensic cases based on microCT and histology of putrefied/dry bone samples with comparison of initial macroscopic fracture classification performed during autopsy. Macroscopic morphological patterns of bone fracture are routinely used in forensic pathology and anthropology to distinguish between antemortem, perimortem and postmortem injuries. Based on macroscopic and microscopic analysis of six craniofacial fractures, our study results illustrate the need to complete macroscopical findings and initial fracture classification with microscopic analysis to avoid any inaccuracy. MicroCT has become a powerful technique to identify early bone healing signs but histology remains the gold standard to estimate the PTST and determine vital fracture based on hemorrhage marker. Raman microspectroscopy can identify a blood clot in the fracture line.

Examining the relationship between obesity and mental health outcomes among individuals admitted to a level I trauma centre

The increased incidence of obesity in the general population translates into clinicians caring for an increased number of trauma patients with obesity. Previous research has documented the unique anatomical and physiological challenges that clinicians face when caring for trauma patients with obesity; however, little is known about psychological challenges that trauma patients with obesity may also experience in the months following injury. The aim of this study is to determine the relationship between obesity and (i) mental health, (ii) demographic and injury-related variables and (iii) quality-of-life outcomes among trauma patients between hospitalization and 3-months post-injury. This is a prospective, longitudinal study conducted between March 2012 and May 2014 at a single, level I trauma centre in the southwest United States. Inclusion criteria for this convenience sample consisted of patients who were admitted to the trauma or orthopaedic trauma service ≥24 h, medically stable, spoke English or Spanish and ≥18 years of age. In total, 455 eligible patients were consented and enrolled; 343 (70.87%) completed 3-month follow-up. The objective of this study is to investigate the relationship between obesity and mental health among trauma patients in the months following injury. Demographic and injury-related data were also collected; patients' height and weight were used to determine body mass index. Health outcomes were assessed during initial hospitalization and at 3-month follow-up and included depression, post-traumatic stress symptoms, pain and return to work. Prior to data collection, it was hypothesized that obesity would have a negative effect on mental health outcomes among patients 3 months post-injury. The final sample consisted of 343 participants; average age was 46.4 ± 17.3 years; majority male (n = 213, 63%) and Caucasian (n = 231, 69%). Patients with obesity had higher odds of screening positive for depression (odds ratio [OR] = 2.36, P = 0.02) and overweight patients had lower odds of returning to work (OR = 0.31, P = 0.01) 3 months post-injury compared to patients of normal weight (65% vs. 40%). No other significant differences were found. Results of the current study are novel in that they identify psychological challenges that overweight and trauma patients with obesity may experience. These results demonstrate the need for mental health professionals to be involved in follow-up care to extending in the months following injury.

Monte Carlo determination of k Q and k Qmsr values for the exradin A26 ionisation chamber for the Varian TrueBeam

We have calculated conversion factors, k _Q for the A26 micro ionisation chamber along with machine specific reference beam quality factors, k _Qmsr, for a number of field sizes and beam qualities for the Varian TrueBeam accelerator. The A12 ionisation chamber was simulated alongside the A26, so as to validate against known literature values. Both ionisation chambers were modelled from manufacturer data sheets and schematics. The egs_chamber Monte Carlo user code was used to simulate each absorbed dose relevant to the beam quality conversion factors k _Q and k _Qmsr. Tabulated spectra for beam energies of 4 through 25 MV were used in the k _Q calculations for both investigated chambers. Varian TrueBeam phase space files for 6 MV flattened as well as 6 and 10 MV unflattened beams were used in the simulations of the A26 chamber in field sizes from 2  ×  2 cm square to 20  ×  20 cm square in order to determine k _Qmsr values. The PDD(10)x  values of the tabulated spectra were found to be within variation between studies, with an average deviance of 0.4% from one prior study. The simulated A12 k _Q values matched the accepted literature values with an average variation of  <0.1%. The A26 k _Q values match the manufacturer provided values to within 0.5%. For all investigated field sizes the k _Qmsr values are within 0.006 of unity. There is no published data for this chamber for a direct comparison, but there is similarity between these results and results from other chambers regularly used in similar circumstances. Furthermore, the agreement of the simulated k _Q values to knowns, and the agreement of the PDD(10)x  factors would suggest the correctness and accuracy of the study.

Assessing the quality, reliability and readability of online health information regarding systemic lupus erythematosus

Introduction Systemic lupus erythematosus (SLE) has complex pathophysiology and treatments, and patients often use the internet to better understand their condition. This report systematically assesses the quality, reliability and readability of online information. Methods The search term 'systemic lupus erythematosus' was used with Google™, Bing™ and Yahoo™ search engines sequentially. The first 25 websites returned ('hits') for each search engine (total 75 websites) were compiled. The search terms 'SLE' and 'lupus' were used in separate Google searches to assess for commonality. After removal of excluded hits, websites were assessed using the DISCERN instrument, Journal of the American Medical Association benchmarks and Gunning Fog Index for quality, reliability and readability and presence of 'Health on the Net Code' (HoN) standardisation recorded. Results There was a large degree of commonality among hits from the three different search engines using the search term 'systemic lupus erythematosus', as well as hits returned for the three different search terms using Google. The mean DISCERN score was 47.7 (SD 13.2) for 'systemic lupus erythematosus', 46.4 (SD 14.2) for 'SLE' and 45.2 (SD 10.1) for 'lupus', with no statistically significant difference. The mean number of JAMA benchmarks (maximum four) present for the 'systemic lupus erythematosus', 'SLE' and 'lupus' searches was 1.3 (SD 1.2), 1.4 (SD 1.3) and 1.2 (SD 1.0), respectively, with no statistically significance difference. The average readability of hits for the three different search terms was 9.3 (SD 3.4), 10.0 (SD 3.1) and 11.1 (SD 2.7), with no statistically significant difference. Conclusion There was a large degree of commonality of hits among the different search engines and the utilised search terms but they are not synonymous. Regardless of search term, the overall quality of websites was fair, whilst reliability was poor. Websites appearing higher in searches did not score better. Presence of the HoN did not represent better quality. Readability was higher than recommended for near-universal understanding. There was no difference in quality, reliability or readability of websites using the search terms 'systemic lupus erythematosus', 'SLE' or 'lupus', with some high-scoring websites appearing in only one search term result. This study reminds clinicians to direct patients to high-quality websites rather than rely on search engines.

Angioedema in chronic spontaneous urticaria is underdiagnosed and has a substantial impact: Analyses from ASSURE-CSU

Background

ASSURE‐CSU revealed differences in physician and patient reporting of angioedema. This post hoc analysis was conducted to evaluate the actual rate of angioedema in the study population and explore differences between patients with and without angioedema.

Methods

This international observational study assessed 673 patients with inadequately controlled chronic spontaneous urticaria (CSU). Physicians abstracted angioedema data from medical records, which were compared with patient‐reported data. Patients in the Yes‐angioedema category had angioedema reported in the medical record and a patient‐reported source. For those in the No‐angioedema category, angioedema was reported in neither the medical record nor a patient‐reported source. Those in the Misaligned category had angioedema reported in only one source. Statistical comparisons between Yes‐angioedema and No‐angioedema categories were conducted for measures of CSU activity, health‐related quality of life (HRQoL), productivity and healthcare resource utilization (HCRU). Regression analyses explored the relationship between Dermatology Life Quality Index (DLQI) score and angioedema, adjusting for important covariates.

Results

Among evaluable patients, 259 (40.3%), 173 (26.9%) and 211 (32.8%) were in the Yes‐angioedema, No‐angioedema and Misaligned category, respectively. CSU activity and impact on HRQoL, productivity, and HCRU was greater for Yes‐angioedema patients than No‐angioedema patients. After covariate adjustment, mean DLQI score was significantly higher (indicating worse HRQoL) for patients with angioedema versus no angioedema (9.88 vs 7.27, P < .001). The Misaligned category had similar results with Yes‐angioedema on all outcomes.

Conclusions

Angioedema in CSU seems to be under‐reported but has significant negative impacts on HRQoL, daily activities, HCRU and work compared with no angioedema.

Indications for delivery in pre-eclampsia

OBJECTIVE

Examine the frequency with which the most accepted indicators for delivery in pre-eclampsia are used in a population with predominantly late-onset (birth > 32 weeks) pre-eclampsia (PE).

METHODS

Retrospective cohort study using the St George Public Hospital (SGH) Hypertension in Pregnancy database. Demographic, pregnancy, and outcome details were extracted and verified by comparison with data collection sheets.

RESULTS

From 2001 to 2013, 908 women (970 babies) with PE were included, of which a subgroup of 303 women (33%) had clearly delineated delivery triggers available. This subgroup of women had similar demographic and outcome characteristics to the total PE population. In this group, the most common maternal trigger for delivery apart from gestational age 37+ weeks was difficult to control/severe hypertension (114 cases, 38%) and the most common fetal trigger intrauterine growth restriction (IUGR: 14 cases, 4%). 78 (35%) of term women had no specific delivery trigger other than gestation. A primary maternal trigger and/or associated complication was slightly more common in those delivering <37 weeks vs 37+ weeks (52 vs 38%, p = .03), while a fetal or combined maternal/fetal complication was over four times more common in preterm women (25 vs 6%, p < .001).

CONCLUSION

In our population of predominantly late-onset PE, maternal triggers for delivery (predominantly severe hypertension) far outweigh fetal triggers (predominantly IUGR). Fetal and mixed indicators for delivery were relatively more common in women delivering preterm, possibly reflecting the severity of placental dysfunction in this subgroup.

Technical Note: Response measurement for select radiation detectors in magnetic fields

PURPOSE

Dose response to applied magnetic fields for ion chambers and solid state detectors has been investigated previously for the anticipated use in linear accelerator-magnetic resonance devices. In this investigation, the authors present the measured response of selected radiation detectors when the magnetic field is applied in the same direction as the radiation beam, i.e., a longitudinal magnetic field, to verify previous simulation only data.

METHODS

The dose response of a PR06C ion chamber, PTW60003 diamond detector, and IBA PFD diode detector is measured in a longitudinal magnetic field. The detectors are irradiated with buildup caps and their long axes either parallel or perpendicular to the incident photon beam. In each case, the magnetic field dose response is reported as the ratio of detector signals with to that without an applied longitudinal magnetic field. The magnetic field dose response for each unique orientation as a function of magnetic field strength was then compared to the previous simulation only studies.

RESULTS

The measured dose response of each detector in longitudinal magnetic fields shows no discernable response up to near 0.21 T. This result was expected and matches the previously published simulation only results, showing no appreciable dose response with magnetic field.

CONCLUSIONS

Low field longitudinal magnetic fields have been shown to have little or no effect on the dose response of the detectors investigated and further lend credibility to previous simulation only studies.

Dose response of selected solid state detectors in applied homogeneous transverse and longitudinal magnetic fields

PURPOSE

MR-Linac devices under development worldwide will require standard calibration, commissioning, and quality assurance. Solid state radiation detectors are often used for dose profiles and percent depth dose measurements. The dose response of selected solid state detectors is therefore evaluated in varying transverse and longitudinal magnetic fields for this purpose.

METHODS

The Monte Carlo code PENELOPE was used to model irradiation of a PTW 60003 diamond detector and IBA PFD diode detector in the presence of a magnetic field. The field itself was varied in strength, and oriented both transversely and longitudinally with respect to the incident photon beam. The long axis of the detectors was oriented either parallel or perpendicular to the photon beam. The dose to the active volume of each detector in air was scored, and its ratio to dose with zero magnetic field strength was determined as the "dose response" in magnetic field. Measurements at low fields for both detectors in transverse magnetic fields were taken to evaluate the accuracy of the simulations. Additional simulations were performed in a water phantom to obtain few representative points for beam profile and percent depth dose measurements.

RESULTS

Simulations show significant dose response as a function of magnetic field in transverse field geometries. This response can be near 20% at 1.5 T, and it is highly dependent on the detectors' relative orientation to the magnetic field, the energy of the photon beam, and detector composition. Measurements at low transverse magnetic fields verify the simulations for both detectors in their relative orientations to radiation beam. Longitudinal magnetic fields, in contrast, show little dose response, rising slowly with magnetic field, and reaching 0.5%-1% at 1.5 T regardless of detector orientation. Water tank and in air simulation results were the same within simulation uncertainty where lateral electronic equilibrium is present and expectedly differed at the beam edge in transverse field orientations only. Due to the difference in design, the two detectors behaved differently.

CONCLUSIONS

When transverse magnetic fields are present, great care must be taken when using diamond or diode detectors. Dose response varies with relative detector orientation, magnetic field strength, and between detectors. This response can be considerable (∼20% for both detectors). Both detectors in longitudinal fields exhibit little to no dose response as a function of magnetic field. Water tank simulations seem to suggest that the diode detector is better suited to general beam commissioning, and each detector must be investigated separately.

Dose response of selected ion chambers in applied homogeneous transverse and longitudinal magnetic fields

PURPOSE

The magnetic fields of an integrated MR-Linac system will alter the paths of electrons that produce ions in the ionization chambers. The dose response of selected ion chambers is evaluated in the presence of varying transverse and longitudinal magnetic fields. The investigation is useful in calibration of therapeutic x-ray beams associated with MR-Linac systems.

METHODS

The Monte Carlo code PENELOPE was used to model the irradiation of NE2571, and PR06C ionization chambers in the presence of a transverse and longitudinal (with respect to the photon beam) magnetic fields of varying magnitude. The long axis of each chamber was simulated both parallel and perpendicular to the incident photon beam for each magnetic field case. The dose deposited in each chamber for each case was compared to the case with zero magnetic field by means of a ratio. The PR06C chamber's response was measured in the presence of a transverse magnetic field with field strengths ranging from 0.0 to 0.2 T to compare to simulated results.

RESULTS

The simulations and measured data show that in the presence of a transverse magnetic field there is a considerable dose response (maximum of 11% near 1.0 T in the ion chambers investigated, which depends on the magnitude of magnetic field, and relative orientation of the magnetic field, radiation beam, and ion chamber. Measurements made with the PR06C chamber verify these results in the region of measurement. In contrast, a longitudinal magnetic field produces only a slight increase in dose response (2% at 1.5 T) that rises slowly with increasing magnetic field and is seemingly independent of chamber orientation. Response trends were similar for the two ion chambers and relative orientations considered, but slight variations are present from chamber to chamber.

CONCLUSIONS

Care must be taken when making ion chamber measurements in a transverse magnetic field. Ion chamber responses vary not only with transverse field strength, but with chamber orientation and type, and can be considerable. Longitudinal magnetic fields influence ion chamber responses relatively little (2% at 1.5 T), and only at field strengths in excess of 1.0 T.