Anatomical Approach to Giant Pituitary Tumors

Introduction

Various classifications of pituitary tumors have been proposed on the basis of size, radiographic appearance, cytogenesis, staining properties, and endocrine function. There is no consensus regarding the terminology to be used in describing “giant” pituitary tumors. Large or giant tumors have been described as invasive adenoma, malignant adenoma, and carcinoma. Hardy classified pituitary tumors on the basis of their biological behavior. Some authors refer to pituitary tumors with size in excess of 30 mm, or those extending less than 6 mm from the foramen of Monro as “giant” irrespective of their invasiveness. Jefferson observed an incidence of giant pituitary tumors in about 6% of all cases. The histological characteristics of giant pituitary tumors do not usually correlate well with their gross features and generally consist of benign cells.

We classified those pituitary tumors, which measured more than 40 mm in maximum diameter, as being “giant. ” These tumors constituted 13.7 % of the total number of pituitary tumors seen during the period. The relatively larger percentage of giant pituitary tumors in our series could be related to ignorance of early symptoms due to the high rate of illiteracy and poverty, and difficulty in obtaining specialized medical help in our country.

Giant and “invasive” pituitary tumors are among the more complex neurosurgical challenges. Despite their histological benign nature, some of these tumors grow into a massive size and invade diverse anatomical structures. Due to the invasiveness and size of such tumors, surgical resection is difficult and on occasion dangerous. The results of radiation therapy are inconsistent.

The diagnosis of the pituitary tumor can usually be made on the clinical features and their classical anatomical extensions seen on imaging. Biopsy of the lesion thus usually has no relevance. The clinical outcome following a successful small or partial resection is generally not satisfactory. Radical resection of the tumor is the most optimum modality of treatment, although it is difficult and in itself dangerous. An exact understanding of the anatomical nature of the tumor and of its extensions is mandatory for planning and execution of surgery. A successful radical resection of the tumor can lead to rapid symptomatic recovery and an excellent long-term clinical outcome. The recurrence rate of the tumor after its radical resection is reported to be low. We had recently discussed our experience with giant pituitary tumors and had presented our surgical policy regarding these patients.

For this presentation, the authors have analyzed a series of 180 cases of giant pituitary tumors encountered during a 10-year period ( Figures 26-1 through 26-25 ). Giant nonfunctioning pituitary tumors are frequently soft in consistency, are slow growing, and therefore considering the massive size they have relatively innocuous presenting symptoms. It was observed that the giant pituitary tumors had a specific and anatomic pattern of extension, a feature that had great relevance in deciding the surgical strategy. The tumors were divided into four grades depending on the relationship to the diaphragma sellae and the cavernous sinus. Patients operated upon earlier and having a giant tumor mass were not included because it was observed that the extensions in some tumors was bizarre and did not follow a defined pattern seen in the nonoperated cases ( Figure 26-6 ).

Grade I Pituitary Tumors ( Figures 26-1 through 26-11 )

The general growth pattern of a pituitary tumor is that after the origin from the pituitary gland, the tumor grows in size and expands within the confines of the sella, which is enlarged and ballooned. The diaphragma sellae is elevated superiorly as the tumor grows in size. Grade I pituitary tumors were those that remained underneath the superiorly elevated diaphragma sellae and did not invade into the cavernous sinus. This group of tumors was confined underneath the elevated diaphragma sellae and laterally was bordered by an intact medial wall of the cavernous sinus. It was observed that the diaphragma sellae had a great degree of resilience and elasticity and transgression of this anatomic membrane was a relatively rare feature of the giant pituitary tumors. It bulged superiorly with the growth of the tumor and on occasion extended up to or even beyond the corpus callosum. The elevation of the diaphragma sellae was not always rounded or smooth, but was sometimes irregular and nodular. The diaphragma sellae was markedly thinned out at places and frequently formed big or small daughter balloons. The rounded superior wall of the tumor and the fact that no blood vessels are identified within the confines of the tumor were suggestive of the fact that the diaphragma sellae was intact. The presence of an intact diaphragma sellae was confirmed during surgery and this feature had surgical relevance because it formed an important protective barrier and such tumors could be resected radically through a relatively small exposure by a transsphenoidal route. The elevation of the diaphragma sellae even extended into the frontal or the temporal brain and in the retrosellar region.

Extension of the tumor into the sphenoid or other paranasal air sinus by transgressing the sellar dura was a relatively rare feature despite the massive tumor sizes seen in the series. In a large proportion of cases, the sellar dura remained intact despite the massive ballooning of the sella. We observed that a thinned out sellar bone was also preserved in most of these cases.

Surgical strategy: Transsphenoidal surgery was effective and relatively safe in grade I pituitary tumors. The surgical strategy was to expose the sellar part of the tumor and debulk the tumor mass. As the tumor mass was debulked, the suprasellar component progressively fell into the operative field. The Valsalva maneuver was used during surgery to facilitate the descent of the tumor mass. Wherever there was difficulty in exposure of the dome of the tumor mass, additional bone in the region of the tuberculum and planum sphenoidale were removed and the dural incision was extended. The understanding of the anatomical fact that the tumors did not transgress the diaphragma sellae, or extended into the supradiaphragmatic space, or encase the small and large blood vessels of the circle of Willis was crucial. Tumor resection is assisted by the pulsatile force of the diaphragma sellae and the diaphragma sellae should bulge in to the sella at the end of the tumor resection. It is necessary to visualize the entire diaphragma sellae at the end of the surgery, as otherwise small or large portions of the tumor can be missed within its folds. The tumor in patients with marked visual deficit is usually under significant pressure and “pours” into the surgical field after the sellar dural incision is made. The soft and necrotic nature of the tumor and the frequently encountered cystic areas within the confines of the tumor are positive features during tumor resection. Many of the tumors are highly vascular and will bleed excessively during the surgery. It is crucial at this stage to continue with the tumor resection and once the entire tumor is resected, hemostasis is relatively easily achieved. Hemostatic agents such as Gelfoam and Surgicel assist in the hemostasis. The author does not believe that cauterization is necessary within the confines of the tumor. If the diaphragma sellae is lacerated during the surgery and cerebrospinal fluid emerges into the surgical field, the Valsalva maneuver becomes ineffective. In such cases, reconstruction with a fat graft obtained from the thigh is mandatory. The sella and the sphenoid air sinus are packed with fat. It is generally better to overpack the region than to underpack it. This is because the fat in the region is vascularized early and chances of infection or rejection are extremely low. On the other hand, a cerebrospinal fluid fistula can be a formidable and life-threatening problem. The author also uses bone pieces and vascularized mucosal flap to strengthen the reconstruction.

None of the patients with grade I pituitary tumors have needed a transcranial operation in the authors’ series during the last 5 years.

The authors use a sublabial-interseptal approach to the sella. Such an approach was found to be versatile in general and particularly in cases with giant pituitary tumors. It provided a quick and wide exposure to the region. The approach avoided the need to work in a relatively small nasal cavity and to make a mucosal incision through hair follicles. Although theoretically, an endoscope can help in identifying a tumor in hidden corners, I have not found this tool extremely effective or even necessary while dealing with giant pituitary tumors.

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