Oncology

Chapter 20


Oncology





INTRODUCTION


The prevalence of tumors in horses is increasing, partly related to animals living to older ages but also to better preventive and therapeutic medical practices. As a result, veterinarians are called upon more frequently to diagnose and manage horses with tumors. Horse owners are more aware of progress in cancer treatment and are becoming more demanding in seeking care for their horses.


More than 80% of tumors in horses affect the skin and subcutaneous tissues. Because lesions involving the skin are easily seen by the owner and so are often rapidly brought to the attention of the veterinarian, the vast majority of equine tumors should be caught early and be highly curable. Unfortunately, mismanagement of equine tumors due to lack of knowledge of the disease process and misuse of conventional treatments may result in euthanasia.


In this chapter the principles of oncology as applied to the equine patient are presented. The intent is to provide equine practitioners an overview of the principles and practices of cancer diagnosis, and treatments. Readers should refer to the Index and the systematic chapters for details of diagnosis and treatments of specific tumor types and locations.


There are five rules for successful tumor management ( Box 20.1).



Box 20.1   Rules of tumor management




A correct assessment of the treatment goal is critical to select the optimal treatment or treatment combination. Overly aggressive treatment plans can expose animals that are not curable to needless morbidity or to prolonged and expensive treatments. On the other hand, therapeutic decisions that are too pessimistic deprive the animal of a chance for cure.



ASSESSMENT OF THE CANCER PATIENT





PHYSICAL EXAMINATION


Before proceeding to a detailed assessment of a tumor, it is essential to examine the whole animal. A complete physical examination will alert the clinician to other masses that may not have been noticed by the owner and will also allow an appraisal of any concurrent disease.


Although the incidence of metastatic disease for malignant tumors in the horse is only 25%, many tumors (including equine sarcoid [q.v.], squamous cell carcinoma [q.v.], and melanoma [q.v.]) frequently arise in multiple sites on the same horse.


For deep-seated tumors, the clinician relies upon clinical manifestations of the disease resulting from a mass effect leading to obstruction or compression, bleeding, and effusion or endocrine syndromes associated with hyperfunctional tumors. Frequently, indirect systemic effects of cancer, known as paraneoplastic syndromes, including cachexia, fever not associated with infection, hypercalcemia, hematologic–hemostatic abnormalities and hypertrophic osteopathy are the first manifestations of an occult tumor.


For superficial or accessible tumors such as cutaneous and endocavitary tumors, the majority of the physical examination can be performed by direct visual assessment and manual palpation. Cutaneous and subcutaneous tumors such as sarcoid tumors, squamous cell carcinomas, basal cell carcinomas, adenocarcinomas (sweat gland), squamous papillomas, melanomas and dermal melanomatosis, cutaneous lymphomas, soft tissue sarcomas including fibrosarcomas, schwannomas, hemangiosarcomas, liposarcomas, lymphangiosarcomas, hamartomas and mast cell tumors (q.v.) should be examined with respect to their appearance (mass or ulcer, symmetry, contour), color, number, size, location, consistency, presence or absence of fixation to underlying tissue, and whether the overlying skin is ulcerated.


The growth pattern of the lesion, i.e. any irregular growth and contour, should be carefully assessed. Based on appearance, the biological behavior of the lesion, and risk factors including age, breed/skin pigmentation, exposure to contributing agents such as viruses, sun exposure, chemicals and trauma, a tentative diagnosis of “tumor” can be made.



DIAGNOSIS


Although the clinical manifestations or appearance, location and growth pattern of a lesion may give the veterinarian a high degree of suspicion as to the type of tumor involved, it is imperative that a definitive diagnosis be attained so as to plan treatment properly and discuss prognosis with the owner. The most common diagnostic procedures are cytology and tissue biopsy.



Aspiration cytology


Aspiration cytology is simple procedure requiring only a 22 G, 25 mm (1 inch) needle and a 6 mL syringe for superficial lesions or 22 G, 65 mm (2.5 inch) spinal needle with stylet for deeper lesions.


The technique can be used for tissues (fine needle aspirate) or body cavity fluid. The site is cleaned with a surgical scrub and alcohol if an abdominal cavity is to be entered. Usually there is no need for sedation or local anesthesia. Topical anesthetic (lidocaine 2.5% and prilocaine 2.5%) may be used on a shaved skin but local anesthesia takes approximately 15 min for mucosa and 30 min for intact skin. The sample is obtained by gentle aspiration, as the needle is advanced through the tissues in several different locations.


All negative pressure must be released before the needle is withdrawn from the tissue. The syringe is detached and the aspirated cells are then expelled onto a microscope slide with a frosted end. A second slide is placed flat on the first and the two are gently slid apart to spread the aspirate. The slide is then allowed to air-dry. If larger volumes of fluid sample are obtained, a small drop can be smeared in a similar manner. Information regarding patient and aspiration site should be written with a pencil on the frosted end.


Separate needles are used for sampling multiple sites. Multiple slide preparation with standard fixative should be made when possible and the cellularity of the specimen should be evaluated prior to submission to a diagnostic laboratory by microscopic examination of a stained slide preparation to ensure sample quality. The air-dried slides are then shipped by courier or mail to a diagnostic laboratory in a polypropylene slide mailer with a snap top lid or Styrofoam container to protect the glass slides.


Fine needle aspirate cytology is a screening tool to differentiate neoplastic from inflammatory lesions. Cytology often helps differentiate between epithelial and connective tissue tumors. Several tumors such as melanomas, lymphomas and mast cell tumors (q.v.) can be accurately diagnosed cytologically. In most cases a positive cytologic finding can only give a tentative diagnosis and a negative cytologic finding should be treated as inconclusive. Aspiration cytology cannot be a substitute for histopathology because it does not provide any information about the biological behavior of the tumor.



Biopsies


Procurement and interpretation of a biopsy specimen is perhaps the most important step in the management of all tumors. Not only will the biopsy provide a definitive diagnosis, it may also be used to predict the biologic behavior of the tumor, which aids in selecting the type and extent of treatment and determining prognosis. It is important to provide the pathologist with a well-fixed (1 part tissue volume to 10 part fixative volume) tissue specimen large enough to represent the disease process. Fixative solutions (10% buffered neutral formalin) are provided in containers of different sizes by diagnostic laboratories upon request. Fixed samples are kept at room temperature and shipped to the local veterinary diagnostic laboratory.


As a rule, any active lesion (growth, appearance change, new lesions) that is not responding or temporarily responding to medical treatment should be biopsied.


There is no evidence that a biopsy can negatively affect the outcome of a patient as long as it is followed by definitive treatment if the diagnosis is tumor. Moreover, in the human and veterinary literature there is no suggestion of tumor transformation or an increased risk of metastasis after a planned biopsy. The advantages of an accurate diagnosis far outweigh the theoretical risk of enhancing metastasis or the unproven risk of transforming a benign lesion into a malignant one. Thus there is no medical reason for not performing a biopsy, and if a client brings an active lesion to a veterinarian’s attention it is worth taking a biopsy. If a veterinarian recommends a biopsy for a suspicious lesion and the client declines the procedure, the reasons for not performing the biopsy should be documented in the medical record for legal purposes and a plan for follow-up established.


The only contraindication for biopsy of an accessible lesion is the lack of commitment of the owner to pursue treatment for financial or emotional reasons because the trauma caused by the biopsy can trigger tumor proliferation of an otherwise slow-growing tumor and precipitate its local evolution. The non-medical reasons for not performing a biopsy should be documented in the medical record and a plan for follow-up established.


An accurate diagnosis should be obtained prior to surgery if the type of treatment or extent of the treatment depends on the tumor type or when the owner’s decision to treat depends on prognosis. Large lesions or lesions in unfavorable locations should be biopsied before any treatment attempt.


The most common techniques of tissue procurement prior to surgery are needle core biopsy, punch, pinch and incisional biopsies (q.v.). All biopsy tracts should be positioned in such a manner that the tracts they create can be excised in continuity with the primary tumor at surgery to prevent tumor seeding. In other words, prior to performing a biopsy, the veterinarian must assume the lesion is a tumor and determine an appropriate surgical approach. Keeping this in mind, the veterinarian must then plan the biopsy sites and tracts within the planned surgical field.



Needle core biopsy

Needle core biopsies of soft tissue tumors may be obtained with Tru-Cut or ABC needles. Palpable lesions may be fixed in place manually and biopsied blindly. Deep-seated lesions must be biopsied under ultrasound guidance. Bone lesions and bone marrow biopsies may be performed with a Jamshidi needle or Michelle trephine.


After sterile skin preparation with surgical scrub, 3–5 mL of buffered lidocaine (3 mL of 2% Xylocaine plus 1 mL of sodium bicarbonate 8.4% added immediately before injection) is injected using a 25 G needle in and around all of the tissues that extend from the skin to the lesion. A small stab incision is made in the overlying skin, and through the same skin hole several needle cores are removed from different sites. Tissue samples recovered from the instrument are fixed in formalin solution.


The biopsy procedure is usually done in the conscious animal using sedation, e.g. detomidine 0.01 mg/kg IV plus butorphanol 0.1 mg/kg IV, or xylazine 0.5 mg IV plus butorphanol 0.1 mg/kg IV, if necessary.


In spite of a small sample size, the structural relationship of the tissue and tumor cells can be determined histologically. The technique is relatively atraumatic but care must be exercised to avoid large blood vessels.



Bone marrow aspiration and biopsy

Bone marrow aspiration, and less commonly biopsy, is an essential procedure for determining cytologic abnormalities of the bone marrow caused by a wide variety of neoplastic and myelodysplastic conditions. It is used for the diagnostic and staging of hematopoietic malignancies (lymphomas and leukemias [q.v.]). A bone marrow biopsy is used when bone marrow aspiration does not yield enough tissue for diagnostic.


The preferred site for bone marrow aspirates is the sternum (ventral midline between front legs) because hematopoietic activity persists throughout life, and it provides an easy access. Other sites include tuber coxae or proximal ribs. The hair is clipped, and the bone marrow aspiration site is prepared with a surgical scrub. Using a 25 G needle, approximately 3–5 mL of buffered lidocaine (3 mL of 2% Xylocaine plus 1 mL of sodium bicarbonate 8.4% added immediately before injection) is injected in and around all of the tissues that extend from the skin where the bone marrow needle is to be introduced to the bone.


The 12 mL collection syringe and bone marrow needle is rinsed with EDTA before the procedure to reduce clotting of the bone marrow sample. The biopsy area is scrubbed a final time and, using sterile technique, the skin is stretched between the thumb and index finger and a small stab incision of the blocked skin is made with a no. 11 surgical blade. The bone marrow needle (50 mm/2 inches long, Illinois or Rosenthal bone marrow needle) with the stylet in place is advanced through the stab incision and through the skin and subcutaneous tissue, down to the bone. With the stylet in place, the bone marrow needle is advanced into the bone, using a corkscrew motion (clockwise and counterclockwise) while applying firm and steady pressure. The instrument should not be allowed to wobble. When the needle is firmly fixed in the bone, the stylet is removed and the syringe is affixed.


The bone marrow sample is aspirated briskly into the 12 mL syringe; usually, 1 mL of marrow is adequate. If a sample is not obtained, the stylet is replaced in the bone marrow needle, and the instrument is then advanced further into the bone for a second attempt at aspirating marrow contents. Once marrow has been obtained, marrow can be spread on a glass slide with a frosted end into a monolayer like an ordinary blood smear and air-dried. Sample shipment and identification are the same as for any aspiration cytology sample.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Oncology

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