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Clinical Investigation |
1 Metabolic and Functional Imaging Centre, Clinical Research Centre, Centre Hospitalier Universitaire de Sherbrooke, Fleurimont, Quebec, Canada; 2 Department of Nuclear Medicine, Centre Hospitalier Universitaire de Montréal, Montréal, Quebec, Canada; and 3 Division of Pneumology, Department of Medicine, Centre Hospitalier Universitaire de Sherbrooke, Fleurimont, Quebec, Canada
Correspondence: For correspondence or reprints contact: François Bénard, MD, Metabolic and Functional Imaging Centre, Clinical Research Centre, Centre Hospitalier Universitaire de Sherbrooke, 3001 12th Ave. N., Fleurimont, Quebec, Canada J1H 5N4. E-mail: francois.benard{at}usherbrooke.ca
| ABSTRACT |
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Key Words: PET prognosis lung cancer survival standardized uptake value bone marrow
| INTRODUCTION |
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According to an extensive review of the literature published recently by Brundage et al. (4), >150 prognostic factors have been reported for NSCLC. Among them, TNM classification has been consistently shown to be the most powerful. Several other factorsclinical (weight loss, performance status, sex, age), biochemical (hypercalcemia, albumin, lactate dehydrogenase [LDH]), hematologic (hemoglobin), or molecular markershave been used as well, with variable power and consistency.
18F-FDG PET as a whole, in relation to its accurate staging abilities (59), has already been shown to be an excellent prognostic factor in NSCLC patients (10). In addition, many authors have already demonstrated that the metabolic activity of the primary NSCLC lesion, as measured by the tumor standardized uptake value (SUVT), can also be used as a prognostic factor (1117). However, the cutoff values that have been used vary considerably, ranging from 5 to 20. The SUV of the primary lesion has been related to the pathologic aggressiveness of pulmonary adenocarcinomas as well (18). The relationship between SUV and histologic type of NSCLC is controversial as one author described significant differences in SUVs between NSCLC subtypes while another found no significant difference.
In our clinical experience, we noticed that some patients, who were referred for 18F-FDG PET in the setting of pulmonary nodule evaluation or staging of NSCLC, showed unexplained, diffusely increased bone marrow metabolic activity (Fig. 1). We also observed that this subgroup of patients seemed to have a very poor outcome, which appeared to be even worse than that of the average patient with NSCLC (19). The objective of this study was to evaluate the prognostic significance of PET parameters relating to the tumor SUV, stage of disease on PET, and bone marrow activity and to assess their independence relative to some well-known prognostic factors. We also searched for potential causal factors that could explain bone marrow hypermetabolism.
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| MATERIALS AND METHODS |
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Data Acquisition
Whole-body 18F-FDG PET scans were acquired on a dedicated PET scanner (Ecat HR+; Siemens) from the neck to the pelvis. Patients were required to be fasting for a minimum of 6 h. Blood glycemia was monitored with a portable capillary glucometer, and a small bolus of intravenous insulin was administered as needed in a few patients with blood glucose levels above 8 mmol/L. A minimal delay of 1 h occurred between the intravenous injection of insulin and 18F-FDG administration. All subjects received a 18F-FDG dose of 7 MBq/kg. The mean delay from 18F-FDG injection to imaging was 91 min. Images were acquired in 2-dimensional mode (with septa) for 810 min per bed position and were reconstructed with an iterative algorithm (ordered-subset expectation maximization; 2 iterations, 16 subsets) with and without attenuation correction. The attenuation map was obtained with a 68Ge transmission source.
The mean and maximum SUVT was readily available as they are routinely reported at the Centre Hospitalier Universitaire de Sherbrooke. The maximum SUVT is obtained by drawing a region of interest (ROI) over the most intense slice of the primary lesion. The mean SUVT was obtained by drawing a ROI whose borders are defined by an automatic isocontour set at 75% of the maximum SUV within the ROI. (The 75% cutoff used for the mean SUV allowed for good reproducibility between subjects and excluded regions of tumor necrosis in order to obtain a measurement most representative of the metabolically active part of the lesions.)
A mean SUV was also obtained for each of the 3 larger homogeneous vertebrae visualized in the field of view (most of the time, L3, L4, and L5, unless showing severe osteoarthritic changes or metastasis). A ROI was drawn over the vertebral body, again using an automatic isocontour ROI set at 75% of the maximum SUV. The bone marrow SUV (BM SUV) was defined as the mean value of the 3 selected vertebrae. Normal reference values for BM SUV (mean ± SD, 1.32 ± 0.23) and for the relative ratio of activity between bone marrow and liver (BM/L) (mean ± SD, 0.94 ± 0.26) were obtained from a group of 20 healthy subjects, without any evidence or past history of oncologic or hematologic disease. All of these reference patients were free of disease after a minimum of 5-y follow-up. A ROI was also drawn on a homogeneous transaxial slice of the liver to obtain the BM/L ratio. All SUVs obtained were corrected for weight and lean body mass (45.5 + 0.91 x [height (cm) 152]).
Statistical Analysis
Statistical analysis was performed on S-Plus 6.0 (Insightful Corp.). Univariate KaplanMeier survival analyses were performed on SUVT, BM SUV, BM/L, and 18F-FDG stage as well as on the accepted prognostic factors that were available from patient records (age, sex, hemoglobin, LDH, calcium, albumin, conventional imaging stage, pathologic stage, and other hematologic parameters such as platelet and white blood cell [WBC] count). In addition to chest radiography and chest CT, conventional imaging included, as needed, abdominal ultrasound, brain CT or MRI, and bone scintigraphy. Some well-known prognostic factors, such as weight loss and performance status, could not be assessed retrospectively from this database as they were not collected systematically.
All variables were dichotomized using cutoffs reported in the literature when available or the best discriminating value as determined by recursive partitioning. Variables shown to be of prognostic significance (defined as a log-rank test P value < 0.05) on univariate survival analysis (KaplanMeier) were then entered into a Cox proportional hazards model using 2 different approaches: stepwise forward selection and backward deletion (20). In forward stepwise selection, the variable with the strongest association with mortality on univariate analysis is entered first, followed by the next strongest until all variables that are significant (at a prespecified level; in this case, P < 0.05 on log-rank test) are entered into the model. Variables entered in the model that become no longer significant are deleted sequentially. In backward deletion, all variables significant on univariate analysis are entered into the model and are sequentially deleted starting with the variable having the weakest association with mortality in the model until all of the variables left are significant at a prespecified level (P < 0.05). Variables related to the pathologic stage were not entered into multivariable models considering that this information is usually not available in clinical practice at the time of decision making and that a considerable portion of our population was composed of patients with advanced stage disease, who were not treated surgically and for whom this information was therefore missing.
Finally, we looked for some possible associations between bone marrow hypermetabolism (increased BM SUV and BM/L ratio) and variables that could presumably be causally related to it. For continuous variables, the Spearman correlation test was used (as BM SUV did not have a normal distribution). For the evaluation of discrete variables, patients were subdivided into 2 subgroupsthat is, normal BM versus bone marrow hypermetabolism, according to either their BM SUV or BM/L ratio, using the best discriminating cutoff value found in survival analysis. Variables were then tested for statistical independence using the Fisher exact test.
| RESULTS |
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Among the clinical and biochemical factors, sex (P = 0.003, log-rank test) and age (<60 y old; P = 0.02, log-rank test) were relatively good mortality predictors in our study. Hypocalcemia (P = 0.02), LDH (P = 0.002), hemoglobin (P = 0.000001), and albumin (P = 0.02, log-rank test) were all shown to be significant predictors of mortality, although, for hemoglobin, a cutoff value of 90 g/L obtained by recursive partitioning was used instead of the usual 110 g/L cutoff.
Survival analysis of some other parameters of the whole-blood count presumably related to bone marrow hypermetabolism revealed unexpectedly that thrombocytopenia (<160 x 1012/L) was the most powerful predictor of mortality of all factors studied (P = 0.0000002). To our knowledge, only 1 study reported thrombocytopenia as a significant predictor of mortality, in patients with advanced small cell lung cancer (21). Thrombocytosis (P = 0.01) and leukocytosis (P = 0.03) showed a weaker association with mortality. Detailed results of all prognostic factors that proved significant and that were included in the multivariate analysis are summarized in Table 2.
Multivariate survival analysis was realized using the 2 different approaches described earlier. In our set of patients, both methods happened to converge to the same model (Table 4), which included 6 variables: bone marrow hypermetabolism (expressed as BM SUV corrected for lean body mass), nodal stage on 18F-FDG PET (stage N0N1 vs. stage N2N3), anemia (hemoglobin, <90 g/L), thrombocytopenia (<150 x 1012/L), thrombocytosis (>340 x 109/L), and leukocytosis (WBCs) (>12.5 x 109/L). BM SUV turned out to be more strongly associated with mortality than the BM/L ratio on multivariate analysis. Surprisingly, according to this model, bone marrow hypermetabolism would be independent of many whole-blood count abnormalities that proved to be significant on univariate analysis. Unlike what was reported previously, the SUV of the primary lesion did not prove to be an independent prognostic factor in our study.
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| DISCUSSION |
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The metabolic activity of the primary lesion was again confirmed to be a predictor of mortality in this study. However, it was not independent of other predictors on multivariate analysis. In previous studies in which it was reported to be an independent prognostic factor, the only factors that remained significant in the Cox models along with SUVT were the stage of disease in 3 studies (11,14,15) and the stage of disease plus performance status in another study (12).
18F-FDG PET staging was also confirmed to be an accurate predictor of mortality. The predictive value of the nodal stage determined on 18F-FDG PET was comparable with the overall pathologic stage and appeared slightly better than the nodal stage determined surgically, but many patients in this series did not undergo extensive surgical sampling. In this study, all prognostic factors not directly related to 18F-FDG PET suffered from a selection bias. With the patient selection being based on a 18F-FDG PET scan database, the PET-related data were more likely to be complete than non-PETrelated variables. Some patients had their CT scans in remote clinics or hospitals and their results were incomplete or missing. This has certainly reduced the relative prognostic value of conventional staging. Also, because the clinicians based their investigation largely on PET scan results, conventional imaging staging was much less extensive than it would have been without 18F-FDG PET. Thus, the lower prognostic significance of conventional imaging compared with 18F-FDG PET could be somewhat artificial. In this patient population, not all lesions reported as suggestive of metastasis on conventional imaging were investigated, because other clinical parameters or test results, including PET, might have led investigators to conclude that the likelihood of metastasis was low. Because a conventional stage had to be assigned without the benefit of the PET results, such findings were considered to indicate metastatic disease on conventional staging, even when further investigation was unavailable. The heterogeneity of the subjects relative to the stage of disease might also have reduced the significance of some prognostic factors that are more specific to either localized disease or advanced disease.
Another limitation of this multivariate analysis is the absence of 2 of the most important clinical parameters in NSCLCweight loss and performance status, which were not systematically recorded in patients' records. Therefore, the results of this analysis must be interpreted with caution. In fact, they are probably appropriate for evaluating the relative value of 18F-FDG PET factors between themselves but they are certainly less accurate for evaluating their relative value compared with clinical factors. A prospective study in which all data are systematically collected could solve many of these problems.
The etiology of bone marrow hypermetabolism observed in some NSCLC patients is unclear. Among the 16 patients with elevated BM SUV, none had received granulocyte colony-stimulating factor (G-CSF). Only 1 patient had a previous history of hematologic disease (myeloproliferative syndrome) and another had a previous history of endometrial cancer. The remaining 12 patients had no previous history of cancer or hematologic disease. A bone marrow aspiration performed on 1 patient with diffuse, but not focal, marrow hypermetabolism showed reduced erythroblasts with otherwise normal cellularity and iron reserves, without evidence of metastatic or proliferative disease. Four of the patients with diffuse marrow hypermetabolism also had focal bone metastases, and another patient developed multiple bone metastases within 6 mo after diagnosis.
Among the possible explanations for bone marrow hypermetabolism are secretion of stimulating cytokines by the primary tumor and invasion of bone marrow by micrometastases. Cytokines such as colony-stimulating factors and interleukin-6 (IL-6) (22) as well as vascular endothelial growth factor (VEGF) (23) can be secreted by tumor cells. Both leukocytosis and thrombocytosis have been associated with NSCLC (2426) and even with poor prognosis (2732). Interestingly, a case of diffuse bone uptake of 201Tl attributed to G-CSF secretion by a large cell carcinoma was reported previously (33). The abnormal uptake disappeared after tumor resection.
With special immunocytochemistry techniques, bone marrow micrometastases and lymph node micrometastases can be detected in up to 31%60% of NSCLC patients without nodal involvement (3436), but this has not always been shown to be of prognostic significance (37). Such immunocytochemistry techniques were not applied to our patients with increased BM SUV.
Further studies are thus required to better identify the exact cause of increased BM SUV. Such studies should include serum levels of cytokines such as VEGF, G-CSF, granulocytemacrophage colony-stimulating factor, and IL-6 as well as immunocytochemistry cytokeratin staining of some bone marrow aspirates. Confirmation of any or both of these hypotheses would certainly provide an additional argument to support administration of neoadjuvant or adjuvant chemotherapy.
Bone marrow hypermetabolism is possibly also related to factors depending on the host rather than on the tumor itself: Other variables that were significantly associated with BM SUV include the PaO2 and the hemoglobin level. The hemoglobin and PaO2 levels represent relative hypoxemic states that can more-or-less directly stimulate hemopoiesis. Bone marrow hypermetabolism could also be influenced by coexisting illnesses indirectly or completely unrelated to cancer.
| CONCLUSION |
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Our Cox model revealed that bone marrow hypermetabolism and the presence of nodal metastases on 18F-FDG PET were prognostic factors independent of anemia, thrombocytopenia, thrombocytosis, and leukocytosis; these results remain to be confirmed in a prospective study that would include the systematic collection of well-recognized factors such as performance status and weight loss. The primary tumor SUV was not an independent prognostic factor from the parameters used in this model.
| References |
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