Nomenclature and Diagnostic Resources in Anatomic Toxicologic Pathology

3.1

Terminology for Nonneoplastic Lesions

For nonneoplastic lesions, morphologic changes often allow the use of qualifiers that specify important characteristics of the morphological disease process (e.g., distribution, severity, and duration) to effectively convey additional information about morphologic changes. For example, “liver, infiltrate, mononuclear cells, focal, moderate” allow some degree of perspective on the distribution and severity of the morphological change noted. It should be noted that each category of qualifiers typically has a large pool of potential terms lending several possible permutations and combinations available for the capture of the final diagnosis. For example, the qualifier for distribution may include such terms as focal, multifocal, locally extensive, or diffuse.

Organ-specific site qualifiers should be used to delineate lesions based on different regional anatomical differences, tissue-specific responses, or biological significance. The choice of site qualifiers can vary with the anatomic complexity of an organ or tissue and the prevalence of treatment-related effects in an organ. For example, at different “levels” along its length, the nasal cavity is lined by squamous, transitional, respiratory, or olfactory surface epithelia. The response(s) of these epithelia to inhaled toxicants may differ by location, and the use of a site qualifier helps characterize site-specific effects. Likewise, chemically induced lesions in the larynx often have site specificity at the base of the epiglottis; thus, the designation “larynx, epiglottis” is appropriate. Other possible examples of site qualifiers include “liver, bile duct”; “nerve, sciatic”; and “kidney, renal tubule.” Alternatively, in some situations, it may be best to refrain from using site qualifiers. As an example, for inflammation in hollow organs or tissues that have surface epithelia (e.g., the nose, stomach, gastrointestinal tract, or urinary bladder), the use of qualifiers that indicate specific sites (e.g., lumen, epithelium, mucosa, submucosa) may not be appropriate, because the presence of small numbers of inflammatory cells around organs associated with points of entry and/or external environment is generally considered to be within normal limits and has no biological relevance. Therefore, unless a site-specific toxicity is suspected, in such instances, characterization of a background lesion is unnecessary, and designating the overall severity of inflammation may be best used to convey differences in lesion distribution.

Distribution qualifiers are used to indicate the pattern of an effect (e.g., diffuse, locally extensive, multifocal, or focal). However, focal and multifocal qualifiers may also be addressed by the severity grade. Distribution modifiers can provide additional site specificity and indicate an inherent feature of a lesion that has a particular biological significance or reflects a specific pathogenesis. For example, centrilobular hepatic necrosis rarely has the same pathogenesis as multifocal, randomly distributed hepatic necrosis; therefore, it is important to include these types of distribution modifiers. Likewise, it may be toxicologically relevant to the interpretation of a study to know whether hyperplasia of the forestomach is focal versus diffuse. However, the use of distribution qualifiers is usually not warranted for common background, spontaneous, and/or age-related lesions specific for certain species and strains, unless an exacerbation of such lesions is suspected. Because the objective of the pathologist is to identify test article–related effects, such common background, spontaneous, and/or age-related findings may typically be cited via references in the pathology report. Severity grading of common background lesions is particularly relevant when such lesions are dose response related, where grading may be the most reliable means of effectively documenting differences between test groups and controls and among test groups, i.e., dose response. For example, certain classes of chemicals can potentiate chronic progressive nephropathy (CPN) as well as alpha _2u -globulin nephropathy in male rats. In such cases, assigning severity grades can provide important information.

Although distribution qualifiers depict the pattern of a lesion with additional specificity, the intensity or magnitude of the lesion is reflected by the use of severity grades. Severity grading is the designation of a semiquantitative score to a lesion or process (e.g., often a four- or five-point scale, depending on the software program used for data entry). It is used to reflect a combination of the amount and distribution of tissue involvement, as well as the actual degree of tissue damage. Typically, these include minimal (1), mild (2), moderate (3), and marked (4), and if a five-point scale is used, severe (5). Regardless of whether a four- or five-point scale is used, general criteria used to define each grade should be included in the pathology report. Severity grading of nonneoplastic lesions can provide useful dose–response information in that a test article may alter incidence of a lesion or its severity, or both. Severity grades are continuous rather than discrete data representing degrees of changes relative to similar lesions in controls and other animals in a study, and thresholds for grading often vary between individual pathologists and or study types.

Severity grading (ordinal data) is reserved for nonneoplastic lesions, whereas neoplasms are considered either present or not present (nominal [diagnosis] or quantal [malignant vs. benign] data). Severity grading adds little valuable information in some nonneoplastic lesions. Examples include calculus in the urinary tract or cysts of the pituitary pars distalis . Severity grades are most useful when making comparisons of qualitative changes, for example, when average severity increases with increasing dose, or when comparing the relative toxicity of structurally related compounds. The generation of data allows statistical analysis and, in some instances, may be the only indication of treatment-related differences among test groups.

Duration qualifiers are more commonly used in association with particular types of inflammatory changes. The commonly used notation of “-itis” in diagnostic pathology is discouraged in toxicologic pathology and replaced with more descriptive terminology. Commonly used duration qualifiers include acute (neutrophils), subacute (lymphocytes), chronic (macrophages or mononuclear cells with or without fibrosis), and chronic active (a mixture of macrophages or mononuclear cells and neutrophils). In histopathological evaluation of toxicological studies, it is recommended to describe the cell type within the morphological description in the pathology narrative rather than duration qualifiers.

3.2

Terminology for Neoplastic Lesions

A few rules exist for the terminology applied to neoplasms. By convention, it is assumed that neoplasms not designated as metastatic are primary or arising at that site. For neoplasms in tissues distant from the primary site, the qualifier “metastatic” is added to the diagnosis, and the site of the primary lesion can be indicated parenthetically (e.g., “lung—hepatocellular carcinoma, metastatic (liver)”). For some neoplasms, the diagnosis should indicate whether the lesion is benign or malignant (e.g., “adrenal gland, medulla—pheochromocytoma, benign” and “adrenal gland, medulla—pheochromocytoma, malignant”). The occurrence of multiple and bilateral neoplasms should be indicated, especially when this occurrence appears to be a treatment-related phenomenon. Examples include “liver—hepatocellular adenoma, multiple” and “kidney, renal tubule—adenoma, bilateral.”

Diagnoses for neoplasms should not contain site qualifiers unless such qualifiers serve to distinguish the histogenesis of one neoplasm from another. For example, the site qualifiers “C-cell” or “follicular cell” should be used to distinguish carcinomas of the thyroid gland.

Finally, inflammation and other types of secondary changes that may result from tissue damage associated with the presence of neoplasms (e.g., a histiocytic infiltrate in the alveoli associated with an alveolar bronchiolar carcinoma) are usually not meaningful to the study and therefore should not be recorded as separate diagnoses, but, if deemed important, they can be described in the narrative section of the pathology report (see Preparation of the Anatomic Pathology Report for a Toxic ity Study , Vol 2, Chap 13).

4.1

Training

Most anatomic toxicologic pathologists are trained in a veterinary diagnostic pathology setting, usually at a college of veterinary medicine or, much less frequently, a medical college. The primary objective in diagnostic pathology is to identify the cause of disease or death for an individual animal and communicate the findings in a detailed fashion (i.e., each disease process is identified and recorded as a separate entity) to the veterinarian or clinician for that single case. In contrast to toxicologic pathology, consistency in morphologic diagnosis terminology across cases is not a major concern.

The emphasis in toxicologic pathology is on identifying changes in relation to a treatment group rather than in an individual animal—dose and response. Specifically, the goal of the toxicologic study pathologist is to determine if there are differences in the incidences and/or severities of lesions in treated groups compared to the control animals (positive and negative controls, where applicable) in order to provide dose–response data, and occasionally, where applicable, to delineate effects of test article from formulation. Study pathologists must identify the various lesions in tissues, but generally have less freedom than a veterinary or medical diagnostic pathologist in the type and number of diagnoses that they may record. For the study pathologist, it is especially important to use consistency and brevity in recording diagnoses in order to facilitate the statistical evaluation (e.g., carcinogenicity studies) and interpretation of the findings.

Diagnostic terminology is influenced by the training and experience in laboratory animal pathology that the toxicologic pathologist may have received. A histopathological diagnosis is primarily a qualitative judgment of the nature of a specific lesion and its apparent or expected biological behavior. Each diagnosis is a subjective observation, the accuracy of which depends on the pathologist's training and experience, the state of knowledge of the specific disease process, and the generally accepted diagnostic criteria and nomenclature within the profession.

The training, qualifications, and experience of the pathologist can influence how lesions are interpreted, and hence, the selection of diagnostic terminology. The majority of veterinary anatomic pathology training occurs in domestic and companion animal settings, with occasional exposure to the common background lesions of laboratory animals; trainees are less often exposed to toxicologic pathology. This may lead to selection of inappropriate terminology by pathologists with less experience evaluating rodent studies. Common issues encountered in evaluations by novice pathologists include the use of multiple morphological descriptive terms or synonyms for a lesion or disease process; inconsistencies in the designation of topography, sites, and subsites; and duplication of diagnoses, any of which may result in misinterpretation of toxicologic data.

Among experienced pathologists, nomenclature is still an issue. It is influenced by their training, as well as by individual and philosophical biases. One example includes terminology to describe inflammation composed primarily of mononuclear cells and fewer neutrophils. Some pathologists prefer the term “chronic active,” while others prefer the term “chronic.” Still others may use the term “cellular infiltrate” rather than “inflammation” with or without cell type modifiers.

Regardless of the chosen terminology, it is important for the study pathologist to identify such diagnostic differences and either resolve the difference, explain the reason for the use of alternate terminology in the pathology narrative, or state that the two different diagnoses reflect the same pathological process. Pathologists with experience evaluating rodents trained in a “basic” experimental research setting may be accustomed to nomenclature used for genetically engineered rodents, which may differ from that used in toxicologic pathology ( ).

Some customized Good Laboratory Practice–compliant computerized systems allow study pathologists to build a selective diagnostic vocabulary. For experienced and novice pathologists alike, the availability of a wide range of possible diagnostic terms can be a source of inconsistency due to overlapping criteria in defining subjective lesions, thus violating the assumptions for the statistical tests used. Additionally, multiple studies conducted with different pathology data capture systems for the same test article often generate nomenclature issues based on the differences in lexicon available within each data capture system. Often, this is mitigated by inclusion of a second-tier evaluation by a peer review pathologist. The peer review process allows the peer review pathologist to evaluate and compare diagnoses with the study pathologist, which results in better standardized nomenclature for the study (see Pathology Peer Review , Vol 1, Chap 26 ). Occasionally, unresolved diagnoses between the study and peer review pathologist are further examined by a Pathology Working Group (PWG) to reach a consensus. Such a process allows for consistency in diagnoses between pathologists and between studies for the same test article.

4.2

Thresholds

Training and/or experience of the pathologist can play a role in the applications of “thresholds” by individual pathologists from a diagnostic perspective. The use of “thresholds” should not be confused with use of this term by toxicologists as relevant in carcinogenicity risk assessment. Histopathology evaluation of subtle lesions poses problems when one pathologist considers a tissue to show variation within normal limits of the species/strain, while another pathologist considers the variation to be above the threshold and, therefore, a lesion. Such differences impede the dose–response assessment.

In general, there exists an inherent spontaneous biological variation in a given tissue that is expected for the species/strain, which may change with age. Pathologists use knowledge of this variation in species/strain, as well as incidence in concurrent control group animals (when appropriate), to ascertain diagnostic criteria to determine thresholds for selecting and differentiating diagnoses. Exacerbation may occur as increased incidences or severities or both. Some pathologists may choose to diagnose all changes considered to be background at the lowest severity grade, and any treatment-related changes with an increased severity grade (where applicable). Alternatively, a diagnosis may be made only when the level of variation is determined to be above a diagnostic “threshold” of the background finding when it exceeds that considered to be within normal limits. For example, peripheral nerve fiber degeneration is a common age-related finding in older rats (especially males). Because an actual treatment-related effect is not diagnostically different from background findings, an increase in severity across dose groups may be the only parameter that captures potential exacerbation at the nomenclature level. Therefore, such triggers during histopathological evaluation should spur reevaluation of the diagnostic threshold initially set for this finding to ensure that exacerbation of the background lesion due to treatment is indeed considered. Further, the pathologist must address this specific finding in the narrative within the context of what is known about background nerve fiber degeneration in rodents, potential differences between sexes, age (a one-month repeat-dose study vs. a carcinogenicity study), and other influencing factors.