Radiation dose and related risk in interventional cardiology

Abstract

Cardiac catheterization procedures are classified as high radiation dose procedures, European Directive 2013/59/Euratom, and may result in skin injuries (deterministic effects) and/or increased cancer risk (stochastic effects). The radiation-induced effects can be expressed differently depending on whether the patient is a child or an adult. In adults there is an increased risk for developing radiation-induced skin injuries while in children the risk for radiation-induced cancer is more prominent. The cancer risk associated with radiation should be communicated to patients [directive 2013/59/Euratom], and patients with increased risk for skin injuries should be included in a follow-up program [ICRP 85]. However, skin dose and cancer risk estimates from cardiac interventional procedures are complicated because the dose indicator on the X-ray equipment only provides information on the total amount of radiation (air kerma-area product, KAP and cumulative air kerma at patient entrance reference point) used, without taking into account different radiation geometries.

To facilitate monitoring of radiation induced effects, conversion coefficients that relate the radiation exposure as expressed by the air kerma-area product (KAP) to maximum entrance skin dose (MESD), equivalent organ dose (HT), effective dose (E), risk for exposure-induced cancer death (REID) and organ-specific risks of exposure-induced cancer death (REIDHT) have been estimated in the current thesis. Based on such coefficients, simple tools are suggested that can be used in the clinical routine with the aim to assess radiation doses and related risks. Two commercial software were used for the estimation of dose and related risks, WinODS (version 1.0a; RADOS Technology Oy, Finland) and PCXMC (v.1.5 and v.2.0; Radiation and Nuclear Safety Authority, Helsinki, Finland). In addition, film dosimetry techniques were been used for skin dose determination. The results from cohort studies on adult patients that underwent cardiac procedures showed that the skin dose (MESD) varies with both the type of cardiac procedure and with the skill of operator. On a patient-by-patient basis, the conversion coefficient relating KAP to skin dose can be used to identify patients with risk for radiation induced skin injury, to be included in follow up programs in accordance with ICRP 85.

The relationship between E and KAP and between HT and KAP demonstrated a need for age-dependent conversion coefficients (E / KAP; HT / KAP) within the paediatric age range. Estimation of these quantities are performed using radiation exposure data retrieved from the patient radiation dose sheet as well as data from the complete radiation dose structured reports (RDSR); the latter contains detailed information on beam geometry and exposure data for the whole procedure. No significant difference between the two methodologies could be demonstrated for the population-averaged conversion coefficients (E / KAP; HT / KAP) except for HT (lung) in new-borns. The importance of this outcome is that hospitals that do not have access to the data retrieval and calculational methods used in this thesis (partly in-house development) can apply less detailed techniques without a significant effect on the estimated population dose, for a specific age group. Furthermore, the relationship between risk for exposure-induced cancer death (REID) and KAP displayed both age- and gender-dependence for paediatric cardiac catheterization procedures. The conversion coefficient (REID / KAP) can be used to assess the population cancer risk. In clinical situations, the estimated population cancer risk for the procedure can guide the operator in communicating risks to the patient/parent. Additionally, it has been shown that the risk organs for adult patients undergoing cardiac catheterizations are lung and bone marrow (leukemia), as well as for children lung and breast.

The thesis introduces a novel concept based on age- and gender-specific risk reference values (RRV) that is related to a specific REID-level. By setting an “acceptable” level on the REID for a given patient group, the corresponding KAP value can be calculated and used for monitoring risk for late effects when performing cardiac intervention. In conclusion, cardiac catheterizations are life-saving procedures and thus the benefit is always considered to outweigh the risk. However, the ALARA principle (as low as reasonably achievable) must still be applied, which means that the patient must not be exposed to a higher radiation dose than is required for the procedure. Conversion coefficients presented in this dissertation can be a support for applying both the ALARA principle in clinical practice as well as for the identification of patients to be included in follow-up programs addressing skin injury and for communicating population cancer risk to patients. An additional intent of this thesis work has been to shed further light on important parameters that affect the radiation dose and risk, such as beam geometry (related to operator skill) and patient age and gender.