¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography and ¹⁸F-fluorodeoxyglucose positron emission tomography/magnetic resonance imaging in lung cancer metastasis after surgery

INTRODUCTION

Lung cancer, now the malignant tumor with the highest incidence and mortality rates, poses a significant threat to public health.^[1] Radical surgery is crucial for early-stage non-small cell lung cancer (NSCLC) patients’ survival rates, yet post-operative recurrence and distant metastasis occur in about 20.3% and 26.6% of stage I cases and up to 45.5% and 47.7% of stage II-III cases, leading to poorer survival and patient outcomes.^[2] Positron emission tomography (PET)/computed tomography (CT) can effectively achieve precise localization and functional imaging of lesions. The National Comprehensive Cancer Network (NCCN) guidelines selected PET/CT as the preferred and important diagnostic and evaluative tool for detecting distant metastasis in lung cancer.^[3] The emergence of PET/magnetic resonance imaging (MRI) provides multi-parameter imaging, excellent soft tissue visualization, and no radiation exposure, which may challenge the current dominance of PET/CT in the diagnosis of lung cancer. In some studies, the use of PET/MRI did not show a significant advantage over PET/CT in detecting early lung cancer.^[4,5] However, MRI has demonstrated outstanding performance in detecting pleural and mediastinal involvement in advanced lung cancer and shows high sensitivity to brain, liver, and bone metastases, indicating that in these cases, PET/MRI may outperform PET/CT, identifying patients who are under- or over-estimated by PET/CT.^[6,7]

From a clinical perspective, it is crucial to select the appropriate PET imaging method to provide optimal clinical recommendations for patients with advanced lung cancer, such as information regarding the patient’s clinical staging, treatment recommendations, and prognosis. This should be done before conducting PET/MRI after the PET/CT examination. This case report highlights a patient with post-operative recurrent lung adenocarcinoma and distant metastasis, emphasizing that PET/MRI can provide more comprehensive lesion information in lung cancer examinations, with greater utility in assessing treatment efficacy and prognosis compared to PET/CT.

CASE REPORT

A 68-year-old female patient with stage Ib adenocarcinoma of the left upper lobe of the lung underwent surgery in another hospital in June 2021 without chemoradiotherapy. On February 24, 2022, a CT scan performed at our hospital revealed multiple solid nodules in both lungs and the pleura. Regular reexaminations, including a biochemical examination on November 4, showed significantly increased levels of carcinoembryonic antigen (139 ug/L), carbohydrate antigen 125 (50.3 KU/L), and ferritin (607 ug/L) [Table 1], suggesting a high suspicion of post-operative lung adenocarcinoma recurrence. CT examination was performed on the same day, and the imaging showed that the patient had soft tissue thickening at the surgical stump of the left upper lobe of the lung with left pleural thickening. At the same time, multiple solid nodules were observed in both lungs and subpleura, with the largest measuring approximately 1.7 cm by 0.6 cm in diameter. Compared with the images of the upper lobe of the left lung [Figure 1a] and the lower lobe of the left lung [Figure 1b] on February 24th, new metastatic lesions were shown in this area on November 4th [Figures 1c and 1d], and no lymph node enlargement was observed in the mediastinum or hilum. Combined with the patient’s previous history of lung adenocarcinoma surgery, and the existing CT imaging and biochemical indicators, there was a high suspicion that the patient had post-operative lung adenocarcinoma recurrence with metastasis. Subsequent biochemical tests on November 22 revealed a further increase in carcinoembryonic antigen levels to 172 ug/L [Table 1], suggesting that the patient’s cancer lesions may be progressing and metastasizing.

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Figure 1:

A 68-year-old woman with recurrence and multiple metastases in various parts of the body after resection of adenocarcinoma in the left upper lobe was examined. The computed tomography (CT) images of the patient on February 24, 2022 and November 4, 2022 were compared. (a) The CT image on February 24 showed no metastasis in the upper lobe of the left lung. (b) The CT image on February 24 showed no metastasis in the lower lobe of the left lung. (c) The CT scan on November 4th revealed a metastatic lesion in the upper left lobe (indicated by the white arrow). (d) The CT imaging on November 4th showed another metastatic lesion in the lower left lobe (indicated by the white arrow).

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To further clarify the patient’s clinical stage, we performed ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET/CT and ¹⁸F-FDG PET/MRI on December 19 [Figure 2]. The maximum intensity projections of PET/CT and PET/MRI [Figures 2a and 2b] showed that the left upper lobe bronchus was truncated, and there was no definite abnormal density or increased radiation absorption at the surgical stump. However, in the left lung, there were multiple nodules of different sizes with increased radioactive uptake, with a maximum diameter of 1.4 × 0.7 cm in the transverse axis and an SUVmax of 1.1. In addition, multiple nodules in the left mediastinal pleura and left subpleural of the left rib were accompanied by a slight increase in radioactive uptake, and the extent of the lesions was difficult to accurately measure because the lesions were close to the pleura, with an SUVmax of 0.8, and multiple bone metastases were found in the C4 spinous process, C7 vertebral body, and right iliac body, and bone destruction with increased radioactive uptake, and the SUVmax was 5.2. In contrast, PET/CT missed some pleural lesions [Figure 2c] and lymph node metastases [Figure 2d], while PET/MRI images showed more pleural lesions [Figure 2e] and lymph node lesions [Figure 2f] with clearer range and edges. Furthermore, two small bilateral iliac body metastases that were missed in the PET/CT examination [Figure 3a] were clearly discovered and displayed by PET/MRI [Figure 3b], and radioactive uptake showed a mild increase. Although ¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI had the same stage for patients, PET/MRI showed better performance for distant metastasis of lung cancer and provided rich imaging information.

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Figure 2:

A 68-year-old woman with recurrent left upper lobe adenocarcinoma and systemic mult-site metastases.(a) Maximal intensity projection of ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography imaging (¹⁸F-FDG PET/CT); (b) Maximal intensity projection of ¹⁸F-fluorodeoxyglucose positron emission tomography/magnetic resonance imaging (¹⁸F-FDG PET/MRI) showed the characteristics of the tumor; (c) ¹⁸F-FDG PET/CT failed to detect pleural metastases; (d) ¹⁸F-FDG PET/CT failed to detect lymph node metastases; (e) ¹⁸F-FDG PET/MRI clearly shows pleural metastasis (white arrow). (f) Lymph node metastasis with increased FDG uptake is demonstrated on ¹⁸F-FDG PET/MRI (white arrow).

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Figure 3:

A 68-year-old female patient with recurrent adenocarcinoma of the upper lobe of the left lung and multiple distant bone metastases throughout the body; (a) ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography imaging (¹⁸F-FDG PET/CT) failed to detect the iliac bone metastasis; (b)¹⁸F-fluorodeoxyglucose positron emission tomography/magnetic resonance imaging (¹⁸F-FDG PET/MRI) identified the iliac bone metastasis(white arrow).

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DISCUSSION

Lung cancer is a common malignant tumor, with 80% of cases being NSCLC, and it has the highest mortality rate worldwide.^{[8] 18}F-FDG PET/CT has played a huge role in the diagnosis, staging, monitoring, and follow-up of lung cancer and has been widely used in clinical practice. At present, the emerging PET/MRI has significant advantages in the detection of various diseases due to its multi-parameter imaging, high soft tissue resolution, and low radiation dose, and it may replace the dominant position of PET/CT. However, the diagnostic efficacy of MRI for lung diseases is often limited by the large amount of air in lung parenchyma (low hydrogen proton density), respiratory motion, and other factors, resulting in poor imaging performance. Interestingly, some studies have shown that PET/MRI has comparable performance to PET/CT in the T and N staging of NSCLC, and its diagnostic ability does not seem to be affected by the limitations of traditional MRI lung imaging.^[4,5]

There is limited research directly comparing the performance of ¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI in detecting distant metastasis of lung cancer. While some studies have suggested that PET/MRI may outperform PET/CT in monitoring distant metastases,^[6] these studies typically focus on specific metastasis sites. In our case, we compared all potential metastasis sites and found that PET/MRI not only detected all the metastases visible on PET/CT but also identified additional pleural and bone metastases with clearer imaging and higher lesion uptake.

Immune checkpoint inhibitors (ICB), representing targeted therapy and immunotherapy, have significantly improved the prognosis of patients with advanced unresectable NSCLC. The programmed death-ligand 1 (PD-L1) tumor proportion score assessed by immunohistochemistry is currently the most commonly used biomarker. However, due to the primary limitations of PD-L1 quantification being a lack of sensitivity and specificity, it is not considered the definitive biomarker.^[9] Dall’Olio et al.^[10] found that in patients with PD-L1 ≥1% and high total metabolic tumor volume (tMTV), the overall survival of the chemotherapy combined with ICB was longer compared to the use of ICB alone, and the levels of tMTV were significantly associated with patient prognosis. Although PET/CT and PET/MRI were consistent in the clinical staging (IVb stage) of the patient, PET/MRI detected more abundant lesion information (more pleural and bone metastatic lesions) through its extremely high soft tissue resolution and multi-parameter imaging advantages and could obtain a higher tMTV quantitative value, thereby precisely evaluating the clinical efficacy and prognosis of the patient and providing a basis for choosing the best treatment plan to prolong survival. In addition, low radiation dose is one of the important clinical advantages of PET/MRI.

The prognosis and treatment options for patients with metastatic lung cancer vary depending on the organs involved and the number of metastases.^[10] In this case, the choice of ¹⁸F-FDG PET/MRI can provide more comprehensive lesion information (such as tMTV), which is crucial for guiding treatment, assessing prognosis, and improving the quality of life for patients.

CONCLUSION

¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI are important diagnostic tools for assessing distant metastasis in NSCLC; however, PET/MRI can provide more accurate lesion information and parameters, which is valuable for the true assessment of patient prognosis and treatment selection, making it the preferred choice for patients with advanced lung cancer metastasis and recurrence.