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Clinical Investigations |
Division of Nuclear Medicine, University of Washington Medical Center, Seattle, Washington
| ABSTRACT |
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75 min after injection. For 20 newly diagnosed, untreated, locally advanced breast cancer patients, both the maximum SUV and the average SUV within the lesion were calculated with and without correction for blood glucose concentration. A linear regression analysis of the portion of the timeactivity curves starting at 27 min after injection was used to estimate the rate of SUV change per minute during the interval from 27 to 75 min. The rate of SUV change with time was compared with the instantaneous SUV obtained at different times from 27 to 75 min. Results: In untreated breast cancer, 18F-FDG SUV values changed approximately linearly after 27 min at a rate ranging from -0.02 to 0.15 per minute. In addition, the rate of SUV change was linearly correlated with the instantaneous SUV measured at different times after injection (r2 ranged from 0.82 to 0.94; P < 0.001). Using this information, an empirical linear model of SUV variation with time from injection to uptake measurement was formulated. The comparison method was then applied prospectively to a second set of 20 locally advanced breast cancer lesions not included in the initial analysis. The average percent error using the method to adjust for time differences was 8% and 5% for maximum SUVs and average SUVs ranging from 2 to 12. Conclusion: In untreated breast cancer, the SUV at any time point approximately predicts the rate of change of SUV over time. A comparison method based on this finding appears feasible and may improve the usefulness of the SUV by providing a means of comparing SUV acquired at different times after injection.
Key Words: standardized uptake value 18F-FDG breast cancer PET
| INTRODUCTION |
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The purpose of this study was to measure how the SUV changes with observation time after injection in breast cancer and to examine the feasibility of an approximate method to compare the SUVs from studies with modest variations in the time between injection and uptake measurement experienced in clinical practice. Thie et al. (8) used dynamic scans of normal tissue and a heterogeneous group of tumors to explore the change in activity over time and to better define the appropriate time for imaging after injection based on optimal contrast ratios. They also suggested a method for correcting uptake measurements to a standardized time. Our study differs in that we analyzed dynamic scans of a homogeneous group of tumors and we also evaluated the potential error of this approach using a second independent study.
| MATERIALS AND METHODS |
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18F-FDG PET imaging was performed using 246393 MBq (6.710.6 mCi) 18F-FDG, prepared using the method of Hamacher et al. (10). In all cases, 18F-FDG radiochemical purity was >95% and specific activity was >47 GBq/µmol. 18F-FDG was infused using a volume of 710 mL over 2 min in the antecubital vein contralateral to the affected breast. All imaging studies were performed using an Advance PET scanner (General Electric Medical Systems) operating in a 2-dimensional high-sensitivity mode with 35 imaging planes covering an axial field of view of 15 cm (4.0-mm axial full width at half maximum at the center of the tomograph) and an intrinsic in-plane resolution of
5 mm (11,12). Before the PET study, patients fasted for a minimum of 6 h and blood glucose levels were measured using a glucose analyzer (Beckman Coulter, Inc.) before 18F-FDG injection.
Dynamic imaging was performed for 60 min after the start of 18F-FDG infusion. For the portion of the scans evaluated in this study, 5-min time bins were used. For 13 of the patients, a 7-min static emission scan, taken as part of a torso survey and starting up to 12 min after the 60-min dynamic study (yielding time points of up to 75 min), was also available and used for data analysis. Imaging data underwent corrections for attenuation, random coincidences, and scattered coincidences and were reconstructed into 35 transverse image planes (128 x 128 pixels) using filtered backprojection with a Hann 10-mm smoothing window. Image count data were converted to kilobecquerels/milliliter using data from calibration vials of known activity measured in a dose calibrator (radioisotope calibrator CRC-7; Capintec, Inc.).
For each lesion, the SUV versus time curves were generated using both the maximum SUV (Smax) and the average SUV (Sav) within a volume of interest (VOI). The VOI consisted of 3 circles of 17 pixels and
1.5-cm diameter each, over 3 contiguous transaxial planes, each 4.5-mm thick, and the middle slice containing the maximum pixel value for the lesion. Both the Smax and the Sav were calculated using the formula:
![]() | (Eq. 1) |
![]() | (Eq. 2) |
The time of each SUV was considered to be the midinterval of each acquisition time bin or frame. As mentioned above, for most studies, an additional SUV measurement was obtained between 71 and 75 min after injection from a subsequent standard clinical scan.
A linear regression analysis of the curves after 27 min following injection was used to estimate the rate of SUV change (dS/dt [min-1]) during the interval from 27 to 75 min for each lesion. The estimated dS/dt was compared with the instantaneous measured SUV at 27, 42, 57, and 75 min after injection. Using a linear model of dS/dt versus SUV(t), an empirical method for comparing SUV for varying times from injection to uptake measurement based on the linear correlation of dS/dt versus SUV(t) was formulated.
To test the validity of this comparison method, we selected a second set of 20 locally advanced breast cancer lesions in patients not included in the initial analysis who were studied using the same the same imaging protocol and data analysis as described above. Using the comparison method based on our linear model, we estimated the Smax and Sav at 7175 min after injection using the known Smax and Sav at 45 min in those patients. The estimated values were compared with the measured values at 7175 min.
| RESULTS |
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SUV Measurements
Figure 1 shows the tumor timeactivity curves of tumor VOIs for all patients. At 57 min, the Smax for the tumors ranged from 1.3 to 12.4 (mean, 6.6) and the Sav ranged from 0.9 and 10.6 (mean, 4.9). When blood glucose correction was applied, the Smax at 57 min ranged from 1.0 to 10.5 (mean, 5.8) and the Sav ranged from 0.7 and 8.6 (mean, 4.3).
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![]() | (Eq. 3) |
2 is the estimated SUV at a desired time t2, S1 is the measured SUV at time t1, and dS/dt is the rate of SUV change at the measurement time t1 for SUV, S1. The value for dS/dt can be obtained from the plots of the linear fits of dS/dt versus S in Figure 4 for the measurement time, t1.
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![]() | (Eq. 4) |
![]() | (Eq. 5) |
12.3. We note that, in principle, any reference time can be used for the values a and b. Alternately, the approximate value for dS/dt can be found graphically using Figure 4.
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2) at 7175 min after injection using the comparison method at a reference time (t0) equal to 57 min and a measured Smax (S1) at 45 min (t1) was compared with the measured Smax at 7175 min. Table 2 shows the average percent error on the Smax and Sav measured at 7175 min. The comparison method was most accurate for SUVs (both Smax and Sav) higher than 5. For SUVs lower than 5, the comparison method had an average percent error comparable to simply ignoring the effect of differences in injection time. As expected, the method performed better (lower average percent error) using Sav due the significantly higher intrinsic statistical noise of Smax as compared with Sav.
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| DISCUSSION |
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It is notable that some tumors in our series with lower metabolic activity undergo a small decline in SUV over time. This could potentially confound dual time point imaging protocols used to distinguish a tumor from inflammatory or benign processes (14,15), because some tumors with lower uptake may have a late uptake-to-early uptake ratio less than unity and would be falsely considered benign.
The wide variation in SUV change among locally advanced breast cancer stresses the importance of consistently acquiring images at the same time after injection. However, in a busy PET service, this is not always possible to achieve. A literature survey of articles published since 1990 was conducted that showed considerable variability of SUV measurement time after injection even within protocols at the same institution (8). A corrective method to standardize the time of SUV estimation would increase the clinical usefulness of SUVs.
The strong linear correlation between the rate of SUV change and the SUV measured at different times after injection is in accordance with the findings of Thie et al. (8), who suggested that more metabolically active tissues can show steeper timeactivity curve slopes. This information can be used to compare the SUV for varying times of uptake by using the linear model and Equation 3. The approach proposed here using Equations 3 and 5 (or Fig. 4) can be used as a simple tool to compare SUV for imaging time variations and to guide clinical interpretation of SUV. The linear model was validated for SUV from 2 to 12 measured between 27 and 75 min and, thus, our method may not apply to tumors with SUVs outside this range or with injection to scan time outside of the specified interval. As shown by the average percent error (Table 2), the comparison method is more useful for SUVs in the upper range (>5) and performed better using the Sav as compared with the Smax. For SUVs in the lower range (<5), the rate of SUV change over time and, thus, the absolute SUV change will be small and adjustment using our method is unnecessary. The accuracy of this comparison method also depends on the uncertainty on SUV measurements, which is larger, while using Smax as compared with Sav.
Some additional sources of variability need to be considered in applying the proposed comparison method. If the method were to be used with tumors of <2 cm in size, underestimation of the SUV due to partial-volume averaging would lead to underestimation of the rate of SUV change over time. Similar to the glucose correction, the use of a different normalization factor (lean body weight or surface area) would simply rescale the SUV versus time curve on the y-axis in a given patient without changing the shape of the curve. Therefore, a similar empirical linear model could be applied to SUV normalized with lean body weight or surface area.
Our model assumes that the SUV curves are approximately linear within the specified time interval. However, the shapes of SUV curves may be different for other types of tumors, which will affect the approximation. For example, tumors having a significant FDG dephosphorylation rate may have significant deviation from linearity of their SUV curves within the time interval studied. Similarly, the shapes of SUV curves may change after treatment (as shown in lung cancer (7)). Consequently, our comparison method may not necessarily apply to tumor types other than breast cancers or to treated breast tumors.
Future studies should examine SUV time dependency in a variety of treated and untreated tumors to refine such a comparison method. Also, studies using kinetic analysis in breast cancer may be helpful to understand the underlying biologic characteristics that explain the 2 levels of approximate linearity observed in our study in the specified time interval.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: David A. Mankoff, MD, PhD, Division of Nuclear Medicine, Box 356113, University of Washington Medical Center, 1959 N.E. Pacific St., Seattle, WA 98195.
E-mail: dam{at}u.washington.edu
| REFERENCES |
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