J Pollard

Radiotherapy-Induced Malfunction in Contemporary Cardiovascular Implantable Electronic Devices: Clinical Incidence and Predictors.

IMPORTANCE Risk stratification and management paradigms for patients with cardiovascular implantable electronic devices (CIEDs) requiring radiotherapy (RT) vary widely and are based on limited clinical data. OBJECTIVE To identify the incidence and predictors of CIED malfunction and describe associated clinical consequences in a large cohort of patients treated with photon- and electron-based RT. DESIGN, SETTING, AND PARTICIPANTS Retrospective analysis of all patients with a functioning CIED who underwent RT between August 2005 and January 2014 with CIED interrogation data following RT at an academic cancer center. We identified 249 courses of photon- and electron-based RT in 215 patients (123 pacemakers [57%]; 92 implantable cardioverter-defibrillators [43%]). Substantial neutron production was generated in 71 courses (29%). EXPOSURE Implantation of CIED with subsequent therapeutic radiation treatment (neutron producing with 15- or 18-MV photons and non-neutron producing with electrons, GammaKnife, or 6-MV photons). MAIN OUTCOMES AND MEASURES Malfunction of CIED, characterized as single-event upset (data loss, parameter resets, unrecoverable resets), and delayed effects including signal interference, pacing threshold changes, and premature battery depletion. RESULTS Malfunction of CIED attributable to RT occurred during 18 courses (7%), with 15 CIEDs experiencing single-event upsets, and 3, transient signal interference. All single-event upsets occurred during neutron-producing RT, at a rate of 21%, 10%, and 34% per neutron-producing course for CIEDs, pacemakers, and implantable cardioverter-defibrillators, respectively. No single-event upsets were found among 178 courses of non-neutron-producing RT. Incident CIED dose did not correlate with device malfunction. Patients treated to the abdomen and pelvis region were more likely to undergo a single-event upset (hazard ratio, 5.2 [95% CI, 1.2-22.6]; P = .03). Six patients with a CIED parameter reset developed clinical symptoms: 3 experienced hypotension and/or bradycardia, 2 experienced abnormal chest ticking consistent with pacemaker syndrome, and 1 developed congestive heart failure. The 3 episodes of signal interference did not result in clinical effects. No delayed malfunctions were directly attributed to RT. CONCLUSIONS AND RELEVANCE In a cohort of contemporary CIEDs, all cases of single-event upset malfunction occurred in the setting of notable neutron production, at a rate of 21% for neutron-producing RT and 0% for non-neutron-producing RT. Where clinically feasible, the use of non-neutron-producing RT is recommended. Given the lack of correlation between CIED malfunction and incident dose observed up to 5.4 Gy, invasive CIED relocation procedures in these settings can be minimized.

Closing the Cancer Divide Through Ubuntu: Information and Communication Technology-Powered Models for Global Radiation Oncology

“The chance for a cure, the chance to live, should no longer remain an accident of geography” (1). This is one of the key messages in “Closing the cancer divide: A blueprint to expand access in low and middle income countries” (1). This article highlights the growing burden of global cancer disparities and makes a compelling case that the time for unified action to close this divide is now. There is growing consensus that information and communication technologies (ICTs) have tremendous potential to catalyze global health collaborations. Advanced ICTs can be used to leverage the recent major upsurge in global health interest into greater space-time flexible collaborative action against cancer and for enhancing greater effectiveness of existing global health initiatives. The recent call for greater action in closing the cancer divide through collaborations, including that in International Journal of Radiation, Oncology, Biology, Physics (IJROBP), inspired the 2015 Global Health Catalyst cancer summit, which brought together a unique combination of global oncology leaders, diaspora leaders, and ICT and palliative care experts, industry, nonprofits, and policy makers. The summit provided a forum for networking, knowledge sharing, and discussion of some of the emerging models for ICT-powered global health collaborations in radiation oncology care, research, and education, as well as avenues for complementary outreach, including engagement with the diaspora. This article summarizes the discussions and recommendations from the summit and highlights the emerging ICT-powered models for radiation oncology global health, avenues for greater outreach (ubuntu, a term signifying the idea that “I am because we are,” or human connectedness [see discussion below]) for greater impact and sustainability, as well as emerging areas for scaling up and increased action toward closing the cancer divide. At the primary level, a distressing illustration of the cancer divide can be seen in Africa, where most of Africa’s more than 2000 languages do not even have a word for cancer (2). Thus, in that geography, many people die painfully of cancer and, sadly, do not know it. In areas more familiar with cancer, a great lack of cancer prevention education or awareness of the importance of early detection contributes to over one third of preventable cancer deaths (3). This problem is further exacerbated by a culture of silence and strong social stigma associated with the disease (4); even young doctors do not want to specialize in oncology, a medical area that talks only about pain and death. The stigma also means that the overwhelming majority of patients only present late with the disease when it is too late to cure them; the ensuing deaths then further reinforce the stigma that cancer is essentially a death sentence. At a secondary level, the cancer divide is illustrated by the lack of capacity to manage patients once their disease is diagnosed, a problem inherent in poor health care systems. For example, approximately half of Africa’s 54 countries still have no radiation therapy services typically needed in the treatment of more than 50% of cancer patients (5). Limitations to radiation therapy in low- and middle-income countries (LMICs) include the number of radiation therapy centers, the number of treatment units, the critical shortage in health care workforce, the lack of safety regulatory infrastructure, and the perception that radiation therapy is a complex and expensive solution. Without greater investment and collaboration in radiation therapy services, this will only exacerbate the burden of cancer and make the cancer divide worse. Meanwhile, at the tertiary level, the cancer divide is appropriately captured by what has been called “the pain divide” (6). Here, many people dying with cancer do so in excruciating pain, due to a lack of basic pain medication and other palliative options. Such harrowing deaths with needless suffering bolster the physical and social trauma of cancer and the reason why many people in LMICs do not even want to talk about cancer. A word of African origin, which people do like to talk about, is ubuntu. Popularized worldwide by African Nobel Prize winners Desmond Tutu and Nelson Mandela, ubuntu signifies the idea that “I am because we are,” or human connectedness. This ethos rings particularly true in today’s hyperconnected world, where we all share in the bounty of the expanding internet or ICTs and where local health has become global health and vice versa. Ubuntu also represents an operating system underlying ICTs used for cloud computing, including in radiation oncology. The recent call for greater action in closing the cancer divide through collaborations (1, 7–9), including more recently in radiation oncology (8), inspired the 2015 Global Health Catalyst (GHC) cancer summit (10), which brought together a unique combination of global oncology leaders, industry, policy makers, and African diaspora leaders. Here the African diaspora refers to Africans settled outside of the African continent. Building on a recent publication (11), a central theme of the summit was the use of ICTs to catalyze high-impact international collaborations in cancer care, research, and education with Africa. This article summarizes the summit proceedings and highlights the emerging ICT-powered models for radiation oncology global health, avenues for greater partnership (ubuntu), and outreach beyond the traditional, as well as emerging areas for scaling up and increased action toward closing the cancer divide.

TU-H-CAMPUS-TeP2-01: A Comparison of Noninvasive Techniques to Assess Radiation-Induced Lung Damage in Mice

PURPOSE To evaluate the use of post-irradiation changes in respiratory rate and CBCT-based morphology as predictors of survival in mice. METHODS C57L/J mice underwent whole-thorax irradiation with a Co-60 beam to four different doses [0Gy (n=3), 9Gy (n=5), 11Gy (n=7), and 13Gy (n=5)] in order to induce varying levels of pneumonitis. Respiratory rate measurements, breath-hold CBCTs, and free-breathing CBCTs were acquired pre-irradiation and at six time points between two and seven months post-irradiation. For respiratory rate measurements, we developed a novel computer-vision-based technique. We recorded mice sleeping in standard laboratory cages with a 30 fps, 1080p webcam (Logitech C920). We calculated respiratory rate using corner detection and optical flow to track cyclical motion in the fur in the recorded video. Breath-hold and free-breathing CBCTs were acquired on the X-RAD225Cx system. For breathhold imaging, the mice were intubated and their breath was held at full-inhale for 20 seconds. Healthy lung tissue was delineated in the scans using auto-threshold contouring (0-0.7 g/cm3 ). The volume of healthy lung was measured in each of the scans. Next, lung density was measured in a 6-mm2 ROI in a fixed anatomic location in each of the scans. RESULTS Day-to-day variability in respiratory rate with our technique was 13%. All metrics except for breath-hold lung volume were correlated with survival: lung density on free-breathing (r=-0.7482,p<0.01) and breath-hold images (r=-0.5864,p<0.01), free-breathing lung volume (r=0.7179,p<0.01), and respiratory rate (r= 0.6953,p<0.01). Lung density on free-breathing scans was correlated with respiratory rate (r=0.7142,p<0.01) and lung density on breath-hold scans (r=0.5543,p<0.01). One significant practical hurdle in the CBCT measurements was that at least one lobe of the lung was collapsed in 36% of free-breathing scans and 45% of breath-hold scans. CONCLUSION Lung density and lung volume on free-breathing CBCTs and respiratory rate outperform breath-hold CBCT measurements as indicators for survival from radiation-induced pneumonitis. This work was partially funded by Elekta.

WE-H-201-01: The Opportunities and Benefits of Helping LMICs: How Helping Them Can Help You

The desperate need for radiotherapy in low and mid-income countries (LMICs) has been well documented. Roughly 60 % of the worldwide incidence of cancer occurs in these resource-limited settings and the international community alongside governmental and non-profit agencies have begun publishing reports and seeking help from qualified volunteers. However, the focus of several reports has been on how dire the situation is and the magnitude of the problem, leaving most to feel overwhelmed and unsure as to how to help and why to get involved. This session will help to explain the specific ways that Medical Physicists can uniquely assist in this grand effort to help bring radiotherapy to grossly-underserved areas. Not only can these experts fulfill an important purpose, they also can benefit professionally, academically, emotionally and socially from the endeavor. By assisting others worldwide with their skillset, Medical Physicists can end up helping themselves. LEARNING OBJECTIVES 1. Understand the need for radiotherapy in LMICs. 2. Understand which agencies are seeking Medical Physicists for help in LMICs. 3. Understand the potential research funding mechanisms are available to establish academic collaborations with LMIC researchers/physicians. 4. Understand the potential social and emotional benefits for both the physicist and the LMIC partners when collaborations are made. 5. Understand the potential for collaboration with other high-income scientists that can develop as the physicist partners with other large institutions to assist LMICs. Wil Ngwa - A recent United Nations Study reports that in developing countries more people have access to cell phones than toilets. In Africa, only 63% of the population has access to piped water, yet, 93% of Africans have cell phone service. Today, these cell phones, Skype, WhatsApp and other information and communication technologies (ICTs) connect us in unprecedented ways and are increasingly recognized as powerful, indispensable to global health. Thanks to ICTs, there are growing opportunities for Medical Physicists to reach out beyond the bunker and impact the world far beyond, without even having to travel. These growing opportunities in global health for Medical Physicists, powered by ICTs, will be highlighted in this presentation, illustrated by high impact examples/models across the globe that are improving patient safety and healthcare outcomes, saving lives. LEARNING OBJECTIVES 1. Published definitions of global health and the emerging field of global radiation oncology 2. Learn about the transformative potential of ICTs in global radiation oncology care, research and education with focus on Medical Physics 3. Learn about high impact examples of ICT-powered global radiation oncology and the increasing opportunities for participation by Medical Physicists. Yakov Pipman - The number and scope of volunteer Medical Physics activities in support of low-to-middle income countries has been increasing gradually. This happens through a variety of formal channels and to some extent through less formal but personal initiatives. A good deal of effort is dedicated by many, but many more could be recruited through a structured framework to volunteer. We will look into typical volunteer activities and how they fit with organizations already involved in advancing Medical Physics in LMIC. We will identify the range of these organizational activities and their scope to reveal areas of further need. We will point to a few key features of MPWB (www.mpwb.org) as a volunteering and collaborating structure and how members can get involved and contribute to these efforts at the grass roots level. The goal is that scarce resources can thus be channeled to complement rather than compete with those already in place. LEARNING OBJECTIVES 1. Understand the strengths and limitations of various organizations that support Medical Physics efforts in LMIC. 2. Learn about ways to volunteer and contribute to Global Health through a grass roots organization focused on Medical Physics in LMIC. Perry Sprawls - With the growing capability and complexity of medical imaging methods in all countries of the world, the expanding role of medical physicists includes optimizing imaging procedures with respect to image quality, radiation dose, and other conflicting factors. With access to appropriate educational resources local medical physicists in all countries can provide direct clinical support and educational for other medical professionals. This is being supported through the process of Collaborative Teaching that combines the capabilities and experience of medical physicists in countries spanning the spectrum of economic, technological, and clinical development. The supporting resources are on the web at: www.sprawls.org/resources. LEARNING OBJECTIVES 1. Identify the medical physics educational needs to support effective and optimized medical imaging procedures. 2. Use collaborative teaching resources to enhance the role of medical physicists in all countries of the world.

WE-H-201-00: Opportunities for Physicists to Support Low and Mid-Income Countries

The desperate need for radiotherapy in low and mid-income countries (LMICs) has been well documented. Roughly 60 % of the worldwide incidence of cancer occurs in these resource-limited settings and the international community alongside governmental and non-profit agencies have begun publishing reports and seeking help from qualified volunteers. However, the focus of several reports has been on how dire the situation is and the magnitude of the problem, leaving most to feel overwhelmed and unsure as to how to help and why to get involved. This session will help to explain the specific ways that Medical Physicists can uniquely assist in this grand effort to help bring radiotherapy to grossly-underserved areas. Not only can these experts fulfill an important purpose, they also can benefit professionally, academically, emotionally and socially from the endeavor. By assisting others worldwide with their skillset, Medical Physicists can end up helping themselves. LEARNING OBJECTIVES 1. Understand the need for radiotherapy in LMICs. 2. Understand which agencies are seeking Medical Physicists for help in LMICs. 3. Understand the potential research funding mechanisms are available to establish academic collaborations with LMIC researchers/physicians. 4. Understand the potential social and emotional benefits for both the physicist and the LMIC partners when collaborations are made. 5. Understand the potential for collaboration with other high-income scientists that can develop as the physicist partners with other large institutions to assist LMICs. Wil Ngwa - A recent United Nations Study reports that in developing countries more people have access to cell phones than toilets. In Africa, only 63% of the population has access to piped water, yet, 93% of Africans have cell phone service. Today, these cell phones, Skype, WhatsApp and other information and communication technologies (ICTs) connect us in unprecedented ways and are increasingly recognized as powerful, indispensable to global health. Thanks to ICTs, there are growing opportunities for Medical Physicists to reach out beyond the bunker and impact the world far beyond, without even having to travel. These growing opportunities in global health for Medical Physicists, powered by ICTs, will be highlighted in this presentation, illustrated by high impact examples/models across the globe that are improving patient safety and healthcare outcomes, saving lives. LEARNING OBJECTIVES 1. Published definitions of global health and the emerging field of global radiation oncology 2. Learn about the transformative potential of ICTs in global radiation oncology care, research and education with focus on Medical Physics 3. Learn about high impact examples of ICT-powered global radiation oncology and the increasing opportunities for participation by Medical Physicists. Yakov Pipman - The number and scope of volunteer Medical Physics activities in support of low-to-middle income countries has been increasing gradually. This happens through a variety of formal channels and to some extent through less formal but personal initiatives. A good deal of effort is dedicated by many, but many more could be recruited through a structured framework to volunteer. We will look into typical volunteer activities and how they fit with organizations already involved in advancing Medical Physics in LMIC. We will identify the range of these organizational activities and their scope to reveal areas of further need. We will point to a few key features of MPWB (www.mpwb.org) as a volunteering and collaborating structure and how members can get involved and contribute to these efforts at the grass roots level. The goal is that scarce resources can thus be channeled to complement rather than compete with those already in place. LEARNING OBJECTIVES 1. Understand the strengths and limitations of various organizations that support Medical Physics efforts in LMIC. 2. Learn about ways to volunteer and contribute to Global Health through a grass roots organization focused on Medical Physics in LMIC. Perry Sprawls - With the growing capability and complexity of medical imaging methods in all countries of the world, the expanding role of medical physicists includes optimizing imaging procedures with respect to image quality, radiation dose, and other conflicting factors. With access to appropriate educational resources local medical physicists in all countries can provide direct clinical support and educational for other medical professionals. This is being supported through the process of Collaborative Teaching that combines the capabilities and experience of medical physicists in countries spanning the spectrum of economic, technological, and clinical development. The supporting resources are on the web at: www.sprawls.org/resources. LEARNING OBJECTIVES 1. Identify the medical physics educational needs to support effective and optimized medical imaging procedures. 2. Use collaborative teaching resources to enhance the role of medical physicists in all countries of the world.

TH‐CD‐BRA‐01: BEST IN PHYSICS (THERAPY): ‐Field‐Induced Dose Effects in a Mouse Lung Phantom: Monte Carlo and Experimental Assessments

PURPOSE To simulate and measure magnetic-field-induced radiation dose effects in a mouse lung phantom. This data will be used to support pre-clinical experiments related to MRI-guided radiation therapy systems. METHODS A mouse lung phantom was constructed out of 1.5×1.5×2.0-cm3 lung-equivalent material (0.3 g/cm3 ) surrounded by a 0.6-cm solid water shell. EBT3 film was inserted into the phantom and the phantom was placed between the poles of an H-frame electromagnet. The phantom was irradiated with a cobalt-60 beam (1.25 MeV) with the electromagnet set to various magnetic field strengths (0T, 0.35T, 0.9T, and 1.5T). These magnetic field strengths correspond to the range of field strengths seen in MRI-guided radiation therapy systems. Dose increases at the solid-water-to-lung-interface and dose decreases at the lung-to-solid-water interface were compared with results of Monte Carlo simulations performed with MCNP6. RESULTS The measured dose to lung at the solid-water-to-lung interface increased by 0%, 16%, and 29% with application of the 0.35T, 0.9T, and 1.5T magnetic fields, respectively. The dose to lung at the lung-to-solid-water interface decreased by 4%, 18%, and 24% with application of the 0.35T, 0.9T, and 1.5T magnetic fields, respectively. Monte Carlo simulations showed dose increases of 0%, 16%, and 31% and dose decreases of 4%, 16%, and 25%. CONCLUSION Only small dose perturbations were observed at the lung-solid-water interfaces for the 0.35T case, while more substantial dose perturbations were observed for the 0.9T and 1.5T cases. There is good agreement between the Monte Carlo calculations and the experimental measurements (within 2%). These measurements will aid in designing pre-clinical studies which investigate the potential biological effects of radiation therapy in the presence of a strong magnetic field. This work was partially funded by Elekta.

SU-G-JeP3-15: Is the Reproducibility with Respect to Bone of Tumor Position at Simulation for Breath Hold CT Scans Correlated to the Reproducibility for Multiple Breath Hold CBCTs at Treatment in SBRT Thoracic Patients?

PURPOSE To evaluate correlation between the reproducibility of tumor position under feedback guided voluntary deep inspiration breath hold gating at simulation and at treatment. METHODS All patients treated with breath hold (BH) have 3-6 BH CTs taken at simulation (sim). In addition, if the relationship between the tumor and nearby bony anatomy on treatment BH CT(or CBCT) is found to be greater than 5 mm different at treatment than it was at sim, a repeat BH CT is taken before treatment. We retrospectively analyzed the sim CTs for 19 patients who received BH SBRT lung treatments and had repeat BH CT on treatment. We evaluated the reproducibility of the tumor position during the simulation CTs and compared this to the reproducibility of the tumor position on the repeat treatment CT with our in-house CT alignment software (CT-Assisted Targeting for Radiotherapy). RESULTS Comparing the tumor position for multiple simulation BH CTs, we calculated: maximum difference (max) = 0.69cm; average difference (x) = 0.28cm; standard deviation (σ) = 0.18cm. Comparing the repeat BH CBCTs on treatment days we calculated: max = 0.44cm; x = 0.16cm; σ = 0.22cm. We also found that for 95% of our BH cases, the absolute variation in tumor position within the same imaging day was within 5mm of the range at the time of simulation and treatment. We found that 75% of the BH cases had less residual tumor motion on treatment days than at simulation. CONCLUSION This suggests that a GTV contour based upon the residual tumor motion in multiple BH datasets plus 2 mm margin should be sufficient to cover the full range of residual tumor motion on treatment days.

SU-E-T-26: A Dosimetric Comparison of Two Treatment Setups for Lung Stereotactic Body Radiation Therapy (SBRT) Patients

Purpose: To assess the feasibility of treating lung SBRT patients with the ipsilateral arm adducted beside the body instead of elevated above the head. Methods: Patients receiving lung SBRT at our institution are typically treated with both arms raised above their head. However, several patients had difficulty maintaining their arms in an elevated position. In this study, lung SBRT patients who underwent PET-CT imaging with an adducted ipsilateral arm were selected to investigate the dosimetric effects of this treatment setup. PET-CT datasets were fused with treatment planning CT images to simulate the adducted arm position. One VMAT treatment plan was created per patient using the Pinnacle treatment planning system. Plans were optimized to achieve minimal dose to the ipsilateral arm while keeping the target coverage and critical structure doses within clinical limits. The target dose coverage, conformity index (CI) for the target, and DVHs of critical structures for the adducted arm plan were calculated. These parameters were compared with the clinical plan and reported along with the mean and maximum doses of the ipsilateral arm. Results: The target coverage, CI and DVHs for the adducted arm plans of two patients (one with peripheral lesion and one with central lesion) were comparable with the clinical plans. Dose constraints for the chest wall limited further reduction of ipsilateral arm doses for the peripheral lesion plan. The mean ipsilateral arm doses for the central and peripheral lesions were 0.33 Gy and 2.4 Gy, respectively. The maximum ipsilateral arm doses for the central and peripheral lesions were 1.0 Gy and 6.2 Gy, respectively. Conclusion: The results suggested patients with central lung SBRT tumors were more suitable for treatment with the adducted arm position. More patients with various lung tumor locations will be studied to find optimal tumor locations for treatment with this arm position.

SU‐E‐T‐245: Detection of the Photon Target Damage in Varian Linac Based On Periodical Quality Assurance Measurements

Purpose: To determine the best dosimetric metric measured by our routine QA devices for diagnosing photon target failure on a Varian C-series linac. Methods: We have retrospectively reviewed and analyzed the dosimetry data from a Varian linac with a target degradation that was undiagnosed for one year. A failure in the daily QA symmetry tests was the first indication of an issue. The beam was steered back to a symmetric shape and water scans indicated the beam energy had changed but stayed within the manufacturers specifications and agreed reasonably with our treatment planning system data. After the problem was identified and the target was replaced, we retrospectively analyzed our QA data including diagonals normalized flatness (F_DN) from the daily device (DQA3), profiles from an ionization chamber array (IC Profiler), as well as profiles and PDDs from a 3D water Scanner (3DS). These metrics were cross-compared to determine which was the best early indicator of target degradation. Results: A 3% change in FDN measured by the DQA3 was found to be an early indicator of target degradation. It is more sensitive than changes in output, symmetry, flatness or PDD. All beam shape metrics (flatness at dmax and 10 cm depth, and F_DN) indicated an energy increase while the PDD indicated an energy decrease. This disagreement between the beam-shape based energy metrics (F_DN and flatness) and PDD based energy metric may indicate target failure as opposed to an energy change resulting from changes in the incident electron energy. Conclusion: Photon target degradation has been identified as a failure mode for linacs. The manufacturers test for this condition is highly invasive and requires machine down time. We have demonstrated that the condition could be caught early based upon data acquired during routine QA activities, such as the F_DN.

SU-E-T-403: Measurement of the Neutron Ambient Dose Equivalent From the TrueBeam Linac Head and Varian 2100 Clinac

PURPOSE High-energy x-ray therapy produces an undesirable source of stray neutron dose to healthy tissues, and thus, poses a risk for second cancer induction years after the primary treatment. Hence, the purpose of this study was to measure the neutron ambient dose equivalent, H*(10), produced from the TrueBeam and Varian 2100 linac heads, respectively. Of particular note is that there is no measured data available in the literature on H*(10) production from the TrueBeam treatment head. METHODS Both linacs were operated in flattening filter mode using a 15 MV x-ray beam on TrueBeam and an 18 MV x-ray beam for the Varian 2100 Clinac with the jaws and multileaf collimators in the fully closed position. A dose delivery rate of 600 MU/min was delivered on the TrueBeam and the Varian 2100 Clinac, respectively and the H*(10) rate was measured in triplicate using the WENDI-2 detector located at multiple positions including isocenter and longitudinal (gun-target) to the isocenter. RESULTS For each measurement, the H*(10) rate was relatively constant with increasing distance away from the isocenter with standard deviations on the order of a tenth of a mSv/h or less for the given beam energy. In general, fluctuations in the longitudinal H*(10) rate between the anterior-posterior couch directions were approximately a percent for both beam energies. CONCLUSION Our preliminary results suggest an H*(10) rate of about 30 mSv/h (40 mSv/h) or less for TrueBeam (Varian Clinac 2100) for all measurements considered in this study indicating a relatively low contribution of produced secondary neutrons to the primary therapeutic beam.