99mTc-DMSA (V) in Evaluation of Osteosarcoma: Comparative Studies with 18F-FDG PET/CT in Detection of Primary and Malignant Lesions

To evaluate the role of 99mTc-DMSA (V) and [18F]FDG PET-CT in management of patients with osteosarcoma, 22 patients were included in our study. All patients underwent both 99mTc-DMSA (V) and whole-body [18F]FDG PET-CT scans within an interval of 1 week. 555–740 MBq of 99mTc-DMSA (V) was injected i.v. the whole-body planar, SPECT images of primary site and chest were performed after 3-4 hours. [18F]FDG PET-CT images were obtained 60 minutes after i.v. injection of 370 MBq of F-18 FDG. Both FDG PET-CT (mean SUVmax = 7.1) and DMSA (V) scans showed abnormal uptake at primary site in all the 22 patients (100% sensitivity for both). Whole-body PET-CT detected metastasis in 11 pts (lung mets in 10 and lung + bone mets in 1 patient). Whole-body planar DMSA (V) and SPECT detected bone metastasis in one patient, lung mets in 7 patients and LN in 1 patient. HRCT of chest confirmed lung mets in 10 patients and inflammatory lesion in one patient. 7 patients positive for mets on DMSA (V) scan had higher uptake in lung lesions as compared to FDG uptake on PET-CT. Three patients who did not show any DMSA uptake had subcentimeter lung nodule. Resuts of both 99mTc-DMSA (V) (whole-body planar and SPECT imaging) and [18F]FDG PET-CT were comparable in evaluation of primary site lesions and metastatic lesions greater than 1 cm. Though 99mTc-DMSA (V) had higher uptake in the lesions as compared to [18F]FDG PET-CT, the only advantage [18F]FDG PET-CT had was that it could also detect subcentimeter lesions.


Introduction
OS is a primary malignant bone tumor characterized by the direct formation of immature bone or osteoid tissue by the tumor cells. It is thought to arise from primitive mesenchymal bone-forming cells. The classic OS is a rare (45% of all malignant bone tumors) highly malignant tumor [1,2], with an estimated incidence of 3 cases/million population/year. OS arises predominantly in the metaphysis of long bones, the most common sites of involvement are femur (42%, 75% of which are distal femur), tibia (19%, 80% of which are proximal tibia), and humerus (10%, 90% of which are proximal humerus). Other significant locations are the skull and jaw (8%) and pelvis (8%). The age at presentation ranges from 10 to 25 years of age.
The aim of our study was to evaluate the role of Pentavalent 99m Tc-Dimercaptosuccinic acid [ 99m Tc-DMSA (V)] and 18 F-Fluoro-2-Deoxy-Glucose 18 F-FDG PET-CT in the 2 ISRN Oncology

Materials and Methods
A total of 22 patients (14 males and 8 females) with biopsy-proven OS were included in this study. Age of the patients ranged from 8-66 yrs (mean age = 22.2 yrs). Preor postchemotherapy patients (both the types of patients) before surgery were included. Patients excluded were those who had undergone surgery before the nuclear medicine examinations and patients with diabetes mellitus or pathologic glucose tolerance. The patients with primary metastasis disease were not excluded.

Radiopharmaceuticals.
We used two types of imaging modalities with two different radiopharmaceuticals, 99m Tc-DMSA (V) for whole-body scintigraphy using gamma camera and 18 F-FDG for PET-CT imaging. 99m Tc is a gamma emitter and generator produced while 18 F is a positron emitter and cyclotron produced.

PET/CT Data Acquisition Protocol.
After fasting for at least 6 hrs, verifying the serum glucose level (that should be below 140 mg/dL) and with patients in a resting state, in a quiet room, a dose of 5-10 mCi of 18 F-FDG was injected intravenously depending on the age and weight of the patient. After a 45-60 minutes uptake period, patient was placed into the scanner. Initial CT acquisition was done without oral or intravenous contrast injection; followed by PET scan. On the PET/CT scanner, CT scan acquisition was performed on spiral dual slice CT with a slice thickness of 4 mm and a pitch of 1. Images were acquired using a matrix of 512 × 512 pixels and pixel size of about 1 mm. After completing the CT, the table is moved towards the field of view of PET. After transmissions scan, 3D PET acquisition was taken for 3-5 minutes per bed position for 5-7 bed positions depending on the patient length. PET data was acquired using a matrix of 128 × 128 pixels, and PET      acquisition of the same axial range, as was with CT, was taken with the patient in the same position on the table. Separate scans of the primary site were taken if it was not coming in the FOV of the whole-body PET-CT acquisition. CT-based attenuation correction of the emission images was done.

Processing Protocol
The CT images were acquired and reconstructed using optimized parameters for attenuation correction. Data obtained from the CT acquisition was used for low noise attenuation correction of PET emission data and for fusion of attenuation corrected PET images with the corresponding CT images. After completion of PET acquisition, images were reconstructed by iterative method ordered subset expectation maximization (2 iterations and 8 subsets) with a filter of 5 mm. The reconstructed attenuation-corrected PET images, CT images and fused images of matching pairs of PET and CT images-was available for review in axial, coronal, and sagittal planes and in maximum intensity projections, three dimensional cine mode. After image reconstruction, a region of interest (ROI) was carefully drawn around the site of the abnormal FDG uptake on lesion/s in the consequent 4-6 PET-CT scan slices. The slice with a maximal FDG uptake in the ROI was chosen for quantitative measurement of metabolic activity of the tracer (SUV). From these ROIs, the SUV was calculated according to the formula described below: whereas "MBq" is the mega-Becquerel and "g" is the grams.

99m
Tc-DMSA (V) Scanning. Patients were injected with 15-20 mCi of 99m Tc-DMSA (V), depending upon the age and weight of the patients. Dual head gamma camera was used and whole-body scans were taken 3-4 hours after the injection. SPECT images of the primary and the chest were also taken. Spot views were also taken if needed.

Acquisition Protocol.
Whole-body planar images were acquired using the continuous mode with a table speed of 20 cms/sec from head to toe. A matrix size of 256 × 1024 was used. The photopeak was kept at 140 keV with a 20% energy window. Scan lengths were taken according to the length of the patients.
SPECT Images of the Primary Site and Chest Region were also Taken. SPECT images were taken with the following protocol.

Data Interpretation.
Two experienced nuclear medicine physicians evaluated the scan findings independently and they were blinded to the structural imaging findings and clinical findings. Images were looked for area of increased radiotracer uptake and corresponding areas in the CT images, and fused PET-CT images were corroborated.

Statistical Analysis.
Multiple regression analysis was performed to find the significance of correlation (r > 0.5 and P < 0.05 were considered significant). The intensity and the extent of uptake in 18 F-FDG-PET-CT versus 99m Tc-DMSA (V) at the primary site were analysed quantitatively and qualitatively. Accompanying lung lesions absent versus present and their uptake imperceptible versus perceptible were compared visually in both the modalities.

Results
In this prospective study a total of 22 patients were included. All the patients were already biopsy-proven cases of OS. All of them underwent thoracic CT within the two weeks before or after 18 F-FDG-PET/CT and 99m Tc-DMSA (V) scan. Those patients who were taken after chemotherapy had undergone therapy at least two weeks before the 18 F-FDG-PET/CT and 99m Tc-DMSA (V) scan. Characteristics of all patients are given in Table 1 with graphical representation in Figure 1.
Age. In 22 patients considered for this study, range of age was 8-66 years, with a mean age of 22.2 years. Maximum number of patients falls within the range of 10-20 years, with 10 patients (45.5%) falling in this interval. Out of these patients  Table 3. Since they were already histopathologically proven, so the sensitivity of 18 F-FDG-PET-CT was 100% in detecting the primary site. No correlation was found between the size of the tumor and the SUV max (r = 0.1209) (Figure 2(a)).

Metastases.
Analyzing the results of whole-body 18 F-FDG-PET-CT scans each patient underwent, foci with increased pulmonary/pleural tracer uptake was found in 11/22 patients. By correlating clinical and other imaging findings, 10/11 (90%) of the examinations were classified as true positive, one as false positive. Out of 11 patients, 8 had bilateral lung nodule with variable degree of FDG uptake (mean SUV max = 1.1). One patient (patient 9) had bilateral lung nodule with no FDG uptake in one nodule. The other remaining two patients had solitary lung nodules (patients 1, 21). Out of these 2, "1" had mild [ 18 F]FDG uptake while the other found to be inflammatory lesion on HRCT (patient 21). So 18 F-FDG-PET-CT was found to have a sensitivity of 100% in detecting the lung mets and specificity of 91% in comparison to the other clinical and imaging modalities findings. Although PET alone was not able to detect many lesions but with the incorporation of CT, sensitivity of PET increased tremendously. With regard to other metastatic locations, abnormal uptake was also detected in one patient (patient 10) at the site of left scapula (coracoid process). No other abnormal uptake was detected at any other site.  Table 3. So 99m Tc-DMSA (V) showed 100% sensitivity in detecting the primary site like 18 F-FDG-PET-CT findings. t/nts of 99m Tc-DMSA (V) showed no statistically significant correlation with spect to the size of the primary tumor. T/NT max (r = 0.0311) (Figure 2(b)) and T/NT avg (r = 0.2490) (Figure 2(c)) were nowhere correlated to the size of the tumor. increased blood pool activity was evident in all the scans, which decreased with the delayed imaging. showed significant response to the chemotherapy. also the overall number of lung lesions detected with 99m Tc-DMSA (V) was less in all the 7 patients.

Other Sites.
Abnormal uptake was also detected in patient 10 in the left scapula region, as was seen in the 18 F-FDG-PET-CT images 99m Tc-DMSA (V) could also show the amount of necrosis and soft tissue involvement at primary site. Patient 9 also showed uptake in the right axillary lymph node. In case of children, there was an increased uptake at the growth plates because of increased growth at these points (patients 3, 5, 7, 11, 14, 16, 17, and 20).

SUV max of 18 F-FDG-PET-CT and T/NTs of 99m
Tc-DMSA (V). No statistically significant correlation was found between SUV max of 18 F-FDG-PET-CT and the T/NTs ratios of 99m Tc-DMSA (V) of primary site with SUV max and T/NT max (r = 0.2711, P = 0.2223), and SUV max and T/NT avg (r = 0.30, P = 0.18) (Figures 3(a) and 3(b)).

Discussion
OS is a rare form of bone cancers, which mainly affects the people in the age group of 10-25 years [3]. The 5-year relative survival for children with bone cancer has improved ISRN Oncology from 49% to 63% in the recent time with the development of improved diagnostic modalities, which helps in the early detection of disease. Early detection of the disease has good prognosis. It usually metastasizes to the lungs and bones which is associated with poor prognosis. So it is essential to know about the metastatic involvement of the disease. In the present study we tried to assess the role of 99m Tc-DMSA (V) in detecting the primary as well as the metastatic involvement of the OS by comparing it with the 18 F-FDG-PET-CT. Role of 18 F-FDG-PET in characterizing the tumor as malignant and benign has already been studied extensively and it was found to be a promising tool to study the nature of tumor noninvasively [4][5][6][7]. 99m Tc-DMSA (V), which is a tumor targeting radiopharmaceutical, has also found application in detection of medullary thyroid carcinoma, amyloidosis, head and neck cancers, and soft tissue tumors. It was found to have higher sensitivity in detecting the bone metastasis and soft tissue tumors than 99m Tc-MDP [8,9]. No study has been done till now to compare the role of 99m Tc-DMSA (V) and 18 F-FDG-PET-CT in imaging the primary tumor of OS. However, one study which compared the 99m Tc-DMSA (V), 99m Tc-MDP, and CT found the scans of 99m Tc-DMSA (V) superior to the 99m Tc-MDP scans and the CT scans in identifying the metastases of OS [10].
In our study we found that both the 18 F-FDG-PET-CT and 99m Tc-DMSA (V) showed intense uptake at the primary site in all the patients (Figures 4, 5, and 7). 99m Tc-DMSA (V) showed equal sensitivity to 18 F-FDG-PET in detecting primary tumor sites. The extent of localization and the intensity of uptake were comparable in both the cases.
Although no significant statistical correlation was found between the SUV max and T/NT ratios of 18 F-FDG and 99m Tc-DMSA (V), respectively. But the detection efficiency of the two modalities was same. We found in our study that there is no significant statistical correlation between the SUV max or the T/NT ratios and the size of the primary tumors. This could be due to the association of primary tumors of OS with extensive tumor necrosis and calcification which causes the nonuniform uptake of radiopharmaceutical.
18 F-FDG-PET has also shown a higher sensitivity (98%) in detecting lung mets, equivalent to CT. A positive 18 F-FDG-PET result can be used to confirm any abnormalities seen on thoracic CT scans as metastatic [11]. In our study 18 F-FDG-PET-CT showed 100% sensitivity in detecting the lung mets. In case of 99m Tc-DMSA (V) only 7 patients were found to have lung mets. The decreased sensitivity of 99m Tc-DMSA (V) could be attributed to the limited resolution of gamma camera (0.9-1.3 cm) as most of the nodules, which were not seen, were subcentimetre in size.
However, PET alone showed abnormal lung uptake in 8 patients only out of 10 proved patients with lung mets. This could be due to the partial-volume averaging effect (which is a reduction in detected activity that occurs when the object size is smaller than the axial or transaxial spacing) [12]. The signal of small lesions could be "diluted" during reconstruction and malignant lesions can result in false negative findings on PET images. Effect of respiratory motion is also well known to cause artifacts on PET images. Although with the introduction of PET-CT, its sensitivity has improved tremendously with the anatomic localization of various structures especially in case of evaluation of childhood sarcomas [13]. Similarly detection efficiency of 99m Tc-DMSA (V) can also be enhanced with the incorporation of SPECT-CT. In most of the cases, uptake in the normal bone was not detectably greater than the surrounding soft tissue. In the entire patients, both the excretion and retention in kidneys were apparent, with excreted radioactivity exclusively in the form of 99m Tc-DMSA (V) [9], but the degree of retention was variable. No area of increased uptake was noted in the thyroid gland. Few females showed increased breast uptake. Increased uptake in the liver was also seen in few patients. Although no correlation was found between the extent of disease and the renal and hepatic uptake, no correlation could also be established between the extent of metastasis and renal and liver uptake.
18 F-FDG-PET has a disadvantage of showing a high number of false positive results because of the increased uptake in many inflammatory and benign nodules (as we had one patient which had false positive uptake in the inflammatory fibronodular lesion on 18 F-FDG scan). Tumor metabolism and, consequently tumor detection with 18 F-FDG-PET, may be highly susceptible to chemotherapy. While 99m Tc-DMSA (V) is a tumor seeking agent and is not metabolism dependent the extent of tumor uptake can differentiate malignant and chondrogenic tumors. 99m Tc-DMSA (V) has an advantage of detecting bone as well as soft tissue tumor [9,14]. Although several instances of falsepositive uptake have occurred in both the soft-tissue as well as skeletal disease but the lesions latter showed relatively low uptake on bone scans. However, no false positive result was obtained in our study with 99m Tc-DMSA (V). We could also detect abnormal uptake in bone mets at the scapula by both the imaging modalities.
Apart from application in imaging tumors, interest in the biodistribution of 99m Tc-DMSA (V) is increasing these days. Since the β-emitting analogues 186/188ReDMSA (V) offer the potential for targeted radiotherapy, the avidity with which the tracer is taken up in most bone metastasis suggests that this could be applied to palliative treatment [15].
Being a technetium labeled compound, 99m Tc-DMSA (V) is more economical and more accessible in comparison to the 18 F-FDG-PET-CT. There is no report of any false positive uptake anywhere else in our study, which further increases its reliability. So the use of 99m Tc-DMSA (V) can be thought over for scanning the patients with OS in detecting the primary as well as the metastatic involvement at least in those areas where 18 F-FDG-PET-CT is not available.

Conclusion
Pentavalent 99m TcDimercaptosuccinic acid appears to be a promising radiopharmaceutical for detecting primary and metastases in patients with osteosarcoma. 18 F-FDG PET-CT also plays an important role in detecting primary and metastases in these patients. When 99m Tc-DMSA (V) is compared with 18F-FDG-PET-CT, it has lower sensitivity in detecting smaller metastatic lesions due to lower spatial resolution of gamma camera as compared to PET-CT. However, it requires costly setup including cyclotron, 18F-FDG synthesizing cell and a PET/CT camera, thereby the overall cost of an 18 F-FDG-PET/CT scan is also more, which may be as much as 15 to 20 times the cost of a 99m Tc-DMSA (V) scan. Although 99m Tc-DMSA (V) was not as sensitive as 18 F-FDG-PET-CT was, in detection of subcentimetre nodules, but it seems to be an good alternative to 18 F-FDG-PET/CT in those areas where PET/CT is not available. Also, the sensitivity of 99m Tc-DMSA (V) scan may be improved for detection of sub centimeter nodules by using the SPECT-CT, which further improves the anatomical location, keeping the cost of scan still significantly less compared to an 18 F-FDG-PET/CT scan. 99m Tc-DMSA (V) also has another advantage over 18 F-FDG-PET/CT. The average whole-body dose delivered to the patient in a 99m Tc-DMSA (V) is found to be approximately 3mSv [16], while in a 18 F-FDG-PET/CT scan, the average dose delivered has been found to be 11-12 mSv. Thus a 99m Tc-DMSA (V) scan delivers 4 times less radiation dose to the patient as compared to a 18 F-FDG-PET/CT scan.