Surgical removal of tumors of the skull presents numerous challenges due to the proximity of vital structures, the potential for significant complications, and the delicacy of proximate structures that may sustain injury during dissection. Although a majority of bony tumors of the skull affect the maxilla [1], this discussion will focus on the challenges, both technical and medical, of bony tumors of the cranium. There are a variety of tumor types that have been reported to affect the cranium including infrequent chrondroma, chrondrosarcoma, squamous cell carcinoma, fibrosarcoma, or other tumor types with osteosarcoma and multilobular osteochrondrosarcoma (MLO) comprising the majority of the tumors encountered [1–6]. In retrospective series, MLO tumors appear to occur somewhat more often in the skull than osteosarcoma although definitive data on the relative frequency of these tumor types is not available.

Osteosarcoma and Multilobular Osteochrondrosarcoma of the Cranium

Osteosarcoma of the cranial bones is a malignancy that presents with proliferative and osteolytic lesions and has been suggested to exhibit a somewhat different biological behavior compared to appendicular osteosarcomas with respect to the incidence of distant metastasis as a cause of death [1]. This may in part be due to their location, where difficulty in achieving large margins of resection due to proximity to critical structures makes local recurrence more frequent (80% of cases in one report) and often a cause of death or euthanasia, possibly before metastasis is detected [1]. Osteosarcomas tend to occur in middle‐aged pure‐bred dogs and frequently do not have pulmonary metastasis, the most common site of distant spread, at the time of diagnosis. Surgical excision with or without adjuvant therapies provides the best outcome in terms of survival time with median survival times reported at 329 days for osteosarcomas of the head, although calvarial lesions had a greater hazard of local recurrence or progression [1, 7, 8].

MLO tumors (synonymous terms used over time for this lesion include chondroma rodens, multilobular osteoma, multilobular chondroma, multilobular tumor of bone, multilobular osteosarcoma, cartilage analogue of fibromatosis, calcifying aponeurotic fibroma, and juvenile aponeurotic fibroma) are relatively uncommon tumors arising from the flat bones of the skull with rare reports of such tumors at other sites. MLO tumors occur primarily in middle aged to older, medium‐ to large‐breed dogs, but also can occur in young and small‐breed dogs as well as occasionally in other species [9–11].

These tumors generally present as a firm well demarcated swelling on the skull although internal expansion into the cranial cavity or sinuses/nasal cavity may mask the presence of these tumors early in the course of the disease as well as obscure the overall extent of tumor growth. Neurological or other clinical signs can occur, which are mostly dependent on the structures impinged upon by the expanding mass. MLOs are graded on a three‐tier histological scale with higher grade lesions showing greater likelihood of local recurrence and distant metastasis [5, 12]. MLOs tend to be slow growing and metastasize later in the course of the disease. Complete excision is the treatment of choice with some studies suggesting adjuvant therapies, such as chemotherapy or radiation, may provide additive benefit to surgical excision [4–8]. Although the database for assessment of adjuvant therapy efficacy is not large, early results are encouraging. Dogs with this type of tumor can have an excellent short to intermediate term prognosis following surgical excision but incomplete excision and local recurrence can be problematic. Depending on the histological grade, MLO lesions are likely to recur locally as well as develop metastatic lesions greater than 50% of the time. However, the long timeline for recurrence in most MLO cases, greater than two years median for grade I and II lesions [4, 6, 9, 12], and the improved quality of life during that time makes surgical treatment an attractive option for many clients. It should be noted that the biological behavior of MLOs can be variable and that in a minority of cases the lesions can be considerably more aggressive with more rapid growth and a shorter time to recurrence.

Diagnosis and Characterization

Diagnosis of tumors of the skull is generally not difficult. Imaging will usually provide an accurate presumptive diagnosis for most due to the characteristic appearance of MLO and osteosarcoma lesions [13]. Most calvarial tumors are readily accessible for incisional biopsy for definitive diagnosis. Biopsy procedures should be planned so that the biopsy tract and related incisions are removed as part of any excisional therapeutic procedure. Imaging plays an important role in both diagnosis and therapeutic planning. Computed tomographic (CT) imaging will provide the most information for diagnostic and therapeutic purposes for tumors with proliferative bony characteristics. In some cases, the unique appearance of MLO tumors, sharply marginated masses with a granular mixed bony appearance, can provide a presumptive diagnosis and be advantageous for lesions in difficult to access locations for biopsy [10, 12, 13]. For bony tumors, magnetic resonance imaging (MRI) is less informative. However, in certain scenarios (see sections below regarding occipital tumors affecting the caudal dorsal sagittal or transverse sinuses), MRI can be essential in formulating a therapeutic approach to excision that minimizes patient risks [14–17].

Surgical Planning and Treatment

Surgical excision is the treatment of choice for both MLO lesions and calvarial osteosarcomas. Follow up chemotherapy or radiation therapy may also be warranted for specific case presentations. There are a variety of challenges that will affect the feasibility and difficulty of excision of MLOs. These challenges are directly related to the location of the tumors and the biological behavior of the lesions in any individual patient. These factors will affect the ability to gain margins of excision free of neoplasia and avoid complications related to damage to nearby structures. Achieving wide margins in many calvarial lesions, at least in some parts of an excisional boundary, is frequently impossible due to the proximity to critical structures and lack of tissues that can be sacrificed to obtain such margins. This is likely a significant reason behind the relatively high incidence of local recurrence for these lesions. However, as noted earlier, the slow growth and long timelines for recurrence still make excisional surgery a reasonable option for many clients. Other factors related to the tumor size, location, and behavior affect the probability of severe complications, including death, and the overall resectability of the masses as well as the likelihood of needing adjunctive therapy, such as radiation, that may increase costs of treatment.

Exposure

The difficulty of exposure of these lesions varies from simple to complex as a direct effect of the tumor location on the skull. As a general strategy, the normal bone completely encircling the mass is cleared of all soft tissue attachments prior to commencing cutting the bone. This will allow the surgeon to complete the initial bone cuts and excision as efficiently as possible so that hemorrhagic structures under the bone that may be difficult to access can then be exposed for establishing complete hemostasis. For MLO tumors, the overlying skin is often unaffected unless there is a biopsy tract that needs removal and can be preserved to facilitate closure. Similarly, for mid‐calvarial and occipital lesions, the temporalis fascia may remain mostly intact, except at midline where it attaches to the dorsal sagittal crest, due to the temporalis muscle often providing a barrier to the tumor. Similarly, in most such cases the bulk of the temporalis muscle can be preserved. However, deep temporalis muscle fibers are often attached to the tumor surface and a layer of muscle should be excised along with the tumor in those instances. Recurrence in these soft tissues with sharply marginated lesions is rare in the author’s experience and is instead usually seen in bone adjacent to the excision. The exception to this concept is in MLO lesions with substantial expansion into the surrounding soft tissues. In those cases, complete excision of tumor remnants in the surrounding musculature is difficult to achieve. It should be noted that access to the normal bone around the base of MLO lesions can be impeded by the tumor itself. MLO lesions not uncommonly expand outside of the skull in a mushroom‐like fashion. In some cases, it is necessary to cut off portions of overhanging tumor tissue to effectively access the normal bone underneath and establish a line of excision. In MLO lesions with high bone content, the hemorrhage associated with this maneuver may be relatively minimal. In other lesions, particularly those with a higher soft tissue component, hemorrhage can ooze from the cavernous‐like cut tissue surface and be significant and continuous. For this reason, the author delays such partial excisional maneuvers until later in the procedure as much as possible. Once made, broad manual pressure with gauze on the cut surface may be the most effective way to reduce blood loss until the en bloc resection is completed.

Tumors on the dorsal calvarium or zygomatic arch are usually simple to access and isolate from surrounding tissues for excision. As tumors arise from more lateralized and ventral locations on the skull, exposure and isolation of the neoplasms becomes more complex. Similarly, tumors affecting the occiput also require complex dissections of overlying soft tissue structures in order to isolate the lesions. It should be noted that these dissections may not necessarily be overtly difficult but can be lengthy and tedious due to the multitude of affected structures, such as the cervical musculature and associated vasculature and the need for meticulous hemostasis. In some instances, overlying structures, such as the zygomatic arch, may need to be moved and either sacrificed or affixed back in place at the end of the procedure in order to obtain adequate exposure. Similarly, for lesions arising from the orbit or toward the base of the skull, the ramus of the mandible may need to be excised to allow access to the lesion. This can be done, particularly above the temporomandibular joint, with minimal functional effects on the patient. Figures 21.1a–c illustrate an example of this approach for an MLO lesion in the caudoventral orbit that was exposed and excised by zygomatic arch excision with removal of the ramus of the mandible above the temporomandibular joint. In this example, the eye was severely proptosed but visual and both the eye and vision were preserved after surgery. For lesions affecting the occiput, careful positioning of the patient to create an angle approaching 90° between the axis of the cervical vertebrae and craniocaudal axis of the skull is needed to facilitate isolation of tumors to the base of the occiput and foramen magnum (Figure 21.2).

Patient positioning may also be advantageously used to reduce blood loss during surgery. In most MLO excisions, bleeding is less often from larger or high pressure vessels which, if encountered, can be quickly controlled. Instead, there is a significant risk of excessive blood loss over the long time periods often required for these surgeries due to low pressure continuous diffuse bleeding from the tumor surface or venous/capillary sources. Positioning the patient with the head elevated relative to the body and avoiding compression of the jugular veins helps to avoid venous congestion and reduces the rate of bleeding from diffuse low pressure sources in the surgical field [14]. In many cases, use of a supportive rack allows both proper positioning and avoidance of venous compression (Figure 21.2).

Challenges in Skull Tumor Resection

Resection of MLO lesions can present various challenges depending on the location of the lesion. In some areas, solutions to specific problems have not been completely described and remain areas for further investigation and innovation. Cutting of the bone around these lesions is usually done with either high speed burrs or ultrasonic or piezoelectric bone scalpel devices. Wetting of the area being cut with normal saline to prevent heat induced tissue injury is standard practice. Burring of the bone tends to be slower than cutting with advanced bone scalpel devices. However, the greater width of a burr excision line, referred to as the kerf, is advantageous for angled introduction of periosteal elevators under the bone to tease the underlying dura mater with the entrained dorsal sagittal sinus off the inner calvarial and tumor surface. In some cases, using a bone scalpel, it may be useful to enlarge the cutting kerf by making two parallel cuts approximately 0.25–0.5 cm apart to more easily allow introduction of an elevator under the calvarial bone without having to exert upward traction on the bone prematurely. In the author’s experience, the dura is easily elevated off the underside of MLO lesions with occasional exceptions in cases with unusual presentations of more diffuse disease (Figure 21.3).

Parietal Calvarial and Dorsal Frontal Bone Lesions

Resections of lesions involving the dorsal calvarium can be some of the least challenging resections within the spectrum of locations where MLO or other bony lesions may arise. Dorsal calvarial lesions are generally straightforward to isolate on their external surface by retracting and elevating the soft tissues away from the lesion and normal bone to expose an area for bone incision. The dura and dorsal sagittal sinus can be elevated off the underside of the bone and lesion relatively easily, usually without significant damage to the dura. Perforations of the dura are preferentially repaired by suturing or, for larger defects, use of a fascial or other patch can be implemented. Repair of perforations is preferred although modest dural perforations may be tolerated. Elevation of the dura in the mid‐dorsal calvarium will invariably encounter modest hemorrhage from a cut unnamed diploic vessel that is consistently encountered entering the dorsal surface of the sagittal sinus. This is best controlled with a partial thickness suture of 6‐0 polyglyconate or polypropylene in a cruciate pattern.

As lesions arise on the more lateral aspect of the calvarium in the parietal and temporal bones, exposure becomes more difficult. Extensive elevation of the temporalis muscle is possible by incising the temporal fascia at the outer border of the muscle and elevating the muscle off the underlying bone. Elevation of the temporalis can be extensive since the blood supply to the muscle arises at its caudoventral aspect from the occipital branch of the caudal auricular artery and the temporal branch of the superficial temporal artery, which arise from the carotid and maxillary arteries, respectively. Caution should be exercised in elevation of the cranial portion of the temporalis muscle to avoid damage to the palpebral nerve and loss of the ipsilateral palpebral reflex, although in some tumor presentations this may be unavoidable. Lesions of the lateral parietal and temporal bones can be excised similar to those on the dorsal surface down to the dorsal margin of the external auditory meatus. Lesions that extend ventral to this area often invade the middle and inner ear where excision may result in severe vestibular dysfunction and should be considered with caution and planned carefully if an excision is to be attempted.

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