Histopathology & Tumor Diagnosis
The gross appearance of specific neoplasms rarely is distinctive. Even differentiating neoplastic from non-neoplastic lesions, based upon gross characteristics, is problematic. Examining impression smears and fine-needle aspirates provides some additional information, but these samples still lack the full context of cellular arrangements and relationships to adjacent structures. Histologic evaluation of solid tissue (biopsy specimens from clinical patients, or specimens taken at necropsy) is a powerful diagnostic tool.
Sampling strategies are dictated by lesion size, location, cosmetic outcome, and diagnostic utility. Small (≤1cm diameter) lesions often can be removed completely, including a normal tissue margin. Overzealous squeezing, crushing, electrocautery, or laser application causes disturbing artifacts in these small samples; handle them gently. Larger lesions may still be removable in toto, and submission of whole masses allows for thorough interpretation of not only the lesion features, but also the excisional completeness. Orienting the specimen to the excision site can be done with simple verbal descriptions, or variously placed sutures, or use of multicolored inks applied at specific edges or surfaces. Such landmarks aid in the proper trimming, sectioning, and evaluation of the whole lesion by the pathology laboratory personnel. Similar distinctive markings can allow proper identification of each specimen, when several lesions are submitted simultaneously in the same container. If you receive a report from a pathologist saying a tumor was removed with the clean margins except in one small area, one of the first things an Oncologist will ask- is what area was that in the patient? Preparing samples before submission makes the information you receive more useful.
The location of some large lesions will preclude complete surgical removal. Clinicians must judge how much tissue can be taken, to allow reasonably functional healing, while providing representative samples for diagnostic purposes. Active edges of lesions typically are more useful than necrotic cores, though the most informative specimens of damaged bone (e.g., osteosarcoma) often come from the lesion center.
Nearly all histopathology can begin with tissues fixed in 10% buffered neutral formalin. Some special fixatives have been touted, but these require additional attention to sampling, fixation time, processing quirks, tissue storage, and fixative disposal. Formalin doesnt penetrate tissues rapidly, and specimens much more than 8mm thick [the other two dimensions are not particularly relevant] will not fix in 24 hours. Use care in slicing a large specimen to enhance fixation; too often, these specimens are nearly shredded, such that interpretation of the whole mass is impossible. Exceptions are eyes, brains, and spinal cords, which are better fixed whole. These tissues are so delicate that slicing them when fresh causes more interpretive problems than just letting them fix slowly. Fixative volume should be 10-20 times the tissue volume. After 18-24 hour formalin immersion, samples can be transferred to a container with just enough formalin to keep the specimen moist, for shipment to a pathology laboratory. There is no need to send large volumes of fixative; it is just more expense, and an opportunity for leakage or breakage. Occasional specimens, such as an entire amputated limb, or a whole, enlarged spleen, are difficult to sample thoroughly, or to fix properly. Those sorts of large, intact specimens can be refrigerated and shipped fresh (unfixed) in an insulated container. The pathologist can select and fix representative samples for histologic evaluation, and retain the rest of the specimen for additional dissection, if needed. Sending the entire organ or mass may sound gruesome but it is far preferable over sending a portion that either leads to no diagnosis or limits what can be said about margins of the resection.
The specimen(s) should be accompanied by a thorough clinical history. Descriptions of location, size, shape, color, consistency, rate of growth, and any effects on the patient should be included for each sample. Most tissue processing on well-fixed soft tissues will yield hematoxylin and eosin-stained glass slides by the next working day, following receipt of the specimen. Samples of bone will require some additional processing time [usually 1-2 days] to allow sectioning of such specimens. Histopathologic results are reported (by telephone, FAX transmission) routinely the same day that the glass slides are received by the pathologist. Some specialized techniques (other histochemical stains, or immunohistochemistry, or molecular diagnostic procedures) are powerful ancillary steps, which may require another day or two of processing.
A final, written histopathology report will include two components; a detailed description of the tissue findings, and (more important for the clinician) an interpretation of the diagnosis and its implications. Such interpretations of neoplasms should include tumor grading and staging, when appropriate.
Veterinarians who use a diagnostic facility for histologic evaluation of neoplasms will become familiar with the particular laboratorys capabilities. When in doubt, call the laboratory. Not every laboratory does everything, and not every laboratory does everything exactly the same way as all other laboratories. Veterinarians who provide histopathology service are interested and integral colleagues in the care and management of patients, especially those with neoplasms.
Written by Dr Charles Leathers, Washington State University.