The Chernobyl Thyroid Cancer Experience: Pathology

Abstract

The Chernobyl accident was followed by a large increase in the incidence of thyroid carcinoma in the areas exposed to high levels of fallout. The Chernobyl Tumor Bank was set up in 1998 to make tumours available for study internationally, and a pathology panel reviewed all the tumours and established an agreed diagnosis. The thyroid tumours that were discovered after the Chernobyl nuclear accident were virtually all (95%) of the papillary carcinoma type. Rare examples of other tumour types were identified. Within the papillary group, several subtypes were noted, including classical or usual type, follicular variant, solid variant and mixed patterns Diffuse sclerosis variant, cribriform/morular type and Warthin-like variant were rare. No tall cell or columnar cell variants were identified. The tumours examined by the Pathology Panel of the Chernobyl Tumor Bank constitute a large representative sample (estimated at about 50%) of the tumours that developed in this population. This overview describes the method adopted by the panel and the different diagnostic categories adopted; illustrates the pathology of these neoplasms; compares the pathological characteristics of the early lesions with those identified after long latency periods and the institution of screening programmes and outlines the possible associated causes for the various morphological patterns seen.

Introduction

About 4 years after the nuclear accident at the Chernobyl nuclear power plant in the north of Ukraine in April 1986, an increased incidence of malignant thyroid tumours was identified in the paediatric population of the exposed regions. The diagnosis of malignancy was confirmed [1] and the tumours were recognised as papillary thyroid carcinomas (PTCs), often of an unusual solid type [2]. In many cases the presence of extra thyroidal extension and cervical nodal metastases were noted. The diagnoses of many of these cases were confirmed and documented by pathologists from western Europe and North America.

The large number of cases led to the realisation that there was a need and an opportunity to preserve material from these tumours in a uniform way and to make them available to the scientific research community to study. The opportunity to improve understanding of thyroid tumorigenesis, radiation-associated tumour formation and carcinogenesis in general was recognised. From these considerations, the realisation that the tumours had to be well characterised pathologically was identified, and a panel of internationally recognised endocrine pathologists was formed, which has met on a regular basis since 1998 to confirm and refine the diagnoses of these tumours. This panel is one of the committees of the Chernobyl Tumor Bank (CTB), which is described in detail in another paper in this special issue of Clinical Oncology [3].

The early experience of the panel led to the realisation that the standard diagnostic classification adopted by the World Health Organization did not cover all the possible outcomes of the panel discussions. This arose because there were tumours where all members agreed it was not possible to be certain of a definitive diagnosis of benign or malignant, sometimes because of a lack of adequate material, and because there were tumours where disagreements between panel members could not be resolved. A modified diagnostic was developed and adopted (Table 1) [4]. The panel was composed of one representative from each of the main thyroid centres serving the exposed areas in Belarus (up to March 2002), Russian Federation and Ukraine, together with one thyroid pathologist each from Europe, Japan and the USA. The representatives of the exposed countries were asked to submit to the panel sections from all the thyroid carcinomas in those under 19 years at the time of the accident from the exposed areas, together with all the cellular thyroid lesions where there was any possible suspicion of malignancy.

The method of working of the panel was to hold 6 monthly meetings at which all the cases were reviewed separately by each member, and then using a multi-headed microscope were reviewed jointly and a consensus diagnosis agreed. There were remarkably few occasions on which the panel disagreed with a submitted diagnosis of malignancy.

Below we will describe the tumours and their subtypes, define the diagnostic categories used by the panel, compare and contrast the tumours in early and later years of the CTB existence, review the differences and similarities in exposed and non-exposed children and summarise the non-neoplastic changes in the non-tumoural thyroids.