Richard A. LeCouteur, BVSc, PhD, DACVIM (Neurology), DECVN
The detection, localization and characterization of brain lesions has been greatly improved through the use of computed tomography (CT) and magnetic resonance (MR) imaging, however in most cases appearance alone provides only a broad list of differential diagnoses for these lesions. There remains a need to obtain an intraoperative neuropathological diagnosis from tissue samples of the lesion. In people the intraoperative cytological evaluation of smear preparations of brain lesions has become a routine procedure, providing a rapid, highly accurate diagnosis. In addition, future therapies may involve intralesional administration of drugs, following results of a brain biopsy. The need to obtain biopsy material for diagnosis and/or to deliver therapeutic agents with precision and without an invasive surgical procedure has stimulated the development and refinement of image-guided brain biopsy.
For a number of reasons stereotactic brain biopsy has been slower to develop in dogs and cats as compared to people where it is viewed as a routine procedure. Most commercially available systems use a cumbersome head-frame, designed specifically for the human skull, and require dedicated, expensive computer software for the planning phase. More recently several different techniques of image-guided brain biopsy have been studied in dogs.
CT-Guided Free-Hand Needle Biopsy
CT-guided, free-hand needle biopsy of brain tumors has been reported in a series of eight dogs with intracranial lesions. The procedure was performed in order to obtain a histological diagnosis prior to initiation of treatment. Results of the study indicated that although free-hand CT-guided needle biopsy was a safe procedure, the diagnostic yield was low, and incorrect (normal tissue) samples were obtained in five of eight dogs. In one dog in which a meningioma was diagnosed by biopsy, necropsy results showed that the tumor was in fact an oligodendroglioma. Error was attributed to an inadequate amount of tissue being obtained on biopsy. The low diagnostic yield was especially disappointing considering the fact that the investigators obtained four tissue samples (each 1-2 mm in diameter) from each dog.
Pelorus Mark III Stereotactic Biopsy System
More recently modifications of the human CT-guided Pelorus Mark III Stereotactic Systema have allowed this system to be used very successfully in obtaining biopsies of brain lesions in dogs and cats. This system has been promoted as being relatively inexpensive, mechanically less complex, and easier to use than other commercially available stereotactic systems. The Pelorus Mark III Stereotactic System differs from other devices in two important aspects that have allowed its adaptation for use in dogs and cats. It employs a simple metal ring as a base plate to attach various imaging and biopsy devices to the patient's skull. Other systems employ a large ring that encircles the patient's head like a crown. Secondly the translation of the target coordinates from CT space to biopsy frame space is accomplished mechanically on a special phantom frame, and does not require any propriety computer software.
Modifications of the human system are necessary to accommodate the 90% shift in orientation of the canine head as compared to the human head during CT imaging, and to facilitate other phases of the biopsy procedure that are affected by the uneven and variable size and shape of canine and feline skulls. This is accomplished by the addition of a plastic adaptor skull ring to the system. This adaptor ring, by having more screw holes, is more easily attached to the dog's skull than the Pelorus aluminum skull ring used in people. Placement of AO spiked washers between the plastic adaptor ring and the skull helped to secure the ring, which is especially important in smaller patients.
The safe and accurate use of this device to perform CT-guided stereotactic brain biopsies in 50 dogs with intracranial lesions has been reported by the author. Since that time it has been used successfully in an additional 70 dogs and cats. In the 50 dogs reported the stereotactic brain biopsy diagnosis was compared with the diagnosis obtained from surgical resection, or necropsy in 22 animals. The stereotactic biopsy provided a correct diagnosis in 20 dogs for an overall diagnostic yield of 91%. This is similar to the diagnostic yield reported in people.
Using this system, brain biopsies were most easily performed on rostral fossa lesions. This is related to the ease of attaching the plastic adaptor ring over the relatively flat surface of the frontal sinuses, and of drilling the biopsy access hole through the skull when the biopsy needle trajectory is nearly perpendicular to the calvarial surface. Biopsy needle placement error was significantly affected by lesion location, with error being largest for caudal fossa lesions, and intermediate for middle fossa lesions.
Intraoperative Diagnosis Using the Smear Technique
The rapid cytological evaluation of a brain lesion from a biopsy sample can provide crucial information on operative management, medical management, chemotherapy, or radiation therapy. In people intraoperative cytological evaluation of smear preparations of brain tumors, supported by frozen and paraffin-embedded tissue, has become a routine procedure, and cytological profiles of smears of various types of human brain tumors have been well described. Smear preparations are generally wet fixed in 95% alcohol and stained with hematoxylin and eosin although toluidine blue, Giemsa, or Papanicolaou's stain may also be used.
In a recent study, tissue samples were obtained from lesions either by CT-guided stereotactic brain biopsy (44 samples) or intraoperatively during craniotomy (49 samples) and the results from the smear technique compared with those from sections of paraffin-embedded tissue. The overall diagnostic accuracy from samples obtained by both craniotomy and stereobiopsy was about 80%. This compares favorably with the 69-94% accuracy reported in some large series of human cases. The main advantages of this method of intraoperative diagnosis are speed, ease of preparation, technical simplicity, need for minimal equipment, high degree of cytological resolution compared to frozen preparations, low cost and small sample size required. A limitation of this system is that it is difficult to prepare adequate smear preparations in certain tough and coherent tumors (e.g., schwannomas, fibrillary astrocytomas, and some meningiomas). Smear preparations provide excellent cytologic detail, however these differ from the conventional histologic appearance of HE-stained paraffin-embedded tissue. Experience is required in the correct interpretation of smear preparations.
a. Pelorus Mark III Stereotactic System, Ohio Medical Instrument Company, Cincinnati, OH.
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2. Koblik PD, LeCouteur RA, Higgins RJ, Bollen AW, Vernau KM, Kortz GD, Ilkiw, JE: CT-guided brain biopsy using a modified Pelorus Mark III stereotactic system: Experience with 50 dogs. Veterinary Radiology & Ultrasound 1999, 40: 434-440.
3. Koblik PD, LeCouteur RA, Higgins RJ, Fick J, Kortz GD. Sturges BK, Pascoe PJ: Modification and application of a Pelorus Mark III Stereotactic system for CT-guided brain biopsy in 50 dogs. Veterinary Radiology & Ultrasound 1999, 40: 424-433.
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