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Clinical Investigations |
1 Department of Nuclear Medicine, University of Tuebingen, Tuebingen, Germany
2 Department of Radiation Oncology, University of Tuebingen, Tuebingen, Germany
3 Section of Radiopharmacy, University of Tuebingen, Tuebingen, Germany
| ABSTRACT |
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Key Words: hypoxia imaging 18F-fluoromisonidazole PET prognosis
| INTRODUCTION |
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However, the oxygen electrode method exhibits quite a few problems. First, it is technically demanding, requires considerable operator skill, and thus is prone to sampling errors. Second, it is useful only for studying accessible (i.e., superficial) tumors. Therefore, and because of its invasive character, the oxygen electrode has not become a general clinical tool for measuring tumor hypoxia. The most promising approach to visualizing hypoxia appears to be imaging by PET, provided a tracer that is a true hypoxia marker is applied. Ken Krohn and his group in Seattle were the first to successfully demonstrate the potential of PET using fluoromisonidazole (FMISO) as the tracer (711). Moreover, the value of PET and FMISO was illustrated by further clinical studies (1214). The tracer most extensively studied both in humans and in animals is 18F-FMISO (1518). Other tracers such as 18F-FAZA and 60Cu-ATSM are also under evaluation (1924).
Previous results for FMISO suggest that it is feasible for imaging hypoxia and shows the regional distribution of tumor hypoxia, usually expressed as fractional hypoxic volume. This information may become more relevant when intensity-modulated radiation therapy (25) is applied (26). Thus, integration of hypoxia data obtained by FMISO PET into the radiation-planning process appears desirable. However, the prognostic value of FMISO has not been validated in clinical studies. Therefore, the aim of our study was to analyze uptake of FMISO in 2 tumor entities that are usually treated by radiationadvanced-stage non-small cell lung cancer (NSCLC) and head and neck cancer (HNC)and to compare uptake with clinical outcome after treatment.
| MATERIALS AND METHODS |
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FMISO PET Scanning and Image Reconstruction
PET investigations were performed with a whole-body PET scanner (Advance; General Electric Medical Systems). We administered 350450 MBq of 18F-FMISO intravenously with an automated bolus injection technique (12 s). In all but 3 patients, dynamic scans were obtained for 14 min (12 frames of 10 s, 8 frames of 15 s, and 10 frames of 1 min). Two and 4 h after injection, static emission scans were obtained. PET scans were processed using iterative reconstruction and measured attenuation correction. Whenever logistically possible, we performed additional fusion imaging with a dual-head coincidence
-camera equipped with an integrated x-ray tube and dedicated fusion software (Millennium VG; General Electric Medical Systems) 3 h after injection of FMISO.
Other Imaging Procedures
All patients underwent CT as part of the pretreatment diagnostic work-up and for radiotherapy planning.
18F-FDG PET was performed on 32 of the 40 patients (13/21 patients with proven outcome). It was not performed on the first 8 patients because it was not part of the original study protocol, but preliminary analysis of the first patients showed that despite image fusion, the tumor delineation was not accurate in all cases because of the limited quality of low-dose CT.
Region-of-Interest (ROI) Analysis and Quantification
Definition of ROI.
Tumor ROIs were drawn manually on late images within the slice with the highest tumor uptake, applying a 50% isocontour technique (50% between maximum and background). When the tumor could not be delineated because of low FMISO uptake, ROIs were drawn using fused low-dose CT or 18F-FDG images.
Reference Tissues.
In HNC, the neck muscle was chosen for reference, and in NSCLC, a tumor-free area in the mediastinum was chosen. Other regions (vertebrae, cerebellum, lung, left ventricle, myocardium) were also assessed exploratively but not used as reference tissue because of much higher interindividual variability. The ROI size was 5 x 3.5 mm (oval) in the neck muscle; for mediastinum, we drew circular ROIs with a diameter of 5 mm.
Quantification.
FMISO uptake 2 and 4 h after injection was quantified by calculation of standardized uptake values (SUV = 1 g/mL x measured radioactivity x body weight/injected radioactivity) and tumortoreference tissue ratios (T/Rs). In the following, T/Mu denotes the tumor-to-muscle ratio (HNC) and T/Me denotes the tumor-to-mediastinum ratio (NSCLC).
Additionally, early FMISO uptake was assessed by SUV 02 min after injection and 514 min after injection. In the following, SUV at 12 min is also referred to as perfusion index, and SUV at 514 min, as nonspecific distribution volume.
TimeActivity Curves 014 min After Injection.
Timeactivity curves were visually classified according to shape as rapid washout type (type 1), intermediate type (type 2), or accumulation type (type 3). Typical examples are shown in Figure 1.
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Analysis of Outcome
For 21 patients, sufficient data about the clinical outcome of radiotherapy were available. They were categorized as responders or nonresponders. Responders (n = 8) had complete remission or near-total remission of the tumor, with steady state (as confirmed by repeated CT) for at least 1 y after treatment. The longest observed interval was 28 mo. Nonresponders (n = 13) had progressive disease, or only partial remission after completion of radiotherapy, or local recurrence within a follow-up period of 1 y. For statistical reasons, we did not separate patients with progressive disease from those with local recurrence.
Statistical Analysis
We tested the hypothesis that SUV at 4 h predicts the response to radiotherapy. Analysis of the other parameters focused on the following questions: Do T/Rs provide information additional to that provided by SUV? Do timeactivity curves provide relevant prognostic information? For how long do we have to study FMISO distribution to obtain optimal results?
To confirm that FMISO uptake has prognostic value (SUV, T/R), we tested the significance of group differences (responders vs. nonresponders) using the Student t test with unequal variances and selected optimal cutoff values for separating responders from nonresponders.
| RESULTS |
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FMISO SUV and T/R.
The SUVs 2 and 4 h after injection (Table 1) in the tumor ranged from 1.1 to 4.3 (maximum in ROI) in HNC. In NSCLC, the SUVs were slightly lower, ranging from 1.2 to 2.7 (SUV at 2 h) and from 1.0 to 3.7 (SUV at 4 h). In HNC, the SUV at 4 h closely correlated with the SUV at 2 h (R2 = 0.93), whereas in NSCLC the correlation was weaker (R2 = 0.63). T/Me (NSCLC) was significantly higher than T/Mu (HNC) (P = 0.002 in T/R at 4 h) (Table 1), as can be attributed to significantly lower SUV in mediastinum (P < 0.001) (SUV at 2 h = 0.85 ± 0.25; SUV at 4 h = 0.77 ± 0.26) than in muscle (SUV at 2 h = 1.19 ± 0.15; SUV at 4 h = 1.07 ± 0.17).
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2 at 4 h (NSCLC) or a cutoff T/Mu of
1.6 at 4 h (HNC) allowed prediction of 10 of 13 tumor recurrences and all responders. The T/R at 2 h was less useful than the T/R at 4 h. Although the mean T/R at 2 h in nonresponders was higher than in responders, a clear cutoff could not be defined. For each tumor entity, the T/R at 4 h had a higher predictive value than did SUV. When the selected cutoff values for SUV and T/R were combined, 11 of 13 patients with and 8 of 8 patients without recurrence could correctly be classified.
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| DISCUSSION |
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In this study, we chose a different approach and assessed various quantitative parameters postulating that both the presence and the magnitude of specific FMISO uptake are of prognostic value. We also visually assessed timeactivity curves of the first 14 min after injection of FMISO. This approach has not been reported for FMISO in the literature because the effects of tissue hypoxia are generally assumed to be more pronounced at later times. Being aware that these curves are hard to interpret because of the many unknown kinetic properties of the tumor (e.g., fractional vascular volume, perfusion, extraction, microvessel density (30), and hypoxia-related and hypoxia-independent metabolism of FMISO), we hypothesized that a fast washout is predictive of absence of relevant tumor hypoxia and thus of good radiosensitivity of the tumor.
Predictive Value of SUV and T/R
In accordance with previous studies, we found a correlation between late uptake parameters of FMISO (SUV and T/R) and tumor recurrence, as both are linked to the presence of hypoxia. However, an important finding was that T/R was more predictive than SUV. This finding might be explained by interindividual variances in plasma clearance of FMISO that affect the SUV in tumor and in reference tissues. In our patients, a T/Me greater than 2.0 or a T/Mu greater than 1.6 was highly predictive of later tumor recurrences. The observed different cutoffs roughly correspond to differences in SUV of the reference tissues.
Early TimeActivity Curves
In our study, we classified timeactivity curves according to 3 types that were derived after analyzing the scans of the first 10 patients. Correlation with clinical outcome was performed prospectively. Therefore, we are confident that the observed differences (type 1: recurrence in 0/4 patients; type 3: recurrence in 5/6 patients) reflect the prognostic relevance of these curve types. Additional kinetic parameters separating responders from nonresponders in patients with curves of type 2 or 3 were found exploratively (SUV at 02 h and at 515 h). Because of the small number of patients (4/14 with type 2 or 3) without recurrence, those findings are preliminary and have to be substantiated by a study with a large number of patients.
Suggested Acquisition Protocol
Early animal studies of Rasey et al. (spontaneous osteosarcoma in dogs) indicated that the most useful PET information is obtained during the first 3 h after drug injection. Further extension of imaging did not yield additional information (10). However, no systematic clinical data have been published on acquisition protocols optimal for predicting the outcome of later radiotherapy. In our study group, the T/R at 4 h allowed better outcome prediction than did the T/R at 2 h. Whether this difference reflects actual superiority of imaging has to be confirmed with larger patient cohorts. Because static images 4 h after injection are feasible using an 18F-labeled compound, we suggest extending current protocols. Because our data indicate a relevant correlation between the type of early FMISO timeactivity curve and clinical outcome, we suggest obtaining dynamic data for at least 14 min after injection whenever possible. We used the FMISO distribution in late images to define representative tumor ROIs. Thus, we do not suggest reducing FMISO imaging to dynamic scans of 015 min instead of sequential scanning for 24 h.
Clinical Consequences
Assuming that FMISO PET has prognostic potential with regard to subsequent radiotherapy, we end up with the questions of how the results of FMISO PET can be used for treatment planning and how FMISO data are to be integrated into radiotherapy-planning systems. On the basis of the findings of this pilot study, FMISO PET might be used to identify patients who need intensified treatment. In our study group, tumor recurred in all patients with an SUV greater than 2.0 and in all patients with a tumor-to-tissue ratio above the threshold of 2.0 (NSCLC) or 1.6 (HNC). Combining these 2 simple criteria would lead to correct prediction of tumor recurrence in 11 of 13 patients. Results become even better if curve-type analysis is considered. Using type 1 as an indicator of good treatment response, and type 3 combined with SUV and T/R above the cutoff as an indicator of poor response, would lead to appropriate classification of 19 of 21 patients with proven clinical outcome (>90%). Assessment of early FMISO uptake (014 min after injection) combined with curve-type analysis was most accurate for discriminating patients with a favorable response from those with a poor response: Nearly all patients (5/6) with type 3 curves and most patients (5/8) with type 2 curves were not treated successfully. Additional analysis of early uptake (014 min after injection) allowed the individual risk of recurrence to be defined in more than 90% of all patients (17/18).
For radiotherapy planning, valid information about the precise location of hypoxic areas is mandatory. Using ROI data as described in this paper may lead to problems because of heterogeneous regional tissue properties. Comparing FMISO distribution 2 and 4 h after injection, we found that the areas of highest FMISO uptake were not constant over time. As a consequence, tumor regions defined as hypoxic on the basis of cutoff values (SUV or T/R) shifted. Regional curve analysis revealed that different curve types often were present in the same tumor. Thus, a regional (voxel-by-voxel) kinetic analysis that takes into account early as well as late FMISO data and that can easily be integrated into radiation-planning systems is needed.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Susanne-Martina Eschmann, MD, Department of Nuclear Medicine, University of Tuebingen, Otfried-Mueller-Strasse 14, D-72076 Tuebingen, Germany.
E-mail: susanne-martina.eschmann{at}med.uni-tuebingen.de
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