Radiation Impacts Early Atherosclerosis by Suppressing Intimal LDL Accumulation.

RATIONALE: Bone marrow transplantation (BMT) is used frequently to study the role of hematopoietic cells in atherosclerosis, but aortic arch lesions are smaller in mice after BMT. OBJECTIVE: To identify the earliest stage of atherosclerosis inhibited by BMT and elucidate potential mechanisms. METHODS AND RESULTS: Ldlr-/- mice underwent total body γ-irradiation, bone marrow reconstitution, and 6-week recovery. Atherosclerosis was studied in the ascending aortic arch and compared with mice without BMT. In BMT mice, neutral lipid and myeloid cell topography were lower in lesions after feeding a cholesterol-rich diet for 3, 6, and 12 weeks. Lesion coalescence and height were suppressed dramatically in mice post-BMT, whereas lateral growth was inhibited minimally. Targeted radiation to the upper thorax alone reproduced the BMT phenotype. Classical monocyte recruitment, intimal myeloid cell proliferation, and apoptosis did not account for the post-BMT phenotype. Neutral lipid accumulation was reduced in 5-day lesions, thus we developed quantitative assays for LDL (low-density lipoprotein) accumulation and paracellular leakage using DiI-labeled human LDL and rhodamine B-labeled 70 kD dextran. LDL accumulation was dramatically higher in the intima of Ldlr-/- relative to Ldlr+/+ mice, and was inhibited by injection of HDL mimics, suggesting a regulated process. LDL, but not dextran, accumulation was lower in mice post-BMT both at baseline and in 5-day lesions. Since the transcript abundance of molecules implicated in LDL transcytosis was not significantly different in the post-BMT intima, transcriptomics from whole aortic arch intima, and at single-cell resolution, was performed to give insights into pathways modulated by BMT. CONCLUSIONS: Radiation exposure inhibits LDL entry into the aortic intima at baseline and the earliest stages of atherosclerosis. Single-cell transcriptomic analysis suggests that LDL uptake by endothelial cells is diverted to lysosomal degradation and reverse cholesterol transport pathways. This reduces intimal accumulation of lipid and impacts lesion initiation and growth.

Association between Radiation Exposure and Endothelium-Dependent Vasodilation: Results from Clinical and Experimental Studies.

PURPOSE: The association between occupational radiation exposure and endothelium-dependent vasodilation (EDV) remains unclear. This study evaluated the association between radiation exposure and EDV among fluoroscopy-guided interventional procedure specialists and explored the possible mechanisms. MATERIALS AND METHODS: Brachial flow-mediated dilation was compared in 21 interventional cardiologists (the radiation group) and 15 noninterventional cardiologists (the nonradiation group). Animal radiation experiments were also performed to observe the impact of radiation on EDV. RESULTS: Flow-mediated dilation in both the left (radiation group, 3.63% vs. nonradiation group, 6.77%; P < .001) and right brachial arteries (5.36% vs. 7.33%, respectively; P = .04) and serum nitric oxide (NO) level (343.69 vs. 427.09 µmol/L, respectively; P = .02) were significantly reduced in the radiation group compared to those in the nonradiation group. EDV was significantly impaired in acetylcholine concentrations of 3 × 10-6 mol/L and 10-5 mol/L (60.09% vs.74.79%, respectively; P = .03; and 62.73% vs. 80.56%, respectively; P = .002), and reactive oxygen species levels in the aorta intima and media layers were significantly increased in mice after a single x-ray exposure, which could be partly rescued by pretreatment with folic acid (P < .05). CONCLUSIONS: Radiation exposure can lead to impairment of flow-mediated vasodilation in human or EDV in mice. In mice acutely exposed to radiation, folic acid alleviated radiation-induced EDV impairment by possible reduction of reactive oxidative species.

Electric Polarization of Soft Biological Tissues Induced by Ultrasound Waves.

Ultrasound irradiation makes it possible to generate alternating electric polarization through the electromechanical coupling of materials. It follows that electromagnetic fields are often emitted to the surrounding environment when materials are acoustically stimulated. We investigate the acoustically stimulated electromagnetic (ASEM) response of soft biological tissues. The ASEM signal is detected through a capacitive resonant antenna tuned to the MHz frequency of the irradiated ultrasound waves. The signal is well explained by the stress-induced polarization, which responds linearly to the applied acoustic stress. Induced polarization is clearly observed in the Achilles tendon, aortic wall, and aortic valve samples, whereas it is small in adipose tissue and myocardium samples, indicating that fibrous tissues exhibit electromechanical coupling.

UVB Exposure Prevents Atherosclerosis by Regulating Immunoinflammatory Responses.

OBJECTIVE: UVB irradiation is an established treatment for immunoinflammatory cutaneous disorders and has been shown to suppress cutaneous and systemic inflammatory diseases through modulation of the adaptive immune response. However, it remains unknown whether UVB irradiation prevents an immunoinflammatory disease of arteries such as atherosclerosis. APPROACH AND RESULTS: Here, we show that UVB exposure inhibits the development and progression of atherosclerosis in atherosclerosis-prone mice by expanding and enhancing the functional capacity of CD4+ forkhead box P3+ regulatory T cells and regulating proatherogenic T-cell responses. Experimental studies in Langerhans cell-depleted mice revealed that epidermal Langerhans cells play a critical role in UVB-dependent induction of CD4+ forkhead box P3+ regulatory T cells, suppression of proatherogenic T-cell responses, and prevention of atherosclerotic plaque development. CONCLUSIONS: Our findings suggest the skin immune system as a novel therapeutic target for atherosclerosis and provide a novel strategy for the treatment and prevention of atherosclerosis.

Ionizing Radiation Enhances Activity of Angiotensin-Converting Enzyme in Rat Aorta.

We analyzed changes in angiotensin-converting enzyme activity in rat aorta at the early terms after irradiation in doses equal to one fraction dose used in tumor radiotherapy. Male Wistar rats were exposed to whole body or local (chest) X-ray irradiation (200 kV, 1-7.5 Gy). The activity of the enzyme in aorta segments was measured in 1-48 h after irradiation by hydrolysis of hippuryl-histidine-leucine. Activity of angiotensin-converting enzyme in rat aorta was increased 1-24 h after whole body irradiation in a dose of 2.5 Gy with a peak in 2 h after exposure. After local exposure, enzyme activity also increased in 2 h, but returned to the control level in 24 h. In 2 h after whole-body irradiation in doses >2.5 Gy, the increase in enzyme activity was less pronounced and after exposure to 7.5 Gy, it did not differ from the control. During local exposure, the effect did not decrease with increasing the irradiation dose. The fraction of blood monocytes adherent to plastic in rats subjected to whole body irradiation decreases with increasing the dose. In rats subjected to local irradiation in a dose of 7.5 Gy, monocyte adhesion to plastic did not differ from the control. These data suggest that the increase in activity of angiotensin-converting enzyme in the aorta after irradiation is determined by monocyte adhesion to the endothelium; the decrease in this effects with increasing the dose can be explained by radiation damage of monocytes.

Dihydroquercetin and Fucoidin Inhibit the Increase of Angiotensin-Converting Enzyme Activity in the Rat Aorta after Irradiation.

The time course of angiotensin-converting enzyme activity in the rat aorta after fractionated exposure to ionizing radiation and the effects of dihydroquercetin and fucoidin on this parameter were studied. Male Wistar rats were exposed to single or repeated (fractionated) X-ray radiation in a dose of 2.5 Gy at 200 kV. Activity of angiotensin-converting enzyme in aorta segments was evaluated 2 h after the last exposure by hydrolysis of hippuryl-histidineleucin substrate. Enzyme activity in the rat aorta was higher than normally after all the studied doses of fractionated exposure (2.5 Gy per fraction) with the maximum effect after the total dose of 7.5 Gy (3 fractions). Fucoidin, a blocker of endothelium receptors realizing the leukocyte adhesion to the endothelium, and flavonoid dihydroquercetin inhibiting expression of adhesion molecules in the endothelium abolished the increase in activity of angiotensinconverting enzyme in the rat aorta after single exposure; moreover, dihydroquercetin reduced significantly the effect of fractionated exposure. These data indicate that leukocyte adhesion to the endothelium is an important factor contributing to the increase of angiotensin-converting enzyme activity in the aorta.

Defining the hypoxic target volume based on positron emission tomography for image guided radiotherapy - the influence of the choice of the reference region and conversion function.

BACKGROUND: Hypoxia imaged by positron emission tomography (PET) is a potential target for optimization in radiotherapy. However, the implementation of this approach with respect to the conversion of intensities in the images into oxygenation and radiosensitivity maps is not straightforward. This study investigated the feasibility of applying two conversion approaches previously derived for 18F-labeled fluoromisonidazole (18F-FMISO)-PET images for the hypoxia tracer 18F-flortanidazole (18F-HX4). MATERIAL AND METHODS: Ten non-small-cell lung cancer patients imaged with 18F-HX4 before the start of radiotherapy were considered in this study. PET image uptake was normalized to a well-oxygenated reference region and subsequently linear and non-linear conversions were used to determine tissue oxygenations maps. These were subsequently used to delineate hypoxic volumes based partial oxygen pressure (pO2) thresholds. The results were compared to hypoxic volumes segmented using a tissue-to-background ratio of 1.4 for 18F-HX4 uptake. RESULTS: While the linear conversion function was not found to result in realistic oxygenation maps, the non-linear function resulted in reasonably sized sub-volumes in good agreement with uptake-based segmented volumes for a limited range of pO2 thresholds. However, the pO2 values corresponding to this range were significantly higher than what is normally considered as hypoxia. The similarity in size, shape, and relative location between uptake-based sub-volumes and volumes based on the conversion to pO2 suggests that the relationship between uptake and pO2 is similar for 18F-FMISO and 18F-HX4, but that the model parameters need to be adjusted for the latter. CONCLUSIONS: A non-linear conversion function between uptake and oxygen partial pressure for 18F-FMISO-PET could be applied to 18F-HX4 images to delineate hypoxic sub-volumes of similar size, shape, and relative location as based directly on the uptake. In order to apply the model for e.g., dose-painting, new parameters need to be derived for the accurate calculation of dose-modifying factors for this tracer.

Taxifolin and Fucoidin Abolish the Irradiation-Induced Increase in the Production of Reactive Oxygen Species in Rat Aorta.

We studied changes in ROS content in the aorta of Wistar rats at early terms after irradiation in doses equal to single fraction used in tumor radiotherapy and the effects of taxifolin and fucoidin, blockers of leukocyte adhesion to endothelium, on ROS content. Male rats were exposed to X-rays (200 kW) in doses of 1-7.5 Gy. ROS production in aorta segments was measured in 1-48 h after irradiation by dichlorodihydrofluorescein oxidation. The content of ROS in the aorta of rats exposed to radiation in doses of 1-2.5 Gy increased in 1-24 h after irradiation, the peak ROS content was found in 2 h after irradiation. Taxifolin (100 µg/kg dihydroquercetin once a day with drinking water) and fucoidin (10 mg/kg, i.v.) abolished ROS accumulation. The content of ROS in rat aorta increased in 1-24 h after irradiation in doses used for tumor radiotherapy and this increase can be determined by leukocyte adhesion to the endothelium.

Features of contrast enhancement in different methods of multislice computed tomography.

The purpose of this study was a comparison of the X-ray density in certain organs and anatomical structures and the determination of the radiation exposure during venous-arterial MSCT scanning and classical two-phase CT of organs of the abdominal cavity. It has been established that the technique of venous-arterial MSCT scanning provided a significant reduction of radiation dose during CT of organs of the abdominal cavity and could be used as an alternative to two-phase examination in the process of dynamic monitoring of cancer patients.

Activation of eNOS in endothelial cells exposed to ionizing radiation involves components of the DNA damage response pathway.

In this study, the involvement of ataxia telangiectasia mutated (ATM) kinase and heat shock protein 90 (HSP90) in endothelial nitric oxide synthase (eNOS) activation was investigated in X-irradiated bovine aortic endothelial cells. The activity of nitric oxide synthase (NOS) and the phosphorylation of serine 1179 of eNOS (eNOS-Ser1179) were significantly increased in irradiated cells. The radiation-induced increases in NOS activity and eNOS-Ser1179 phosphorylation levels were significantly reduced by treatment with either an ATM inhibitor (Ku-60019) or an HSP90 inhibitor (geldanamycin). Geldanamycin was furthermore found to suppress the radiation-induced phosphorylation of ATM-Ser1181. Our results indicate that the radiation-induced eNOS activation in bovine aortic endothelial cells is regulated by ATM and HSP90.

High-Pitch Dual-Source MDCT for Imaging of the Thoracoabdominal Aorta: Relationships Among Radiation Dose, Noise, Pitch, and Body Size in a Phantom Experiment and Clinical Study.

OBJECTIVE: The purpose of this study was to investigate, both in a phantom experiment and a within-patient clinical study the relationships among radiation dose, image noise, pitch, and body size in MDCT angiography of the thoracoabdominal aorta, with the use of high-pitch dual-source and standard-pitch single-source acquisitions. MATERIALS AND METHODS: A proprietary tapered phantom consisting of four ultrahigh-molecular-weight polyethylene cylinders was used to mimic the body size ranges (small, medium, large, and extra large) of patients in the United States. The phantom was imaged using both standard-pitch (0.8) and various high-pitch (range, 2.0-3.2 [in increments of 0.4]) settings. Standard-pitch and high-pitch acquisitions were also performed in 45 patients (27 men, 18 women; mean age, 67.6 years). RESULTS: At standard pitch, the volume CT dose index (CTDIvol) increased with phantom size, in a logistic sigmoid relationship. At high-pitch settings, the CTDIvol increased gradually in relation to phantom size, up to a threshold (denoted by tCTDI[pitch] ≈ 48.3-7.5 pitch), which linearly decreased (R(2) = 0.99) with pitch (maximum CTDIvol output at pitch [maxCTDI(pitch)] ≈ 18.9-3.9 pitch). A linear decrease in the size-specific dose estimate (SSDE) was observed beyond phantom size thresholds (tSSDE[pitch] ≈ 47.6-8.6 pitch) linearly decreasing (R(2) = 0.98) with pitch (maximum SSDE output at pitch [maxSSDE(pitch)] ≈ 15.5-1.3 pitch). Image noise was statistically significantly lower at standard pitch than at high-pitch settings (p = 0.01). In patients, statistically significant differences were noted between standard and high-pitch settings in the mean CTDIvol(10.8 ± 2.6 and 8.3 ± 0.7 mGy, respectively), SSDE (11.3 ± 2.1 and 8.8 ± 1.5 mGy, respectively), and noise (9.7 ± 2.2 and 14 ± 4.2, respectively) (p < .0001, for all comparisons). CONCLUSION: Lower radiation dose levels achieved with the use of a high-pitch technique reflect limitations in tube output occurring for medium to large body sizes, with an associated exponential increase in noise. The standard- and high-pitch techniques yield similar radiation dose levels for small body sizes.

Low-frequency ultrasound induces apoptosis of rat aortic smooth muscle cells (A7r5) via the intrinsic apoptotic pathway.

Ultrasound, a non-invasive therapy method, is a potential tool for medical applications, but its biological effects on vascular smooth muscle cells (VSMCs) have not been characterized. The aim of this study was to explore the effect and possible apoptotic mechanism of VSMCs that were induced by low-frequency ultrasound (LFU). Cell viability and apoptosis of A7r5 cells were evaluated after treating A7r5 cells with a continuous 45-kHz 1.0-W/cm(2) ultrasound (exposure time of 0, 10, 20, 30, and 35 s) by MTT assay and flow cytometry. At the optimum ultrasound exposure condition (30 s), gene chip analysis was performed, and the apoptotic signaling pathway was confirmed by reverse transcription-polymerase chain reaction and Western blot. As measured by flow cytometry, LFU significantly induced A7r5 cell apoptosis. Comparing the ultrasound group with the control group, the protein expression of caspase-9 and caspase-3 was increased by 50 and 57%, respectively; the caspase-3 mRNA level was increased by 37.5%. These findings indicate that an intrinsic pathway plays a major role in apoptosis that is induced by LFU and that LFU can induce A7r5 cell apoptosis via caspase-9- and caspase-3-dependent pathways.

Raw data-based iterative reconstruction in body CTA: evaluation of radiation dose saving potential.

OBJECTIVE: To evaluate prospectively, in patients undergoing body CTA, the radiation dose saving potential of raw data-based iterative reconstruction as compared to filtered back projection (FBP). METHODS: Twenty-five patients underwent thoraco-abdominal CTA with 128-slice dual-source CT, operating both tubes at 120 kV. Full-dose (FD) images were reconstructed with FBP and were compared to half-dose (HD) images with FBP and HD-images with sinogram-affirmed iterative reconstruction (SAFIRE), both reconstructed using data from only one tube-detector-system. Image quality and sharpness of the aortic contour were assessed. Vessel attenuation and noise were measured, contrast-to-noise-ratio was calculated. RESULTS: Noise as image quality deteriorating artefact occurred in 24/25 (96%) HD-FBP but not in FD-FBP and HD-raw data-based iterative reconstruction datasets (p < 0.001). Other artefacts occurred with similar prevalence among the datasets. Sharpness of the aortic contour was higher for FD-FBP and HD-raw data-based iterative reconstruction as compared to HD-FBP (p < 0.001). Aortoiliac attenuation was similar among all datasets (p > 0.05). Lowest noise was found for HD-raw data-based iterative reconstruction (7.23HU), being 9.4% lower than that in FD-FBP (7.98HU, p < 0.05) and 30.8% lower than in HD-FBP images (10.44HU, p < 0.001). Contrast-to-noise-ratio was lower in HD-FBP (p < 0.001) and higher in HD-raw data-based iterative reconstruction (p < 0.001) as compared to FD-FBP. CONCLUSION: Intra-individual comparisons of image quality of body CTA suggest that raw data-based iterative reconstruction allows for dose reduction >50% while maintaining image quality. Key Points • Raw data-based iterative reconstruction reduces image noise and improves image quality as compared to filtered back projection • At a similar radiation dose, raw data-based iterative reconstruction improves the sharpness of vessel contours • In body CTA a dose reduction of >50% might be possible when using raw data-based iterative reconstructions, while image quality can be maintained.

[Effect of preparation "Korargin" on metabolic syndrome manifestations in adult and old rats exposed to x-ray irradiation].

Introduction to the diet of adult and old rats for 30 days after R-irradiation at a dose of 5 Gy drug "Korargin" prevented the development of some manifestations of radioinduced metabolic syndrome (MS) in animals of both age groups: in adult irradiated animals--a tendency to decrease insulin levels in blood plasma, increase of cholesterol levels in liver tissue, significantly increased serum high density lipoprotein as compared both to controls and irradiated animals, prevented the decrease in the levels of anions NO2- and NO3- in tissue of the aorta and anions NO2- in tissue of the heart; in old irradiated animals--prevented tended to increase of body weight, the increase in insulinresistance (index HOMA), the decrease in the level of anions NO2- in the aorta tissue, increased levels of anions NO2- and NO3- in the heart tissue compared to controls. Prevention through drug "Korargin" some manifestations of radioinduced MS in adult and old animals, indicates the prospects of exploring the possibility of correction of the violations of carbohydrate and lipid metabolism caused by exposure to ionizing radiation, in individuals of both age groups.

Black tea is more effective than green tea in prevention of radiation-induced oxidative stress in the aorta of rats.

In the work, the effect of black tea on oxidative stress induced in the aorta by irradiation was studied. The efficiency of black and green tea types was compared, and the effect of the main green tea components (-)-epigallocatechin galate (EGCG) and (-)-epigallocatechin (EGC) on the aorta was studied. The activity of ACE in rat aorta segments was determined by measuring the hydrolysis of hippuryl-L-histidyl-L-leucine, and the production of ROS was estimated from the oxidation of dichlorodihydrofluorescein. Black tea prevented the radiation-induced activation of the ACE and suppressed increased ROS production in the aorta of irradiated rats. The IC50 value for the suppression of the irradiation-induced increase in ACE activity is 1 ml of black tea brewed at a rate of 0.17 g/100 ml. Black tea is 12 times more effective than green tea. The administration of both catechin derivatives from green tea to rats leads to an increase in the activity of ACE and the formation of ROS in the aorta. The dose that provided half maximum activation of ACE (EC50) on intraperitoneal (i. p.) injection of galloylated catechins was found to be the same, 0.06-0.07 µg/kg of body weight. Upon intragastric gavage of EGCG, the EC50 value was by one order of magnitude higher, 0.8 µg/kg. Black tea was more effective than green tea in prevention a radiation-induced increase of ACE activity and oxidative stress in the aorta. This difference was explained by a low content of galloylated catechins in black tea.

Radiation-induced lipoprotein-associated phospholipase A2 increases lysophosphatidylcholine and induces endothelial cell damage.

The cardiotoxicity of various anticancer therapies, including radiotherapy, can lead to cardiovascular complications. These complications can range from damaging cardiac tissues within the irradiation field to increasing the long-term risks of developing heart failure, coronary artery disease, and myocardial infarction. We analyzed radiation-induced metabolites capable of mediating critical biological processes, such as inflammation, senescence, and apoptosis. Previously, by applying QTOF-MASS analysis to irradiated human fibroblasts, we identified that metabolite sets of lysophosphatidylcholine (LPC) were increased in these cells. In this study, radiation-induced LPC accumulation in human aortic endothelial cells (HAECs) increased reactive oxygen species (ROS) production and senescence-associated-beta-galactosidase staining, in addition to decreasing their tube-forming ability. Knockdown of lipoprotein-associated phospholipase A2 (Lp-PLA2) with small interfering RNA (siRNA) inhibited the increased LPC production induced by radiation, and reduced the radiation-induced cell damage produced by ROS and oxidized low-density lipoprotein (LDL). Lp-PLA2 depletion abolished the induction of proinflammatory factors, such as interleukin 1ß, tumor necrosis factor-alpha, matrix metalloproteinase 2, and matrix metalloproteinase 9, as well as adhesion molecules, such as intercellular adhesion molecule 1 (ICAM-1) and E-selection. Likewise, we showed that Lp-PLA2 expression was upregulated in the vasculature of irradiated rat, resulting in increased LPC production and LDL oxidation. Our data demonstrate that radiation-induced LPC production is a potential risk factor for cardiotoxicity that is mediated by Lp-PLA2 activity, suggesting that LPC and Lp-PLA2 offer potential diagnostic and therapeutic approaches to cardiovascular damage during radiotherapy.

Single exposure to radiation produces early anti-angiogenic effects in mouse aorta.

Radiation exposure can increase the risk for many non-malignant physiological complications, including cardiovascular disease. We have previously demonstrated that ionizing radiation can induce endothelial dysfunction, which contributes to increased vascular stiffness. In this study, we demonstrate that gamma radiation exposure reduced endothelial cell viability or proliferative capacity using an in vitro aortic angiogenesis assay. Segments of mouse aorta were embedded in a Matrigel-media matrix 1 day after mice received whole-body gamma irradiation between 0 and 20 Gy. Using three-dimensional phase contrast microscopy, we quantified cellular outgrowth from the aorta. Through fluorescent imaging of embedded aortas from Tie2GFP transgenic mice, we determined that the cellular outgrowth is primarily of endothelial cell origin. Significantly less endothelial cell outgrowth was observed in aortas of mice receiving radiation of 5, 10, and 20 Gy radiation, suggesting radiation-induced endothelial injury. Following 0.5 and 1 Gy doses of whole-body irradiation, reduced outgrowth was still detected. Furthermore, outgrowth was not affected by the location of the aortic segments excised along the descending aorta. In conclusion, a single exposure to gamma radiation significantly reduces endothelial cell outgrowth in a dose-dependent manner. Consequently, radiation exposure may inhibit re-endothelialization or angiogenesis after a vascular injury, which would impede vascular recovery.

Novel Methodology to Investigate the Effect of Radiation Dose to Heart Substructures on Overall Survival.

PURPOSE: For patients with lung cancer treated with radiation therapy, a dose to the heart is associated with excess mortality; however, it is often not feasible to spare the whole heart. Our aim is to define cardiac substructures and dose thresholds that optimally reduce early mortality. METHODS AND MATERIALS: Fourteen cardiac substructures were delineated on 5 template patients with representative anatomies. One thousand one hundred sixty-one patients with non-small cell lung cancer were registered nonrigidly to these 5 template anatomies, and their radiation therapy doses were mapped. Mean and maximum dose to each substructure were extracted, and the means were evaluated as input to prediction models. The cohort was bootstrapped into 2 variable reduction techniques: elastic net least absolute shrinkage and selection operator and the random survival forest model. Each method was optimized to extract variables contributing most to overall survival, and model coefficients were evaluated to select these substructures. The most important variables common to both models were selected and evaluated in multivariable Cox-proportional hazard models. A threshold dose was defined, and Kaplan-Meier survival curves plotted. RESULTS: Nine hundred seventy-eight patients remained after visual quality assurance of the registration. Ranking the model coefficients across the bootstraps selected the maximum dose to the right atrium, right coronary artery, and ascending aorta as the most important factors associated with survival. The maximum dose to the combined cardiac region showed significance in the multivariable model, a hazard ratio of 1.01/Gy, and P = .03 after accounting for tumor volume (P < .001), N stage (P < .01), and performance status (P = .01). The optimal threshold for the maximum dose, equivalent dose in 2-Gy fractions, was 23 Gy. Kaplan-Meier survival curves showed a significant split (log-rank P = .008). CONCLUSIONS: The maximum dose to the combined cardiac region encompassing the right atrium, right coronary artery, and ascending aorta was found to have the greatest effect on patient survival. A maximum equivalent dose in 2-Gy fractions of 23 Gy was identified for consideration as a dose limit in future studies.