Patient Radiation Exposure: Imaging During Radiation Oncology Procedures: Executive Summary of NCRP Report No. 184

Can the grading of mild cardiac allograft vasculopathy be further refined? An angiographic and physiologic assessment of heart transplant recipients with ISHLT CAV 1

BACKGROUND

Cardiac allograft vasculopathy (CAV) results in impaired blood flow in both epicardial vessels and the microvasculature and is a leading cause of poor outcomes in heart transplant (HT) recipients. Most patients have mild (International Society for Heart and Lung Transplantation [ISHLT] CAV 1) disease. This study examined outcomes among those with ISHLT CAV 1 and investigated the value of physiologic assessment via cardiac positron emission tomography/computed tomography (PET/CT) for added risk stratification.

METHODS

CAV was graded using ISHLT criteria. Those with CAV 1 were further subgrouped into CAV 1a (maximal lesion <30% stenosis) or CAV 1b (maximal lesion ≥30% stenosis).

RESULTS

299 HT recipients underwent invasive coronary angiography for CAV assessment with a median follow-up of 4.7 years. ISHLT CAV 1 was associated with a 2.9-fold risk of death/retransplantation compared to ISHLT CAV 0 (95% confidence interval [CI] 1.7-5.3, p < 0.001). Of those with ISHLT CAV 1, 12% had ISHLT CAV 1b, which was associated with a 2.8 times greater risk of death/retransplantation compared to CAV 1a (95% CI 1.4-5.9, p = 0.003). In a subgroup of 158 patients with contemporary cardiac PET/CT, among those with CAV 1a, a myocardial blood flow reserve (MBFR) ≤2 was associated with a 4.6-fold risk of death/retransplantation compared to a normal MBFR (95% CI 1.7-12.6, p = 0.001).

CONCLUSION

Patients with CAV 1b had worse outcomes than those with CAV 1a. Among those with CAV 1a, the poorer outcomes than ISHLT CAV 0 observed were predominantly associated with reduced MBFR. These data suggest additional anatomic classification and physiologic assessment can further risk stratify those with ISHLT CAV 1.

PET/CT with Myocardial Blood Flow Assessment Is Prognostic of Cardiac Allograft Vasculopathy Progression and Clinical Outcomes

Cardiac allograft vasculopathy (CAV) causes impaired blood flow in both epicardial vessels and microvasculature and remains a leading cause of posttransplant morbidity and mortality. This study examined the prognostic value and outcomes of CAV, assessed by 13N-ammonia PET/CT myocardial perfusion imaging in heart transplant recipients. Methods: PET/CT and invasive coronary angiography (ICA) were graded using validated scales. CAV progression was assessed using intrapatient sequences: baseline ICA, interval PET/CT with myocardial blood flow reserve, and subsequent ICA. Intervals between ICAs of 600, 900, and 1200 d were included, and for each, the negative predictive value (NPV) of CAV development was assessed. Results: In total, 344 heart transplant recipients underwent PET/CT for CAV assessment with a median follow-up of 4.8 y. PET CAV grade 0/1 had an NPV of 0.93, 0.95, and 0.95 at each respective time point for developing an International Society for Heart and Lung Transplantation CAV 2/3 on subsequent ICA. Compared with PET CAV 0, PET CAV 2/3 was associated with a 2.9-fold increased risk of all-cause mortality (hazard ratio, 2.86; 95% CI, 1.36-6.00; P = 0.006). PET CAV 1 had a numerically increased risk (hazard ratio, 2.03; 95% CI, 0.99-4.15; P = 0.054). In a sensitivity analysis of 135 patients with stable International Society for Heart and Lung Transplantation CAV over successive ICA, PET CAV 2/3 remained associated with increased risk of death or retransplantation (hazard ratio, 3.20; 95% CI, 1.18-8.69; P = 0.03). Conclusion: Noninvasive CAV assessment by PET/CT and myocardial blood flow reserve provides prognostic information and robust NPVs for development of moderate to severe CAV over intervals up to 4 y. These data suggest that, for certain patients, intervals between invasive screenings may be extended.

Added prognostic value of visually estimated coronary artery calcium among heart transplant recipients

BACKGROUND

Cardiac hybrid positron emission tomography/computed tomography (PET/CT) has become a valid screening modality for cardiac allograft vasculopathy (CAV) following heart transplantation (HT). Visually estimated coronary artery calcium (VECAC) can be quantified from CT images obtained as part of PET/CT and has been shown to be associated with adverse cardiovascular outcomes in coronary artery disease. We investigated the prognostic value of VECAC following HT.

METHODS

A retrospective analysis of 430 consecutive adult HT patients who underwent 13N-ammonia cardiac PET/CT from 2016 to 2019 with follow-up through October 15, 2022, was performed. VECAC categories included: VECAC 0, VECAC 1-9, VECAC 10-99, and VECAC 100+. The association between VECAC categories and outcomes was assessed using univariable and multivariable proportional hazards regression. The primary outcome was death/retransplantation.

RESULTS

The cohort was 73% male, 33% had diabetes, 67% had estimated glomerular filtration rate <60 ml/min, median age was 61 years, and median time since HT was 7.5 years. VECAC alone was insufficiently sensitive to screen for CAV. During a median follow-up of 4.2 years ninety patients experienced death or retransplantation. Compared with those with VECAC 0, patients VECAC 10-99 (HR 2.25, 95% CI 1.23-4.14, p = 0.009) and VECAC 100+ (HR 3.42, 95% CI 1.96-5.99, p < 0.001) experienced an increased risk of death/retransplantation. The association was similar for cardiovascular death and cardiovascular hospitalization. After adjusting for other predictors of death/retransplantation, VECAC 10-99 (VECAC 10-99: aHR 1.95, 95% CI 1.03-3.71 p = 0.04) and VECAC 100+ (VECAC 100+: aHR 2.33, 95% CI 1.17-4.63, p = 0.02) remained independently associated with death/retransplantation.

CONCLUSIONS

VECAC is an independent prognostic marker of death/retransplantation following HT and merits inclusion as a part of post-HT surveillance PET/CT.

Cumulative Dose from Recurrent CT Scans: Exploring the DNA Damage Response in Human Non-Transformed Cells

Recurrent computed tomography (CT) examination has become a common diagnostic procedure for several diseases and injuries. Though each singular CT scan exposes individuals at low doses of low linear energy transfer (LET) radiation, the cumulative dose received from recurrent CT scans poses an increasing concern for potential health risks. Here, we evaluated the biological effects of recurrent CT scans on the DNA damage response (DDR) in human fibroblasts and retinal pigment epithelial cells maintained in culture for five months and subjected to four CT scans, one every four weeks. DDR kinetics and eventual accumulation of persistent-radiation-induced foci (P-RIF) were assessed by combined immunofluorescence for γH2AX and 53BP1, i.e., γH2AX/53BP1 foci. We found that CT scan repetitions significantly increased both the number and size of γH2AX/53BP1 foci. In particular, after the third CT scan, we observed the appearance of giant foci that might result from the overlapping of individual small foci and that do not associate with irreversible growth arrest, as shown by DNA replication in the foci-carrying cells. Whether these giant foci represent coalescence of unrepaired DNA damage as reported following single exposition to high doses of high LET radiation is still unclear. However, morphologically, these giant foci resemble the recently described compartmentalization of damaged DNA that should facilitate the repair of DNA double-strand breaks but also increase the risk of chromosomal translocations. Overall, these results indicate that for a correct evaluation of the damage following recurrent CT examinations, it is necessary to consider the size and composition of the foci in addition to their number.

Patient Exposure from Radiologic and Nuclear Medicine Procedures in the United States and Worldwide: 2009-2018

The U.S. National Council on Radiation Protection and Measurements (NCRP) conducted a retrospective assessment of the U.S. data, and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) performed a similar worldwide assessment for 2009-2018 (with most data from 2014 to 2017). Using the data from those reports, the frequency of radiologic and nuclear medicine studies, annual collective, and per capita effective dose in the United States for 2016 were compared with worldwide estimates from 2009 to 2018. There were an estimated 691 million radiologic, CT, dental, and nuclear medicine studies performed in the United States in 2016, which represented 16.5% of the 4.2 billion performed worldwide. The United States also accounted for 74 million CT procedures (18% of the world's estimated total), 275 million conventional radiology procedures (11% of the world's total), 8.1 million interventional radiologic procedures (34% of the world's total), 320 million dental radiography procedures (29% of the world's total), and 13.5 million nuclear medicine procedures (34% of the world's total). The U.S. collective effective dose was 717 000 person-sieverts (17.6% of the world's total). The average annual individual effective dose in the United States was 2.2 mSv compared with 0.56 mSv worldwide. The United States accounts for a large and disproportionate share of global medical radiation procedures and collective effective dose, but use of CT has increased more in other countries compared with the United States.

The why, who, how, and what of communicating CT radiation risks to patients and healthcare providers

Computed tomography (CT) has witnessed tremendous growth in utilization. Despite its immense benefits, there is a growing concern from the general public and the medical community about the detrimental consequences of ionizing radiation from CT. Anxiety from the perceived risks associated with CT can deter referring physicians from ordering clinically indicated CT scans and patients from undergoing medically necessary exams. This article discusses various strategies for educating patients and healthcare providers on the benefits and risks of CT scanning and salient techniques for effective communication.

Typical effective dose values from diagnostic and interventional radiology

Effective dose (E) in medical procedures is of practical value for comparing doses from different types of procedures which is not possible with the different measurable dose quantities. In this survey we estimated and compared the values of E in 12 medical imaging procedures. The mean E for conventional X-ray in mSv were ranged between 0.01 for skull lateral to 0.5 for abdominal while the mean E for cardiac interventional radiology in mSv was up to 16 for percutaneous transluminal coronary angioplasty (PTCA). For dental radiology the mean E in μSv was 1.2 for intraoral and 10 for panoramic. In mammography the mean E was 0.5 mSv. Compared with the literature, chest postero-anterior (PA), lumbar spine AP, PTCA and mammography procedures had shown larger E values. The obtained results can help in justification of techniques.

Review of compounds that exhibit radioprotective and/or mitigatory effects after application of diagnostic or therapeutic ionizing radiation

PURPOSE

Exposure to ionizing radiation can be accidental or for medical purposes. Analyzes of the frequency of radiation damage in the general population, it has been determined that by far the most common are those that occur as a result of diagnostic or therapeutic procedures. Damage caused by radiation, either accidentally or for therapeutic purposes, can be reduced by the use of radioprotectors, mitigators or other therapeutic agents. A detailed research of the available literature shows that there is little systematized data of potentially radioprotective and/or mitigating effects of drugs from the personal therapy of patients during the application of therapeutic ionizing radiation. The aim of this paper is to present review of compounds, especially personal therapy drugs, that exhibit radioprotective and/or mitigating effects after the application of diagnostic or therapeutic ionizing radiation.

CONCLUSIONS

Given the widespread use of ionizing radiation for diagnostic and therapeutic purposes, there is a clear need to create a strategy and recommendations of relevant institutions for the use of radioprotectors and mitigators in everyday clinical practice, with individual evaluation of the patient's condition and selection of the compounds that will show the greatest benefit in terms of radioprotection.

Radioprotection and Radiomitigation: From the Bench to Clinical Practice

The development of protective agents against harmful radiations has been a subject of investigation for decades. However, effective (ideal) radioprotectors and radiomitigators remain an unsolved problem. Because ionizing radiation-induced cellular damage is primarily attributed to free radicals, radical scavengers are promising as potential radioprotectors. Early development of such agents focused on thiol synthetic compounds, e.g., amifostine (2-(3-aminopropylamino) ethylsulfanylphosphonic acid), approved as a radioprotector by the Food and Drug Administration (FDA, USA) but for limited clinical indications and not for nonclinical uses. To date, no new chemical entity has been approved by the FDA as a radiation countermeasure for acute radiation syndrome (ARS). All FDA-approved radiation countermeasures (filgrastim, a recombinant DNA form of the naturally occurring granulocyte colony-stimulating factor, G-CSF; pegfilgrastim, a PEGylated form of the recombinant human G-CSF; sargramostim, a recombinant granulocyte macrophage colony-stimulating factor, GM-CSF) are classified as radiomitigators. No radioprotector that can be administered prior to exposure has been approved for ARS. This differentiates radioprotectors (reduce direct damage caused by radiation) and radiomitigators (minimize toxicity even after radiation has been delivered). Molecules under development with the aim of reaching clinical practice and other nonclinical applications are discussed. Assays to evaluate the biological effects of ionizing radiations are also analyzed.