What type of skull tumor is most likely and what additional test would you recommend confirming your suspicion?
The primary differentials for the mass included a multilobular osteochondrosarcoma (MLO) or a more aggressive osteosarcoma. The typical CT appearance of an MLO on a bone window includes a nonhomogeneous granular mass with well-defined margins. The normal bone will be destroyed by the tumor 1. In contrast, an osteosarcoma contains more lytic and less defined areas with loss of mineralized bone structures and strong periosteal reactions. The osteosclerotic forms of an osteosarcoma can look very similar to an MLO and in general, a biopsy is recommended to confirm the diagnosis. The biopsy results in our patient confirmed the presence of an MLO also known as a multilobular tumor of the bone while ruling out an osteosarcoma.
What are the possible treatment options for an MLO?
MLO is the most common tumor of the skull in older, medium to large breed dogs. The biological behavior may range from benign to more malignant with 23% of the tumors being more aggressive. In malignant tumors, there is extensive local invasion of surrounding structures and the reported incidence of metastatic disease is as high as 56-58% 2,3. Typically, at the time of diagnosis, only a small to medium-sized skull exostosis is noted on physical examination. Later, advanced imaging (MRI or CT scan) often shows an extensive lesion which is widely dispersed throughout the skull and is often compressing multiple brain structures. Surgical resection is an optimal treatment choice in many cases 4 though tumor grade and completeness of resection are important prognostic factors that ultimately determine the long-term outcome. Two studies with a limited number of dogs with skull involvement reported a disease-free interval of 12 and a median survival time of 17.6 to 21 months after surgical resection 2,4. The most challenging part of a surgical resection is the reconstruction of the skull. Tumor removal leaves large skull defects and an exposed brain. Traditionally, the bone defects were covered with bone cement [Poly(methyl methacrylate) (PMMA)] implants 5. More recently, the use of titanium mesh and screws of the Low-Profile Neuro Plating System® (DePuy Synthes) has offered more repair options and a better integration of the implants into the remaining tissues 6,7. Radiation therapy alone can shrink the tumor volume anywhere between 26-87% but will not eliminate the mass. The reported median survival time for stereotactic radiation was 11 months 8. Chemotherapy has been shown to have a limited effect and median survival time was 4.8 months 9. A combination of surgical resection, skull reconstruction and definitive conventional radiation therapy produced an average survival time of 17.1 months 10.
Is the 5.6 years old Golden retriever a good surgical candidate?
Based on this survival time information, surgical resection was planned for our patient. A normal metabolic function and the absence of metastatic disease made this dog a good candidate. A larger skull defect after tumor removal was expected but advanced skull reconstruction techniques made a closure of the gap very likely. The tumor was removed in two pieces due to its large size. Complete margins could not be safely achieved due to the extensive attachment and integration of the tumor to the brain, but the grossly defined portions of the mass were completely removed. The surface of exposed brain was covered with fascia of the temporal muscle. The skull surface and the frontal sinus was reconstructed with the Low-Profile Neuro Plating System® (DePuy Synthes). The titanium mesh was contoured to the previous skull shape (Figure 3). The reconstruction was covered with the temporal muscle and skin. The dog recovered well and was discharged 3 days after the procedure. The histopathological analysis confirmed a low grade MLO. Nine months after the procedure, the patient presented in excellent condition with a very good quality of life. Eleven months post-surgery the dog was still disease free (Figure 4).