Clinical reliability of posterior fossa scintigraphy

Clin. Radiol. (1976) 27, 473-481

CLINICAL

RELIABILITY

OF POSTERIOR

FOSSA

SCINTIGRAPHY

E. H. BURROWS From the Wessex Neurological Centre, Southampton

Radioisotope imaging of the brain is considerably less useful in posterior fossa lesions than in supratentorial diagnosis, due to the peculiar technical difficulties and biological characteristics of infratentorial lesions. Exceptions are acoustic neurofibromas and probably meningiomas, which can be reliably detected. The importance is stressed of making skull and chest radiographs at the time of the examination, in order to enhance the diagnostic accuracy of the scintigraphic result.

THE disappointing results of radioisotope imaging of space-occupying intracranial lesions on the floor of the skull contrast significantly with the high accuracy of this technique in detecting hemispherical and convexity lesions. The patchy yield of accurate clinical information recorded by most authors in infratentorial lesions (Table 1) reduces its use as a screening test in patients suspected of harbouring a posterior fossa mass (Fig. 1; Burrows, 1972a, b). This marked difference appears to be caused by the scintigraphically unfavourable topography of the infratentorial compartment. However, certain tumours appear to be exceptions, notably-acoustic neurofibromas, in which highly reliable results have been reported (Baum et al., 1972; Burrows, 1975a). A review is presented in this paper of 82 proven infratentorial lesions examined by conventional radioisotope scanning, in order to attempt to define diagnostically beneficial clinical areas which justify the use of this technique in patients with posterior fossa signs. SERIES AND RESULTS All 82 patients were admitted to the Wessex Neurological Centre, Southampton, where the diagnosis was satisfactorily verified; in all but a few a histological result was obtained. The patients do not form a consecutive series of posterior fossa cases admitted into the Centre, since the more acute ones, including all children with medulloblastomas, had to be excluded for reasons of neurosurgical urgency. Most examinations were performed with 99mTcpertechnetate (6-12 millicuries) after a blocking dose of potassium perchlorate (200 mg). About a dozen of the earlier examinations were carried out with

197Hg-chlormerodrin (0'5 millicurie). In most cases a single-detector rectilinear scanner was used (Picker Magnascanner V, Picker Nuclear) and two views of the head were made, viz. the particular lateral that seemed most appropriate clinically, and always the tilted posterior view. The final 15 cases in the series were examined with a scintillation camera (Pho-Gamma 1206, Nuclear Chicago) and five routine views were produced. Full technical details have been given elsewhere (Burrows, 1973). All the images were produced on full-size radiographs, the camera images being magnified on X-ray film by means of the Photoscope attachment. No computer aids were used, either at the print-out stage or during interpretation, and each image was recorded simple as 'convincingly positive' or 'negative' after study under standard viewing conditions. Skull and chest radiographs were made on all patients, and viewing of the brain scan was delayed until all these films could be viewed together. The results are analysed in Table 2 according to the nature of the lesions and in Table 3 according to their site. SCINTIGRAPHIC ANATOMY AND TECHNIQUE The intracranial venous sinuses and the large pool of blood within the thick layer of neck muscles create a pattern of scintigraphic landmarks which accurately delineate the posterior cranial fossa (Fig. 2). In the lateral view, the torcular Herophili is usually dense, reflecting the pool of blood at the confluence of the sinuses. In an anterior direction from it, the following structures can be identified: ipsilateral lateral sinus, transverse sinus, sigmoid curve and the superficial petrosal

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474

CLINICAL

RADIOLOGY

sinus. Along the skull base, the uptake is most intense over the parotid glands, which lie in front and below the external auditory canals (and thus the cerebellopontine angles). No increased activity is normally present in the relatively 'bare' area bounded by the transverse sinus and the skull base. In the tilted posterior view, the important landmarks are the torcular Herophili and the transverse sinuses, which represent the roof of the posterior fossa since they lie in the groove of attachment of the tentorium cerebelli. Laterally, the external auditory canals are superimposed on the areas of intense parotid activity. No increased activity is normally present in the relatively 'bare' space beneath the transverse sinuses. Positioning the patient's head relative to the detector is critical for optimal results: the posterior fossa should be as close to the detector as possible. The following views are made: (1) The tilted-posterior view, which is the recognised method of examining the posterior fossa (Witcofski and Roper, 1965). It can only be made if the patient's head is well flexed on the neck preferably with the collimator angled cephalad, in order to 'lift up' the venous sinuses and expose the infratentorial compartment to full view. Unless a conscious effort is made to open up the posterior fossa in this way, the activity in the transverse sinuses will blend with the normal increased activity

TABLE 1 DIAGNOSTIC ACCURACY oF POSTERIOR-FOSSA SCINTIGRAPHY

Takahashi (1965) 29/41 cases (71%) De Roo (1967) (72%) Lincke (1968) 39/56 cases (59 %) Hirschbiegel and BSckern (1969) 41/54 cases (76%) Rasmussen et al. (1970) 20/48 cases (42 %) Steinhoff (1972) 7/20 cases (35 %) Moody et al. (1972) 29/37 cases (78 %) Klaus et al. (1972) 36/42 cases (86%) Ostertag and Mtindinger (1972) 95/179 cases (53 %) Burrows (present series) 40/82 cases (49 %) in the neck and the infratentorial chamber will be obscured. The increased mobility and speed of the scintillation camera makes it superior to the rectilinear scanner in this respect: the posterior fossa is easier to examine properly (Fig. 3), and the results are better (Fig. 4). (2) The Appropriate Lateral View. - When using a rectilinear scanner care should be taken to include the entire skull base, i.e. not to curtail the examination prematurely. 99roTe pertechnetate remains the most widely used radioisotope for imaging the brain. 197Hg chlormerodrin, despite its physical and biological and clinical disadvantages, including an obligatory delay of four hours after injection, was initially hailed as a highly favourable substance for posterior

7.1%

51% FIG. 1 Topography of false-negative radioisotope scans, based on 847 proven lesions. Shaded area includes the mediobasal part of the cerebrum as well as the posterior cranial fossa.

CLINICAL

RELIABILITY

OF

POSTERIOR

FOSSA

SCINTIGRAPHY

475

T~LF2

S C I N ~ G ~ P H Y ~ 8 2 P R O ~ N I N F ~ T E N T O ~ A L ~SIONS

Positive M e t a s t a s e s - 38 c a s e s A c o u s t i c n e u r o f i b r o m a - 15 c a s e s Cerebellar astrocytoma - 7 cases Capillary haemangioblastoma - 8 cases Meningioma - 4 cases Pontine glioma - 3 cases Epidermoid - 3 cases Hypoglossal neurofibroma - 1 case Arteriovenous malformation - 2 cases Infarction- 1 case

16 12 5 3 2

T o t a l - 82 c a s e s

40

% I Fa&enegative Accuracy

1

1

22 3 2 5 2 2 3 1 1 1

42 80 71 35

42

49

TABLE 3

SITES OF 82 INFRATENTORIALLESIONS

Positive

Falsenegative Accuracy

Cerebellopontine angle - 22 cases A x i a l r e g i o n - 25 c a s e s C e r e b e l l a r h e m i s p h e r e - 35 c a s e s

13 7 20

9 18 15

59 28 57

T o t a l - 82 c a s e s

40

42

49

1

i~

2

lJ

FIG. 2 1234-

Scintigraphic anatomy of the posterior cranial fossa. superior sagittal sinus straight sinus torcular Herophili transverse sinus

5 678-

slgmoid curve superficial petrosal sinus parotid gland internal auditory canal (with seventh and eighth cranial nerves)

476

CLINICAL RADIOLOGY

FIG. 3 The tilted posterior view made with the scintillation camera. Uncooperative patients may be examined supine with the head tilted forwards; even better results are obtained with the patient seated.

fossa scintigraphy because of its low background radiation (due to absorption of radionuclide from the venous sinuses) (Bender and Williams, 1966; Frigeni et al., 1967; Decker and Backmund, 1975). However, no convincing evidence has been produced, either in the present study or in the literature, of its superiority over less potentially harmful radionuclides. Indeed, recent study on autopsy tumour material has shown a less favourable tumour: non-tumour ratio for X97Hg chlormerodrin compared to 99mTc pertechnetate (Pink et al., 1973). I N T E R P R E T A T I O N OF RESULTS The three scintigraphic patterns for differentiating posterior fossa neoplasms described by DeLand et al. (1970) were found to be the most useful basis for analysing the 40 abnormal radioisotope scans in this series (Table 3). Each scan could be classified easily into one of the three topographical groups, and no further subdivision appeared to be practically feasible. The three groups are (1) cerebellopontineangle; (2) axial region and (3) cerebellar hemisphere. (1) Cerebellopontine Angle. - Of the 22 cases of proven cerebellopontine-angle lesions, 13 were abnormal. In the lateral view, the abnormal activity occupies a characteristic position behind and above

the parotid gland and the temporalis muscle. The intensity of uptake is only slightly less than that of the parotid glands; in all 13 cases it was notably homogeneous and possessed a smooth crescentic or pointed upper border (Fig. 4). Physiological activity in the sigmoid sinus, which overlies the cerebellopontine angle in this view, may lead to error if both appropriate views are not interpreted together (Pertuiset, 1970). In the posterior tilted view, the paramedian situation of the abnormal activity arising from the floor of the posterior fossa and occupying the area usually 'bare', is characteristic of a cerebellopontine-angle tumour. The 13 abnormal cases comprised 12 out of 15 acoustic neurofibromas and one out of two meningiomas. The false negative cases, in addition to the three acoustic neurofibromas and one meningioma, were three epidermoids, one neoplastic metastasis and one case of adhesive arachnoiditis. All 12 positive acoustic neurofibromas exhibited characteristic funnelling of the appropriate internal auditory canal, while the three false negative cases showed no such erosion. In two of the latter three, the turnout removed at operation was found to be less than 2 cm in diameter. Thus, a histological diagnosis of acoustic neurofibroma could be made in 12 of the 22 cerebellopontine-

C L I N I C A L R E L I A B I L I T Y OF P O S T E R I O R FOSSA S C I N T I G R A P H Y

FIG. 4 Right acoustic neurofibroma, posterior tilted and right lateral views made with the rectilinear scanner (above) and the scintillation camera (below). Right parietal burr-hole activity present in camera views. The additional angulation possible with the camera has the effect of raising the level of the venous sinuses in the posterior tilted view, thus ensuring a more convincing diagnostic result.

477

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CLINICAL RADIOLOGY

FIG. 5 Capillary haemangioblastoma in the upper vermis, extending through the tentorial hiatus.

angle lesions on the basis of interpretation of the plain petrous radiographs and the radioisotope scan. A rough correlation appears to exist between the size of an acoustic neurofibroma, the presence of petrous-bone erosion and a positive scintigraphic result (Baum et al., 1972; Burrows, 1975a). (2) Axial Region. - The midline location of increased activity in the posterior tilted view is the diagnostic sine qua non of this group : the focus may arise from the normal basal uptake on the floor of the skull or it may be separated from it and be misinterpreted as normal torcular activity (see Fig. 5). Similarly on the lateral view the increased activity (which is equally intense in either lateral) may lie anterior and low and resemble a cerebellopontineangle tumour; or posterior, either high or low, and resemble a hemispherical tumour. Of the 25 patients with proven lesions of the vermis, fourth ventricle, pons and medulla, only seven could be identified scintigraphically; thus nearly three-quarters were false negative, confirming the poor results reported in the literature (Lincke, 1968; Ostertag et aI., 1974; Decker and Backmund, 1975). The 25 cases were: neoplastic m e t a s t a s e s - 15 (4 positive); capillary haemangioblastoma - 3 (1 positive); arteriovenous malformation - 1 (positive); glioma of p o n s - 3 (1 positive); infarction - 1

(negative); hypoglossal neurofibroma - 1 (negative); m e n i n g i o m a - 1 (negative). Mealey in 1966 wrote 'In a child with clinical signs of a cerebellar lesion and a normal radioisotope scan, a medulloblastoma is more likely than a cerebellar astrocytoma or an ependymoma'. While cases of medulloblastoma detected by scintigraphy have been reported in the literature since that time, the mediobasal situation of these large, highly malignant tumours of childhood is technically unfavourable, and Mealey's conclusion retains its validity. Indeed Decker and Backmund (1975) observe that a positive result should prompt consideration of an alternative diagnosis, e.g. a cerebellopontine-angle tumour, and these authors report a case to support their view. (3) Cerebellar Hemisphere. - Cerebellar hemisphere activity may be high or low: usually a clear area is present around it, separating it from the basal or sinus activity, always convincingly clear of the midline in the tilted posterior view (Fig. 6). In the appropriate lateral view, the activity may be indistinguishable from lesions in the cerebellopontine angle or the midline. Of the 35 patients with proven turnouts of a cerebellar hemisphere in this series, 20 (57 ~ ) were identified scintigraphically. The 35 cases corn-

C L I N I C A L R E L I A B I L I T Y OF P O S T E R I O R FOSSA S C I N T I G R A P H Y

prised: neoplastic metastases - 22 (12 positive); astrocytoma - 7 (5 positive); capillary haemangioblastoma - 5 (2 positive), meningioma - 1 (positive). DISCUSSION The clinical yield of this study, viz. a detection rate of 49 %, should be viewed in the context of the overall results obtained in a series of 847 proved intracranial or scalp lesions from Southampton (Burrows, 1972a), which included the majority of the present 82 cases. This large series contained 118 false negative cases (13.9%); in particular, the diagnostic accuracy in detecting supratentorial lesions exceeded 90% (Fig. 1). The significant discrepancy between supratentorial and infratentorial lesions can probably be explained by the peculiar technical problems and biological characteristics of pathological processes in the posterior fossa. Firstly, the topography of the infratentorial compartment is unfavourable to the scintigraphic technique - its relatively small volume, at least 50 % of which is obscured by the blood pool in the muscle layers of the neck, which possesses a high level of normal background activity of gamma radiation, and the difficulty of positioning the patient's head so that the deep-seated posterior fossa structures are sufficiently close to the scintillation detector (Klaus et al., 1972). The posterior tilted projection is the most difficult view of the'

479

brain to perform correctly, since it depends on the patient's co-operation: a satisfactory result may be impossible through unconsciousness, cardiac failure, physical disabilities such as a stiff neck or a severe thoracic kyphosis, especially with a rectilinear scanner. Evidence is accumulating that the results of static imaging of the brain with a scintillation camera are actually better, not merely nearly as good as those obtained with the rectilinear scanner (Van Eck and Penning, 1970; Burrows, 1973), because of its greater speed and flexibility of operation. Secondly, the biological characteristics of infratentorial lesions do not make them suited to detection by the scintigraphic method. In this respect, both the size and the histology of the lesion are relevant: given optimal collimation and the most favourable tumour possible - say, a parietal meningioma - no intracranial focus under 2 cm in diameter is likely to be demonstrated. It is well known that infratentorial tumours provoke neurological symptoms and signs earlier, i.e. before they have grown larger, than similar tumours above the tentorium cerebelli, due to the more critical relationships that exist between brain, cranial nerves and skull in the posterior fossa. ACOUSTIC NEUROFIBROMAS are the commonest lesion of the cerebellopontine angle. The majority exceed 2 cm at operation. The high proportion d e t e c t e d in this series, namely 80 ~ (Table 2), has

Fl6.6 Metastasis from breast carcinoma in left cerebellar hemisphere.

480

CLINICAL RADIOLOGY

recently been confirmed by the combined results from three British neurosurgical centres (57 out of 68 cases, 84 ~ ) and by a review of the world literature (106 out of 138 cases, 77 ~). Only one of the 13 cases in this series presenting as an abnormal focus in a cerebellopontine angle was not an acoustic neurofibroma; indeed acoustic neurofibromas accounted for nearly one-third of all the abnormal posterior-fossa lesions detected. The selective affinity of conventional radionuclides for this benign brain tumour is ill understood, especially since lesions of comparable biological type such as trigeminal and hypoglossal neurofibromas and meningiomas, give less consistent results. However, this affinity has been well proved clinically and its great importance in differential diagnosis, especially if interpretation i s combined with that of special radiographs of the petrous bones, is discussed elsewhere (Burrows, 1975b). NEOPLASTIC METASTASES. -- The disappointing diagnostic yield contrasts with the high success rate reported by some authors (DeLand et al., 1970), but the present figures may reflect earlier diagnosis due to the greater occurrence of metastatic malignancy in the patients of this series. Nearly half harboured metastases, and more than half of these - 22 out of 38 - had primary bronchial cancer. Several of the latter presented initially with headache, ataxia and other evidence of posterior fossa involvement, and the primary diagnosis was made from the chest radiograph made routinely at the time of the radioisotope scan. In 13 of the 22 patients in whom the scan was false-negative, a definite diagnosis of lung cancer could be made from the chest radiograph. Combined interpretation of the radioisotope scan and the chest radiograph thus raised the possibility of making a diagnosis of metastatic disease from 42 to 76 ~ . MENINGIOMAS. - One of the four meningiomas that gave a false-negative result was a walnut sized mass in a cerebellopontine angle, therefore beyond detection because of its small size. The other false negative meningioma, however, was a huge inoperable 'pancake' mass on the floor of the posterior fossa, which produced considerable displacement of vessels but showed no 'blush' on vertebral angiography. The third case, a large cerebellopontine angIe meningioma, produced an area of dense circumscribed activity indistinguishable from an acoustic neurofibroma; however, the petrous radiographs showed no evidence of bone erosion. Consistently good results have been reported by other workers in larger series of posterior fossa

mertingiomas (Ancri, 1970; Klaus et al., 1972; Moody et al., 1972) - an accuracy of detection paraUelling that of acoustic neurofibromas. CAPILLARY HAEMANGIOBLASTOMAS. - T w o o u t o f

the three tumours of this type that gave a positive result were solid and the third one cystic; four of the five that were false negative were cystic and the fifth solid. Thus there is no evidence to support the suggestion made by Klaus et al. (1972) that the cystic variety of this tumour, rather than the solid, is more likely to give positive results. GLIAL TUMOtrRS. - A large proportion of these lesions, especially those in children, lie in the midline - a site that appears to be notoriously insensitive scintigraphically. In this series, only slightly more than one in four was demonstrable in this situation- a figure rather higher than that given by other workers with large series of cases (Planiol and Oberson, 1965; Lincke, 1968; Klaus etal., 1972). The relatively small number of glial tumours included in this series reflects conventional neurosurgical practice, which requires preoperative ventriculography in patients with suspected medulloblastoma or some other infratentorial glial turnout. In the posterior fossa, the lack of specificity of a focus of abnormal radioactivity deters surgeons who might have been prepared to carry out a craniotomy, were the focus situated in the supratentorial compartment. Although posterior fossa craniotomies have been performed for more than a decade on the basis of a clinical picture and an abnormal radioisotope scan alone (Brinkman et al., 1962; Matson, 1968) most neurosurgeons prefer to confirm the presence of a true space-occupying lesion by ventriculography and to rule out a brainstem infarction or an outlet-obstruction as the cause of the focal abnormal activity. For this practical reason, radioisotope imaging is at present less useful in the preoperative evaluation of these patients, and it is therefore less often carried out. REFERENCES ANGRI, D. (1972). Les meningiomes de la fosse postrrieure. Nouvelle Presse Mddicale, 4, 233-238. BAUM, S., ROTHBALLER,A. B., SHIFFMAN,F. & GIROLAMO, R. F. (1972). Brain scanning in the diagnosis of acoustic neuromas. Journal of Neurosurgery, 36, 141-147. BENDER, C. E, & WILLIAMS, C. M. (1966). The value of radioactive chlormerodrin for the posterior fossa brain scan. American Journal of Roentgenology, 96, 698-705. BRINKMAN, C. A., WEGST, A. V. • KAHN, E. A. (1962). Brain scanning with mercury 203 labelled Neohydrin. Journal of Neurosurgery, 19, 644-648. BURROWS, E. H. (1972a). False-negative results in brain scanning. British Medical Journal, 1, 473-476. BURROWS, E. H. (1972b). The clinical utility of brain-

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MOODY, R. A., OLSEN, J. O., GOTTSCHALI(,A. & HOFFER, P. B. (1972). Brain scans of the posterior fossa. Journal of Neurosurgery, 36, 148-151. OSTERTAG, C. & MUNDII~IGER, F. (1972). Die Gammaenzephalographie bei pathologischen Prozessen der hinteren SchS.delgrube. Medizinbche Klink, 67, 1447-1451. OSTERTAG, C., MUNDINGER,F., MCDONNELL,D. & HOEFER, T (1974). Detection of 247 midline and posterior fossa tumours by combined scintigraphic and digital gamma encephalography. Journal of Neurosurgery, 39, 224-229. PERTtnSET, B. (1970). Les neurinomes de l'acoustique d6veloppes dans l'angle pontoe6r6belleux. Neurochirurgie, 16, 68-73. PINK, V., LANG, G., CORRENS, H. J_, SCHWESINGER, G. & KIRSCH, G. (1973). Bestimmung der Tumor-HirnQuotienten von 131J-HSA, 99mTc-Pertechnetat und 197HgNeohydrin an Operationspreparaten. Radiologica Diagnostica (Berlin), 14, 771-772. PLANIOL, T. & OBERSON, R. (1966). La gamma-enc6phalographie en pathologie des tumeurs sous-tentorielles. Etudes des 160 cas. Presse Mddicale, 74, 361-366. RASMUSSEN,P., BUHL, J., BUSCH, H., HAASE,J. & HARMSEN, A. (1970). Brain scanning - cerebral scintigraphy. Acta Neurochirurgica, 23, 103-119. STEINHOE•, H. (1972). Die Leistungsf'ahigkeit der Hirnszintigraphie in der Differentialdiagnostik intrakranieller Prozesse. Acta Neurochirurgica, 26, 99-120. TAKAHASm,M. (1965). Comparison of scintillation scanning with other neuroradiologic procedures in the diagnosis of posterior fossa tumors. Journal of the Canadian Association of Radiologists, 16, 248-253. VAN ECK, J. H. M. & PENNING, L. (1970). Comparison of rectilinear scanning and scintiphotography for the detection of brain lesions. Neuroradiology, 1, 107-111. WITCOVSKI, R. L. & ROVER, T. J. (1965). A technique for scanning the posterior fossa. Journal of Nuclear Medicine, 6, 754-761.