Use of ²⁰¹Tl SPECT for Evaluation of Biologic Behavior in Pituitary Adenomas

MATERIALS AND METHODS

We studied 22 of 28 patients with pituitary adenomas who were surgically treated in our hospital between January 1997 and June 1999. ²⁰¹Tl SPECT was performed preoperatively on all 22 patients (11 men, 11 women; age range, 18–65 y; mean age, 45 y). ²⁰¹Tl SPECT was not performed on the other 6 patients, who were excluded from the study. All patients had early SPECT imaging 15 min after intravenous injection of 148 MBq ²⁰¹Tl and delayed SPECT imaging 4 h after injection. All images were obtained with a continuous rotation of 4°/60 s, with a total of 120° per detector by means of a three-detector system with fanbeam collimators (Multispect 3; Siemens, Erlangen, Germany). We used 15% and 20% symmetric windows at 166 and 70 keV, respectively. A 128 × 128 matrix with a Butterworth filter was used for correction, and images were reconstructed on the transverse plane with a ramp filter. A transverse line was made that passed over the tumor on the axial image, and counts on the line were plotted. The ²⁰¹Tl uptake index was then obtained by dividing the maximum count on the curve by the mean value of the nontumor area (Fig. 1).

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FIGURE 1.

Calculation of ²⁰¹Tl uptake index. Transverse line was made that passed over tumor, and counts on line were plotted as curve. Maximum count (C_max) of tumor part was divided by mean count of nontumor area (B₁ and B₂).

Cavernous sinus invasion by tumor was defined on the basis of MRI findings. Those tumors with a grade higher than 2 by the classification of Knosp et al. (12) were defined as invasive.

Tumor tissues were fixed in neutral buffered formalin and embedded in paraffin. Four-micrometer-thick sections were mounted onto glass slides and dried. After deparaffinization and rehydration, endogenous peroxidase activity was blocked. Slide-mounted sections were incubated in 0.01 mmol/L sodium citrate buffer (pH 6.0) and then microwaved for 20 min at 500 W. Sections were incubated overnight at 4°C in MIB1 (1:100 dilution; DAKO, Santa Barbara, CA). Sections were then incubated with biotin-conjugated secondary antibody, followed by peroxidase-conjugated streptavidin. Antigen–antibody complexes were detected with 3,3′-diaminobenzidine/H₂O₂. Slides were counterstained with hematoxylin, dehydrated, cleared, and mounted. In each case, >1,000 tumor cells were counted by light microscopy, and the MIB1 labeling index was expressed as a percentage.

All statistical analyses were performed with StatView, version 5.0, computer software (SAS Institute, Inc., Cary, NC). Differences in the ²⁰¹Tl uptake index between the invasive tumor group and the noninvasive tumor group were evaluated by the Mann–Whitney U test.

RESULTS

Among the 22 cases of pituitary adenoma, 12 were invasive tumors and 10 were noninvasive tumors. Significant differences in the ²⁰¹Tl uptake indices were found between the invasive tumor group and the noninvasive tumor group, including both the early images (Tl^e index) (mean ± SE, 11.40 ± 2.49 and 4.26 ± 0.41, respectively) and the delayed images (Tl^d index) (6.69 ± 0.71 and 3.76 ± 0.37, respectively) (Fig. 2). Threshold values were defined as 5.0 on the Tl^e index and 4.5 on the Tl^d index. Among the12 invasive tumors, 11 were >5.0 on the Tl^e index and 10 were >4.5 on the Tl^d index (sensitivity, 92% and 83%, respectively). Conversely, among the 10 noninvasive tumors, only 3 were >5.0 on the Tl^e index and 1 was >4.5 on the Tl^d index (specificity, 70% and 90%, respectively).

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FIGURE 2.

(A) Tl^e index was significantly greater in invasive tumor group (mean ± SE, 11.40 ± 2.49) than in noninvasive tumor group (4.26 ± 0.41) (P = 0.0010). (B) Tl^d index was also significantly greater in invasive tumor group (mean ± SE, 6.69 ± 0.71) than in noninvasive tumor group (3.76 ± 0.37) (P = 0.0019).

Three prolactinomas and five growth hormone–producing adenomas were included in this study. However, no correlation was found between the ²⁰¹Tl index and the functional type of pituitary adenomas. No significant difference was found between the invasive tumor group and the noninvasive tumor group on the MIB1 labeling index (Table 1).

The Tl^d index correlated significantly with the MIB1 labeling index (P = 0.0107) (Fig. 3). This finding means that the delayed image indicates proliferative activity of the tumors. A representative case is presented in Figure 4.

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FIGURE 3.

Tl^d index correlated significantly with MIB1 labeling index (P = 0.0107). Pearson’s r = 0.527.

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FIGURE 4.

Patient 21. (A) MR coronal T1-weighted image shows macroadenoma that extended bilateral cavernous sinuses. (B) ²⁰¹Tl SPECT early image shows marked concentration of ²⁰¹Tl in tumor. (C) On ²⁰¹Tl SPECT delayed image, ²⁰¹Tl was retained in tumor. (D) Light-microscopic section (×400). MIB1 immunostaining shows relatively high percentage of MIB1-positive cells (arrows).

DISCUSSION

Despite being regarded as benign tumors, pituitary adenomas frequently invade surrounding structures or rapidly regrow after the first operation. In therapeutic planning it is very important to predict such aggressive behavior, especially in cases of nonsymptomatic tumor found incidentally. In this study, we investigated whether the invasiveness and proliferative activity of pituitary adenomas can be predicted with ²⁰¹Tl SPECT. Significant differences were found between the invasive tumor group and the noninvasive tumor group in the Tl^e index and the Tl^d index. Moreover, the MIB1 labeling index, which is a marker of tumor proliferative activity, correlated significantly with the Tl^d index.

Early ²⁰¹Tl concentration in tumors is related to tumor vascularity and destruction of the blood–brain barrier (2). However, histologically, pituitary tissue does not have a blood–brain barrier, so even normal pituitary tissue exhibits twice the counts on ²⁰¹Tl SPECT compared with surrounding normal brain structures. Therefore, in pituitary adenomas, early ²⁰¹Tl concentration in the tumors is related primarily to tumor vascularity. When ²⁰¹Tl is not taken into cells, it is washed out by blood flow. But when ²⁰¹Tl is taken into tumor cells by active transport with Na⁺,K⁺ adenosine triphosphatase (ATPase), it is retained in the tumor cells. Thus, retention of ²⁰¹Tl by the tumor is related to the activity of Na⁺,K⁺-ATPase in tumor cells (6). Therefore, the Tl^d index on the delayed image indicates viability of the tumor. In this study, tumor invasion was related to both the early image and the delayed image, but the MIB1 labeling index was related only to the delayed image. The explanation for these findings is that tumor invasion is related to both tumor vascularity and the proliferative activity of tumor cells (11,13), but the MIB1 labeling index is related only to the proliferative activity of tumor cells (8,10). Consequently, we can detect two different characteristics of tumors by studying the early and the delayed images of ²⁰¹Tl SPECT.

Knowing the biologic characteristics of a tumor preoperatively can be significantly beneficial for therapeutic decision making; hence, many studies on the biologic behavior of pituitary adenomas have been reported (1,2,8,10,14–16). However, because most of these studies used tumor tissue specimens such as MIB1 indices, they could not be used before surgery as an index for deciding whether tumors should be removed surgically.

Although ²⁰¹Tl SPECT has been reported to be useful for detecting tumor viability or tumor recurrence in malignant glioma (1,3–5,17), to our knowledge, no such study has been reported concerning pituitary adenoma. ²⁰¹Tl SPECT has not been used to evaluate pituitary adenomas because ²⁰¹Tl is taken up only by viable tumor cells; therefore, it has not been regarded as a useful study for benign tumors. However, in this study we performed ²⁰¹Tl SPECT on patients with pituitary adenoma, calculated ²⁰¹Tl uptake indices, and found that it is possible to predict the biologic behavior of these tumors. Therefore, ²⁰¹Tl SPECT may be helpful to guide therapy and surgical decision making, especially for patients in whom pituitary adenoma is found incidentally.

CONCLUSION

²⁰¹Tl SPECT makes prediction of the biologic behavior of pituitary adenomas potentially possible. Its clinical use may be helpful to guide therapeutic planning, especially in the case of incidentally discovered pituitary adenomas.