|Home | About | Journals | Submit | Contact Us | Français|
To assess amide proton transfer-weighted (APTW) imaging features in patients with malignant gliomas after chemoradiation and the diagnostic performance of APT imaging for distinguishing true progression from pseudoprogression.
After approval by the Institutional Review Board, 32 patients with clinically suspected tumor progression in the first three months after chemoradiation were enrolled and scanned at 3T. Longitudinal routine MRI changes and medical records were assessed to confirm true progression versus pseudoprogression. True progression was defined as lesions progressing on serial imaging over six months, and pseudoprogression was defined as lesions stabilizing or regressing without intervention. The APTWmean and APTWmax signals were obtained from three to five regions of interests for each patient and compared between the true progression and pseudoprogression groups. The diagnostic performance was assessed with receiver operating characteristic curve analysis.
The true progression was associated with APTW hyperintensity (APTWmean = 2.75% ± 0.42%), while pseudoprogression was associated with APTW isointensity to mild hyperintensity (APTWmean = 1.56% ± 0.42%). The APTW signal intensities were significantly higher in the true progression group (n = 20) than in the pseudoprogression group (P < 0.001; n = 12). The cutoff APTWmean and APTWmax intensity values to distinguish between true progression and pseudoprogression were 2.42% (with a sensitivity of 85.0% and a specificity of 100%) and 2.54% (with a sensitivity of 95.0% and a specificity of 91.7%), respectively.
The APTW-MRI signal is a valuable imaging biomarker for distinguishing pseudoprogression from true progression in glioma patients.
Malignant gliomas are the most common and deadly primary brain tumors in adults.1 Current standard therapy for high-grade gliomas (anaplastic astrocytoma and glioblastoma) involves maximal safe tumor resection, followed by radiotherapy with concurrent temozolomide (chemoradiation), followed by adjuvant monthly temozolomide.1 Although this regimen is associated with improved overall survival versus radiotherapy alone for patients with glioblastoma, it is associated with a high incidence of treatment-related imaging and clinical changes, termed pseudoprogression.2 Pseudoprogression appears most commonly in the first three months after completion of chemoradiation, but can present long after completion of chemoradiation.3 Pseudoprogression mimics true tumor progression, clinically and radiographically. A diagnosis of true progression requires a change in therapeutic approach, while a diagnosis of pseudoprogression supports continuation of current anti-cancer therapy. Hence, it is important to distinguish pseudoprogression from true progression in order to provide appropriate clinical management.
MRI is the most common image technique to evaluate pseudoprogression versus tumor progression. However, the standard clinical MRI sequences, including T2-weighted (T2W), fluid-attenuated inversion recovery (FLAIR), and gadolinium (Gd)-enhanced T1-weighted (T1W), do not have the specificity required to distinguish pseudoprogression from true progression.4 Consequently, patients with changes on MRI post-chemoradiation are required to undergo longitudinal MRI observation for several months or repeat surgery to confirm the diagnosis.5, 6 This process is resource-intensive, as well as burdensome for the patients, and the longitudinal clinical MRI observation also delays definitive treatment. A variety of advanced functional and molecular MR techniques and nuclear medicine approaches are being investigated in the effort to identify a more accurate imaging marker for tumor tissues. Although promising, results have been mixed, and there is currently no standard imaging modality available for differentiating between true tumor progression and pseudoprogression in the clinic.7
Amide proton transfer (APT) imaging is a molecular MRI technique at the protein level that generates image contrast using endogenous mobile proteins and peptides in tissue.8, 9 Several previous reports have indicated that the APT-weighted (APTW) imaging signal could differentiate between malignant brain tumor and peritumoral edema, between pathological grades, as well as between primary central nervous system lymphomas and high-grade gliomas in both preclinical models and in patients.10–15 We hypothesized that treatment-related changes associated with pseudoprogression would be APTW iso-intense to normal brain tissue, whereas active glioma would be associated with high APTW signals, allowing meaningful separation of recurrent tumor from treatment effect. Indeed, several recently published preclinical results have proven that APTW imaging could distinguish active brain tumor from treatment-induced necrosis, more reliably and earlier than conventional MRI sequences.16–18 In addition, the APTW imaging signal in tumor tissue was shown to be sensitive to high-intensity focused ultrasound-induced necrotic changes.19 Clinical APT imaging studies have also shown promise in detecting treatment effect in non-glioma conditions, such as arteriovenous malformations and breast cancer.20, 21 This study explored the ability of APT imaging to distinguish pseudoprogression from true progression in patients who developed non-specific MRI changes suspicious for progressive malignant glioma after completing standard chemoradiation.
The study was approved by the Johns Hopkins Medicine Institutional Review Board. All patients provided written, informed consent. A total of 32 glioma patients with suspected tumor progression after chemoradiation were recruited (21 males, 11 females; median age, 56.5 years; age range, 22–78 years; Table 1), according to the following eligibility criteria: (i) initial diagnosis of histologically proven malignant gliomas; (ii) completed chemoradiation; (iii) had standard clinical MRI before and after chemoradiation; (iv) developed new or enlarged Gd-enhancing lesions within the first three months after chemoradiation; (v) completed APT MRI studies within 12 months after chemoradiation; and (vi) had longitudinal clinical follow-up MRI for more than six months.
All studies were performed on a 3T human MRI scanner (Achieva; Philips Medical Systems, Best, The Netherlands), using a body coil excite and a 32-channel phased-array coil for reception (Invivo, Inc., Gainesville, FL). A fast three-dimensional (3D) APT imaging sequence reported previously22 was used in this study. This sequence consists of radiofrequency saturation (four block radiofrequency pulses of 200 ms duration and 2 μT amplitude), lipid suppression, and 3D gradient- and spin-echo image acquisition. Image parameters used were: field of view, 212×186 mm2; resolution, 2.2×2.2 mm2; 15 slices; thickness, 4.4 mm; sensitivity-encoding acceleration factor, 2, in the right-left direction; repetition time, 3 s; echo time, 17 ms; and specific absorption rate, 1.1 W/kg. Localized shimming using a field map was performed. The calculated optimal shim parameters and scanner’s center frequency were applied to the subsequent APT and other MRI scans. To correct for residual B0 inhomogeneity effects, APT imaging was acquired with a six-offset protocol (unsaturated S0, ± 3, ± 3.5, ± 4 ppm from water; 1, 1, 4, 1 averages, respectively). The total scan time was 10 min 42 s. The water saturation shift-referencing method was used to determine B0 maps (range, −1.5 to 1.5 ppm; interval, 0.125 ppm; saturation power, 0.5 μT; saturation time, 400 ms; repetition time, 1.25 s; scan time, 2 min 15 s).
Several conventional MRI scans were performed simultaneously, with the following parameters: T2W (repetition time, 4 s; echo time, 80 ms; 60 slices; thickness, 2.2 mm); FLAIR (repetition time, 11 s; echo time, 120 ms; inversion recovery time, 2.8 s; 60 slices; thickness, 2.2 mm); and Gd-enhanced T1W (3D magnetization-prepared rapid-gradient-echo sequence; repetition time, 3 s; echo time, 3.7 ms; inversion recovery time, 843 ms; flip angle, 8°; 150 slices; isotropic voxel, 1.1 mm3). The Gd-enhanced T1W imaging (0.2 mL/kg body weight; Magnevist; Berlex, Montville, NJ, USA) was the last sequence acquired.
The image analysis was performed using an interactive data language (IDL, Version 7; Exelis Visual Information Solutions, Inc., Boulder, CO, USA). The acquired APT image data was registered to the saturated image at the offset of 3.5 ppm23 and corrected for the B0 inhomogeneity effect, using the determined B0 map from the water saturation shift-referencing method. The APTW image was then constructed with the so-called magnetization transfer-ratio asymmetry at the offsets of ±3.5 ppm with respect to the water signal:10–15 MTRasym(3.5 ppm) = Ssat(−3.5 ppm)/S0 − Ssat(+3.5 ppm)/S0, where Ssat and S0 are the imaging signal intensities with and without selective radiofrequency irradiation, respectively. The APTW intensity is a continuum and is reported as a percentage change in the bulk water signal intensity. To account for the contribution of the possible nuclear Overhauser enhancement effect at −3.5 ppm to MTRasym(3.5ppm), the calculated MTRasym(3.5ppm) image is generally called the APTw image.
Two neuroradiologists (B.M. and S.J., with 14 and 6 years of neuroradiology experience, respectively) analyzed the conventional MRI characteristics (T2W, FLAIR, and Gd-enhanced T1W), acquired before and after the end of chemoradiation, according to the updated Response Assessment in Neuro-Oncology (RANO) criteria for gliomas.24 During the minimum six-month follow-up period after completion of chemoradiation, patients with lesions that continued to increase in size or with new contrast enhancement were defined as having true progression (Supporting Fig. S1), and patients with stable or regressing lesions without any change in therapy regimen were diagnosed as having pseudoprogression (Supporting Fig. S2).25–27 Of the 32 patients enrolled, 20 patients were classified as having true progression, and the other 12 patients were classified as having pseudoprogression.
For the quantitative MRI analysis, regions of interest (ROIs) were defined according to the signal abnormalities on conventional MRI sequences. For each patient, three to five ROIs (according to the volume of the lesion, 50 to 100 voxels each) were drawn on the Gd-enhanced T1W images (Fig. 1), independently by two neuroradiologists (B.M. and S.J.). These ROIs were then transferred to the corresponding regions on the co-registered unsaturated S0 images to obtain the corresponding APTW signal intensities. Areas of large liquefactive necrosis, hemorrhages, or large vessels evident on standard MRI sequences were excluded from analysis. Special attention was paid to avoid possible artifacts in the areas near skull, ventricles, and air-tissue interfaces. The contralateral normal-appearing white matter was used for control.
For each patient, the mean and maximum APTW (APTWmean, APTWmax) imaging intensities for all selected ROIs were reported, and the results from two observers were averaged. The unpaired Student’s t-test was used to compare APTW imaging intensities in the true progression and pseudoprogression groups. The receiver operating characteristic (ROC) curve was constructed to differentiate true progression from pseudoprogression. The sensitivity and specificity were calculated, and the best cutoff values were determined by maximizing the sum of sensitivity and specificity. All statistical analyses were performed with SPSS version 18.0. The results with P values lower than 0.05 were considered statistically significant.
In the true progression group (n = 20, 14 males, 6 females; median age, 56 years; age range, 22–78 years; Table 1), all patients had chemoradiation for either malignant glioma (anaplastic astrocytoma in five patients and glioblastoma in 13 patients) or for progressive low-grade glioma with presumed malignant transformation (two patients). The median time between completion of chemoradiation and the performance of APT imaging was four months (range, 1–12 months). In the pseudoprogression group (n = 12, 7 males, 5 females; median age, 61 years; age range 43–75 years), all patients had chemoradiation for either malignant glioma (anaplastic astrocytoma in two patients and glioblastoma in nine patients) or for progressive low-grade glioma presumed to have undergone malignant transformation (one patient). The median time between completion of chemoradiation and the performance of APT imaging was two months (range, 1–12 months).
Figure 2 shows the APTW image, together with the conventional MR (T2W, FLAIR, and Gd-enhanced T1W) images, acquired at the same time point, for a typical case in the true progression group. A heterogeneous Gd-enhancing mass, with marked surrounding edema and a slight midline shift, was observed in the left frontal lobe. For all cases who had a clinical course consistent with true progression, the Gd-enhancing areas on the Gd-enhanced T1W images were hyperintense on the APTW images, compared with the contralateral brain area. The abnormal areas on the APTW images were smaller than those on the T2W and FLAIR images.
Figure 3 shows a representative example of a patient who had a clinical course of pseudoprogression, in which a heterogeneous Gd-enhancing mass with central necrosis was observed in the left frontoparietal region. On the T2W and FLAIR images, the mass was quite heterogeneous, compared with the contralateral normal white matter. On the APTW images, the Gd-enhancing lesion appeared isointense on APTW, with punctate APTW hyperintensity scattered within the lesion, with respect to the contralateral normal white matter, and the edema around the lesion was also APTW-isointense.
The APTW imaging intensities (APTWmean, APTWmax) in the true progression and pseudoprogression groups were compared in a quantitative manner (Fig. 4). Inside the lesions, the measured APTWmean signal intensity was 2.75% ± 0.42% in the true progression group, significantly higher than that in the pseudoprogression group (1.56% ± 0.42%; P < 0.001). Moreover, the measured APTWmax signal intensity in the true progression group (3.29% ± 0.61%) was significantly higher than that in the pseudoprogression group (1.95% ± 0.44%; P < 0.001). In the contralateral normal white matter, there was no significant difference in the APTW signal intensities between these two groups (0.21% ± 0.19% vs. 0.22% ± 0.25%; P = 0.86).
Based on the ROC analysis (Fig. 5), the best cutoff value for APTWmean to predict true progression versus pseudoprogression was 2.42%, with a sensitivity of 85.0%, a specificity of 100%, and an area under the curve (AUC) of 0.98. The best cutoff value for APTWmax to predict true progression versus pseudoprogression was 2.54%, with a sensitivity of 95.0%, a specificity of 91.7%, and an AUC of 0.97. Taken together, these data suggested that the APTW signal intensities are relatively reliable imaging biomarkers for distinguishing these two pathologies, with an operational threshold of 2.42.5% APTW signal intensity.
We applied the 3D APT image technique, along with several conventional MRI sequences (T2W, FLAIR, and Gd-enhanced T1W), to assess the radiographic characteristics of true progression versus pseudoprogression in patients with malignant gliomas treated with chemoradiation. The conventional MRI features are similar in these two pathological groups, and Gd-enhancing lesions with T2W and FLAIR hyperintensities were observed in both true progression and pseudoprogression. In contrast, APTW hyperintensity (relative to the contralateral normal brain tissue) was observed predominantly in patients with true progression, and APTW iso-intensity to mild hyperintensity was more commonly seen with pseudoprogression. The quantitative results suggested that the APTW images, when compared with the conventional MRI sequences, can greatly improve the value of MRI in distinguishing treatment effects from true progression.
Distinguishing tumor progression from treatment effects, such as pseudoprogression, following brain tumor therapy remains a major clinical challenge. Patients with possible pseudoprogression versus true progression may end a potentially effective therapy early (if it is pseudoprogression) or continue a potentially ineffective therapy too long (if it is true progression). Currently, these patients typically undergo a biopsy for pathological confirmation or a longitudinal MRI follow-up over several months. APT imaging could provide an important biomarker for brain tumors that could be used to guide biopsies, which would greatly increase the accuracy of pathology. Further, the APTW-MRI signal as a surrogate biomarker of actively growing tumor could potentially reduce the necessity for repeated biopsies and their associated risks, thereby improving the quality of life for patients and decreasing the cost of care. The APT method is currently available only within the research domain. The next required steps for expanding this promising technology into the standard clinical care for patients with malignant gliomas are: (i) a manufacture-provided automated pulse sequence package to be validated in a multicenter study and (ii) approval by the Food and Drug Administration.
The APTW-MRI signal, based on the chemical exchange-dependent saturation transfer mechanism,28 is measured with a reduction in bulk water intensity due to chemical exchange with magnetically labeled backbone amide protons of endogenous mobile proteins and peptides in tissue (such as those in the cytoplasm).8 According to this theory, the APTW signal in tissue is primarily related to the mobile amide proton content and the amide proton exchange rate (a parameter that depends on tissue pH). Several previous proteomics29 and MR spectroscopy30 studies have shown that active tumor tissues express more protein species and higher protein levels. On the other hand, treatment effect is thought to be induced by a treatment-related local tissue reaction, accompanied by an inflammatory process that manifests as edema, mass effect, and a transient increase in the permeability of the blood-brain barrier.31 In contrast to true progression, there are fewer mobile cytosolic proteins and peptides in regions of brain injury associated with pseudoprogression, due to lower cellular density and disrupted cytoplasm.32, 33 Accordingly, the APTW signal intensity in the true progression group is higher, likely due to hypercellularity and abundant cytoplasm in tumor cells, while the APTW signal intensity in the pseudoprogression group is relatively low, presumably due to decreased cellularity.
Rat models have previously demonstrated that the APTW signal intensity is hypointense to isointense in areas with pure radiation necrosis34. In this study, the observed APTW signal intensity in people deemed to have pseudoprogression was isointense to mildly hyperintense with respect to the contralateral white matter. The most possible explanation for this discrepancy is that frank tissue necrosis is an extreme and rare clinical event. A mixture of glioma cells and treatment-related injury is often observed on histologic samples in patients. There are findings suggestive of cellularity and low mitotic rates in pathologic samples labeled as treatment effect.6 In most pseudoprogression cases, the measured APTW imaging signal may be an average of both treatment effect and residual malignant gliomas. Despite such interference, the difference in APTW was still obviously significant between the clinically significant groups of patients who had true progression and pseudoprogression, despite the complexity of human malignant gliomas.
There were two limitations to this study. First, the inclusion criteria allowed a variety of malignant glioma subtypes (including clinically presumed transformation from low-grade to malignant gliomas). Given that the goal of the study was to assess the sensitivity and specificity of APTW imaging to distinguish between two clinical subgroups of gliomas treated with chemoradiation, and not to assess survival, this limitation should not influence the assessment of sensitivity and specificity of APT in this clinical setting. Second, when the magnetization transfer-ratio asymmetry analysis is used, the quantified APTW signal is contaminated with the nuclear Overhauser enhancement signal. However, it has been shown recently35, 36 that the APTW image contrast in the tumor is generally predominantly contributed by the APT effect. To quantify the pure APT effect, numerous different APT imaging analysis35–38 or acquisition39, 40 approaches have been proposed recently. However, these alternative methods can be complicated and may require a longer acquisition time, and their use for routine practice requires further validation.
In conclusion, APTW imaging offers a more specific characterization of true progression versus pseudoprogression in malignant glioma patients treated with chemoradiation than several conventional water-based MRI sequences. This is consistent with the previous preclinical findings in various glioma models and models of treatment-induced necrosis in rats. Without the need for exogenous contrast, APT imaging can be readily incorporated into standard clinical protocols to provide a tool for rapid, non-invasive, and specific diagnosis of true progression versus pseudoprogression. We foresee that the availability of APTW MRI for the routine clinical use may improve the care of patients with gliomas, who have non-specific changes on their MRI and currently undergo repeat surgery or spend precious months in an indeterminate diagnostic state.
Figure S1. Longitudinal follow-up MR images of a 69-year-old woman with a pathologically proven glioblastoma who had undergone surgical resection and chemoradiation. MRI showed a newly developed Gd-enhancing lesion ~1 month after chemoradiation (31 days). The lesion enlarged gradually (157 days and 248 days) on the Gd-enhanced T1W, T2W, and FLAIR images. Accordingly, the patient was defined as having true progression.
Figure S2. Longitudinal follow-up MR images of a 49-year-old man with a pathologically proven glioblastoma who had undergone surgical resection and chemoradiation. MRI showed a newly developed Gd-enhancing lesion ~1 month after chemoradiation (35 days). As time went on, the MRI signal intensities increased (84 days) and then decreased (259 days) on Gd-enhanced T1W, but remained steady (49 day) and then decreased (224 day) on FLAIR and T2W without any changes in the therapy regimen. The clinical diagnosis for the patient was pseudoprogression.
This work was supported in part by grants from the National Institutes of Health (R01EB009731, R01CA166171, R01EB015032, R21EB015555, and P41EB015909).
The authors thank Ms. Mary McAllister for editorial assistance.
Additional Supporting Information may be found in the online version of this article.
Conflicts of Interest: J.Z. and P.v.Z. are co-inventors on a patent at the US Patent and Trademark Office for the APT-MRI technology. This patent is owned and managed by Johns Hopkins University.