Gamma-irradiated Calvarium Allograft Cranioplasty in a Cat Following Brain Tumor Removal

Introduction

Meningioma is the most common primary intracranial neoplasia of cats, representing 58.1% of reported cases.¹ Surgical removal of such tumors has been associated with long-term remission following partial excision or a cure with complete excision. Surgical resection is considered beneficial for the treatment of intracranial meningioma in cats, and it carries a median survival time of 20.6 months.^2,3 Based on tumor location and associated dural or bone involvement, surgical access may require sizable portions of the skull to be removed. In situations where extensive dorsal skull removal is necessary for tumor access, surgical scarring may result in morbidity, neurological deficits or seizures, or mortality during craniectomy healing.⁴ To avoid such complications in the case described herein, an allograft cranioplasty was performed following meningioma excision to repair the bilateral calvarial defect created by the surgical approach.

Case Report

A 14-year-old, spayed female domestic shorthair cat was presented to the Neurology/Neurosurgery service at the Veterinary Teaching Hospital at Washington State University for evaluation of head pressing, circling, and abnormal mentation of 3 months’ duration. The cat had been previously diagnosed with diabetes mellitus and hyperthyroidism. Both of these conditions were well controlled and played no role in the referral to the Neurology/Neurosurgery service.

On physical examination, the cat had a body condition score of 3/9, with mild generalized muscle atrophy. On neurological assessment, the cat circled to the left and was tetraparetic. Segmental spinal reflexes (patellar and withdrawal) were normal, and direct and consensual pupillary light reflexes were normal; however, the menace response was absent bilaterally. Assessment of all other cranial nerves was normal. Neuroanatomical localization suggested an abnormality in the supratentorial area, predominating on the left side.

Complete blood count revealed a lymphopenia (715 cells/μL; reference range 1.5 to 1.7 × 10³ cells/μL) and a thrombocytosis (534,000 cells/μL; reference range 157,000 to 394,000 cells/μL). Serum biochemical profile identified increased alkaline phosphatase (34 IU/L; reference range 11 to 28 IU/L), increased serum creatinine (1.2 mg/dL; reference range 0.7 to 1.1 mg/dL), and increased glucose (379 mg/dL; reference range 67 to 126 mg/dL). A total thyroxine was within normal limits (1.30 μg/dL; reference range 1.00 to 4.00 μg/dL).

Based on the concern for a structural intracranial abnormality, magnetic resonance (MR) imaging of the head was performed to evaluate the intracranial structures. The cat was premedicated with acepromazine maleate^a (0.02 mg/kg intramuscularly [IM] and butorphanol tartrate^b (0.2 mg/kg IM), and general anesthesia was induced with thiopental^c (10 mg/kg intravenously [IV] to effect). Anesthesia was maintained with isoflurane,^d and the cat was positioned in sternal recumbency within a MR scanner.^e A flexible surface coil was used for image acquisition. Imaging sequences were obtained before and after contrast medium^f administration IV in sagittal, dorsal, and/or axial planes. The sequences included T2-weighted, fluid-attenuated inversion recovery (FLAIR), gradient-echo (GRE), and T1-weighted images.

This study showed the presence of an extensive, bilateral, extraaxial mass centered on the dorsal midline at the level of the frontoparietal cortex. The mass extended from the level of the rostral frontal lobes to the caudal aspect of × the corpus callosum. The mass was approximately 2.6 2.3 × 2.6 cm [Figures 1A, 1B⇓]. A mass effect resulted in compression of the right and left cerebral hemispheres. The calvarium adjacent to the mass was thickened and hypointense relative to the brain parenchymal signal [Figures 1A, 1B⇓]. Blood flow was not apparent in the dorsal sagittal sinus and proximal transverse sinuses when evaluated with an MR venogram, suggesting occlusion of these sinuses. On FLAIR and GRE MR images, the mass was heterogeneously hyperintense and isointense to the surrounding brain parenchyma. After gadolinium administration, heterogenous contrast enhancement of the mass was seen on the T2-weighted images [Figure 2⇓]. Based on these MR imaging characteristics, neoplasia (most likely a meningioma) was suspected. Surgical removal of the mass was planned.

Prior to surgery, a dorsal calvarial allograft was collected from a similar-sized, donor domestic shorthair cat euthanized for reasons unrelated to this case. Immunological testing^g prior to euthanasia revealed the donor cat was negative for feline leukemia and feline immunodeficiency viruses (FeLV and FIV). Immediately after euthanasia, the calvarial allograft was aseptically harvested using a high-speed drill and rongeurs. Soft tissues were removed from the collected calvarium, and the graft was washed with 0.9% sodium chloride^h and frozen at −80°C. Prior to irradiation, the graft was thawed and placed in a clean plastic container. The container and graft were irradiated with 18 kilogray (kGy) from a cobalt-60 source.ⁱ

Surgery was performed 8 days after MR imaging. The cat was premedicated with diazepam^j (0.2 mg/kg IM) and hydromorphone^k (0.06 mg/kg IM). Anesthetic induction was with thiopental (10 mg/kg IV to effect) and maintained with isoflurane and oxygen. Methylprednisolone sodium succinate^l (30 mg/kg IV) and mannitol^m (1 g/kg IV) were administered as a bolus prior to the beginning of the operation to preemptively treat for brain swelling. In addition, cefazolinⁿ (22 mg/kg IV) was administered prior to the skin incision and repeated every 90 minutes for the remainder of the operation. A constant-rate infusion of fentanyl^o was administered intraoperatively at a rate of 3 μg/kg per hour IV to decrease the required isoflurane levels and minimize the risk for increased intracranial pressure. Isotonic crystalloids^p (15 mL per hour IV) and dopamine^q (5 to 10 μg/kg per minute IV) were administered to maintain systolic blood pressure.

The cat was placed in sternal recumbency. A routine mid-line skin incision was made, extending from between the eyes to 3 cm caudal to the occipital protuberance. The underlying interscutularis and temporalis muscles were elevated and retracted, exposing the dorsal and lateral aspects of the frontal, parietal, and most of the occipital bones of the skull.⁴ A combined bilateral rostrotentorial (dorsal) craniectomy and modified frontal craniectomy was performed using a high-speed air drill.⁴ The cranial defect was chosen with the following margins: rostrally to the level approximately 0.5 cm from the caudal border of the frontal sinus region; caudally to the level approximately 0.5 cm from the external occipital protuberance; and bilaterally and ventrally to the level of the dorsal zygomatic arch in the horizontal plane. The calvarium was freed from the underlying dura with an Adson periosteal elevator. The mass was adherent to the calvarium [Figure 3⇓].

The mass was subsequently dissected away and removed from the brain parenchyma using a combination of sharp and blunt dissection. The meninges surrounding the mass were removed with micro scissors and a no. 11 scalpel blade to increase the surgical margins. The surgical site was copiously lavaged with warm 0.9% sodium chloride, and a temporalis fascial graft was used to cover the brain. A lattice comprised of two, 3-0 nylon^r sutures was created to support the calvarial allograft [Figure 4⇓]. The suture lattice was tied to holes created in the recipient’s remaining calvarium in a craniocaudal direction. Absorbable, gelatin-compressed sponges^s were packed within the frontal sinuses and also placed immediately dorsal to the brain parenchyma and fascial graft. The irradiated skull allograft was contoured to fit the craniectomy defect using a rongeur and high-speed drill, leaving an approximately 2-mm overlap between the donor and recipient skulls. Burr holes were created in the allograft, which were aligned with the anchor-burr holes created in the recipient calvarium, and the allograft was sutured in place with 2-0 nylon sutures in a simple interrupted pattern [Figure 5⇓]. The temporalis muscles were apposed using 2-0 polydioxanone^t in a simple continuous pattern. The subcutaneous tissues were closed using 3-0 poliglecaprone 25^u in a simple continuous pattern. The skin was apposed using 3-0 nylon in a simple interrupted pattern. The mass with the attached calvarium was submitted for routine cytology and histopathology.

Postoperative care included the administration of isotonic crystalloid fluids (8 mL per hour IV) and morphine sulfate^v (0.3 mg/kg IM q 4 to 6 hours). A bandage was placed over the skin incision. The cat was discharged from the hospital 3 days after surgery. At the time of discharge, the cat’s neurological examination was within normal limits, except for the persistent lack of a menace response bilaterally.

Ten days postsurgically, during the scheduled reexamination, a small, 1.5-cm area of wound dehiscence at the cranial border of the skin incision was noted. The site was lavaged with a 0.05% chlorhexidine diacetate solution,^w and amoxicillin/clavulanic acid^x (62.5 mg per os [PO] q 12 hours for 10 days) was prescribed. A clinical examination was performed 1 month after surgery, and the neurological status was considered improved. A small seroma was noted at the previous site of dehiscence. Amoxicillin/clavulanic acid (62.5 mg PO q 12 hours) was prescribed for an additional 4 weeks. A routine follow-up examination 14 months postcranioplasty revealed no physical defects in the skull and no neurological deficits.

The final diagnosis of the mass was psammomatous meningioma. Histologically, the cells of the mass were arranged in bundles and whorls, with abundant collagenous stroma throughout and small aggregates of basophilic mineral at the centers of several whorls (psammoma bodies). The individual cells were spindle shaped and had a moderate amount of eosinophilic cytoplasm and poorly defined cellular borders. The nuclei were round to oval and had finely stippled chromatin and small nucleoli. Mitotic figures were not observed in the representative sections.

Discussion

Calvarial allografts are described for veterinary patients; however, information on technique and outcome is sparse.⁵ Cranioplasty was first described over 300 years ago by Meekeran, who in 1668 used a xenograft canine skull to repair a soldier’s battle injury.⁶ Allografts may be harvested and prepared for utilization in different ways. For example, grafts can be harvested under surgical conditions and transplanted directly into a recipient or frozen for later use.

Sterilization techniques have been investigated to minimize the risk of transmitting disease to the recipient of a graft. The type of sterilization has an effect on the biological and mechanical properties of the graft. Current methods of terminal sterilization include irradiation, ethylene oxide, hydrogen peroxide, and pasteurization.^7–10 The mechanical strength of a gamma-irradiated allograft decreases in a dose-dependent manner as radiation fraction increases.¹¹ Some studies report that the standard 25-kGy dose significantly reduces graft strength, and higher doses reduce graft strength to unacceptable levels.^11,12 However, other studies show that, in a clinical setting, the rates of fracture and nonunion of allografts irradiated at 25 kGy were similar to the fracture and nonunion rates of nonirradiated allografts.^11,13 In evaluating the influence of graft processing on the host osteoblast attachment and function, significant attachment and differentiation have been found in irradiated, ethylene oxide-, and autoclave-sterilized cortical bone specimens.¹⁴ The osteoblastic activities, however, are decreased compared to fresh-frozen samples.¹⁴ Ethylene oxide sterilization was shown to impair allograft ingrowth compared to gamma irradiation and hydrogen peroxide.¹⁵

For human cortical bone allograft irradiation, the “standard dose” of 25 kGy is ineffective at inactivating viral pathogens; therefore, a dose of 34 kGy has been proposed by one group,¹⁶ despite the fact that a dose of 15 to 25 kGy is reportedly highly effective against bacterial organisms.¹⁷ The dose of 18 kGy was chosen for the allograft used in the feline case described herein to retain maximum graft strength while simultaneously achieving bacterial sterilization. Because the FeLV/FIV status of the donor cat was known to be negative, irradiating the allograft at a higher dose (effective against viral pathogens) was of less concern in this instance. Using gamma irradiation in the 15 to 25 kGy range to sterilize an allograft may not be sufficient to inactivate FeLV/FIV; therefore, serological screening of the donor for these viral agents should be routinely performed prior to graft collection.

To our knowledge, no other reports of allograft cranioplasty performed in the cat have been described. Reports have described feline cranioplasty using hydroxyapatite, plaster of Paris, and polymethylmethacrylate (PMMA), but the largest cranial defect repaired in these studies was only 2.5 cm in diameter.^18,19 In addition, reports of cranioplasty in dogs are available, but only one case report involving an irradiated calvarial allograft in a dog appears to exist.^5,20–22 While a nonbiological PMMA graft has been used in some instances to repair cranial defects, we prefer biological allografts for this location.^5,21,23

Complications associated with allografts include rejection of the implant, nonunion of the graft, and postoperative infection. Allograft rejection is significantly reduced by irradiation, as all living cells are destroyed by the sterilization procedure.^24,25 As previously mentioned, the fracture and nonunion rates are similar between irradiated and non-irradiated cortical allografts.^11,13 While infection of the allograft site is always a concern, postoperative infection rates with allografts are more favorable than the 7.1% reported when using autologous calvarial grafts.^26,27 The cat described in this report had a minor incision-related problem postoperatively; however, no other complications were noted.

Conclusion

Use of irradiated calvarial allografts is a viable alternative to using PMMA to repair calvarial defects in cats. Further research is needed to determine if one method is superior to another.