Answers:
a. and c. The patient had a second resection that demonstrated
glioblastoma multiforme in the left parietal lobe.
Discussion
The fortuitous discovery of primary lung cancers in
patients undergoing cardiac perfusion studies has brought much attention
to thallium 201 (
201Tl) and more recently
99mTc hexakis-2-
methoxyisobutyl isonitrile (MIBI) as potential agents for the imaging of occult tumor and
for distinguishing between viable tumor and necrotic tissue.
1,2
Thallium 201 is a monovalent cationic radionuclide
handled similarly to potassium, crossing the cell membrane via the Na-K-ATPase
pump. Unlike potassium, 201Tl has two binding sites on the enzyme
system, which may explain its prolonged clearance from the cell. Uptake
requires cell viability and has been shown to be greater in tumor cells
than in normal connective tissue or inflammatory cells and is less in areas
of necrotic tissue. Corticosteroid administration does not meaningfully
alter uptake, at least in the brain. The typical background scan features
high abdominal and pelvic activity, with high activity also in the heart
and the thyroid and salivary glands.3 Correspondingly, the most
promising oncologic applications of this radionuclide involve the brain
and extremities, areas where background activity is relatively low.
Early reports of 201Tl imaging of primary
brain tumors confirm the expectation that uptake corresponds to the histologic
aggressiveness of the tumor. Reliable interpretations can be made using
the contralateral homologous area of brain for comparison. In fact, using
a simple ratio (thallium index) of average counts per pixel in corresponding
regions, Black and associates4 found this ratio to predict clinical
outcome better than did the histologic grade of biopsy tissue; they surmise
that the radionuclide study provides a global measure of metabolic activity
and is not prone to sampling error, as is biopsy. The mean thallium index
of 14 low-grade gliomas was 1.27 ± 0.40, while the mean index of
11 high-grade gliomas was 2.40 ± 0.61 (P<.0005). Schwartz
et al5 used a ratio based on the activity in the contralateral
scalp -- which they found initially to be less variable than that of contralateral
brain -- and found excellent accuracy for both recognizing recurrence and
distinguishing recurrence from brain necrosis. They report a dual-isotope
method with 99mTc hexamethylpropilene amine oxime (HMPAO) to
be helpful in patients with intermediate ratios.
In our institutional experience, we have found contralateral
homologous brain to be more reliable than contralateral scalp as a basis
for interpreting 201Tl uptake. We have found 99mTc
HMPAO scanning to be fraught with difficulties in interpretation, and we
do not use it in this setting.6
References
1. Cox PH, Belfer AJ, van der Pompe WB. Thallium-201 chloride uptake
in tumours: a possible complication in heart scintigraphy. Br J Radiol.
1976;49:767-768.
2. Wackers FJ, Berman DS, Maddahi J, et al. Technetium-99m-hexakis2-methoxyiso-butyl
isonitrile: human biodistribution, dosimetry, safety and preliminary comparison
to thallium-201 for myocardial perfusion imaging. J Nucl Med. 1989;30:301-311.
3. Ando A, Ando I, Katayama M, et al. Biodistribution of 201T1 in tumor
bearing animals and inflammatory lesions in induced animals. Eur J Nucl
Med. 1987;12: 567-572.
4. Black KL, Hawkins RA, Kim KT, et al. Use of thallium-201 SPECT to
quantitate malignancy grade of gliomas. J Neurosurg. 1989;71:342-346.
5. Schwartz RB, Carvalho PA. Radiation necrosis vs high-grade recurrent
glioma: differentiation by using dual-isotope SPECT with 201T1 and 99mTc-HMPAO.
AJR Am J Roentgenol. 1992;158:399-404.
6. Bartolini A, Gasparetto B, Albano C, et al. Assessment of 99mTc-HM-PAO
parameters with SPECT for improved evaluation of blood flow in cerebral
lesions. Comput Med Imaging Graph. 1991;15:303-309.