A head-to-head comparison of 2-[18F]FDG PET/CT and 2-[18F]FDG PET/MR in patients with nasopharyngeal carcinoma under different disease settings

doi:10.21203/rs.3.rs-4093928/v1

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Research Article

A head-to-head comparison of 2-[18F]FDG PET/CT and 2-[18F]FDG PET/MR in patients with nasopharyngeal carcinoma under different disease settings

https://doi.org/10.21203/rs.3.rs-4093928/v1

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Objectives

MRI is indispensable for staging of nasopharyngeal carcinoma (NPC) as it offers superior soft-tissue contrast. PET/CT and MRI are complementary in accurate staging of NPC. The combination of MRI and functional imaging from PET in PET/MR is promising in NPC management. We compared the diagnostic performance of PET/CT and PET/MR in 46 patients with NPC under different disease scenarios, including primary nonmetastatic cases, primary metastatic cases, recurrence and/or metastasis after treatment, and post-treatment follow-up cases.

Methods

Forty-six patients (37 males and 9 females) underwent both PET/CT and PET/MR within one day (median age: 54.5 years). Primary tumor extension into risk-stratified anatomic structures, retropharyngeal and cervical lymph node metastasis, distant metastasis and post-treatment follow-up results, as well as maximum of standardized uptake value (SUVmax) were evaluated and compared. Bland-Altman analysis was conducted to assess reproducibility of SUVmax between the two modalities. P < 0.05 was considered statistically significant.

Results

For high-risk structures, PET/MR detected two more sides of tensor/levator veli palatine muscle involvement, one more case of clivus involvement, and ruled out 12 false-positive sides of prevertebral muscle involvement by PET/CT. For medium-risk structures, PET/MR detected four more sides of medial pterygoid muscle involvement. For low-risk structures, abnormal signal on massa lateralis atlantis was detected by PET/MR. PET/MR detected 14 more positive retropharyngeal lymph nodes and more liver micrometastases than PET/CT. Overall, PET/MR changed two patients’ T staging. Furthermore, SUVmax showed high reproducibility between PET/CT and PET/MR (P < 0.001).

Conclusions

PET/MR outperforms PET/CT in delineating muscle, skull-base bone, and nodal involvement, and identifying liver micrometastases, may serve as a single-step staging modality for NPC.

Nasopharyngeal carcinoma

PET/CT

PET/MR

diagnostic performance

head-to-head comparison

Nasopharyngeal carcinoma (NPC) is the most common type of head and neck cancer in Southeast Asia, especially in South China(1). Radiotherapy is the mainstay treatment for nonmetastatic NPC, as it is high sensitive to radiation(1). However, due to the complex anatomic structures surrounding the nasopharynx, delineating the target of NPC can be challenging. Liang et al. investigated the local extension patterns of NPC and classified the anatomic sites around the nasopharynx into three risk groups based on the incidence rates of tumor invasion: high risk (≥ 35%, including parapharyngeal space, levator/tensor veli palatine muscle, clivus, basis of sphenoid bone, nasal cavity, pterygoid process, medial pterygoid plate, foramen lacerum, etc.), medium risk (≥ 5–35%, including medial pterygoid muscle, sphenoidal sinus, pterygopalatine fossa, foramen ovale, the great wing of sphenoid bone, etc.), and low risk (< 5%, including cervical vertebrae, temporal lobe, orbital apex, maxillary sinus, meninges, etc.)(2). Accurate identification of tumor extension into these risk-stratified structures and staging are crucial for treatment selection, particularly in terms of delineation and subsequent radiotherapy planning.

National Comprehensive Cancer Network (NCCN) guidelines recommend contrast-enhanced magnetic resonance (ceMRI) imaging of the head and neck along with 18-fluorine fluorodeoxyglucose (2-[18F]FDG) positron emission tomography and computed tomography (PET/CT) imaging for diagnosis and staging of NPC. While PET/CT is particularly helpful in identifying nodal and distant metastasis(3), its sensitivity in detecting primary tumor extension is limited due to the inadequate soft tissue resolution of CT(4, 5). Simultaneous positron emission tomography and magnetic resonance (PET/MR) imaging, offers multiparametric MRI and PET images that provide improved soft tissue contrast and additional functional information, which may allow for better staging of NPC.

Several studies reported the use of PET/MR in the diagnosis and staging of NPC. A prospective study involving 113 patients with newly diagnosed NPC found that PET/MR was more accurate than the combination of head and neck MRI and PET/CT(6). Cao et al. defined the locoregional extension pattern of NPC by PET/MR in 331 nonmetastatic NPC patients, demonstrating that the spread of the primary tumor and regional lymph node follows an orderly pattern, the skip metastasis of the lymph node was uncommon(7). Cheng et al. performed PET/CT and PET/MR on 35 patients with NPC and showed that PET/MR provided better image quality, lesion conspicuity, and diagnostic confidence for NPC than PET/CT(8). Another study based on 60 NPC cases reported that the overall accuracy of PET/MR for the staging of recurrent or metastatic NPC was 88.3%, indicating that it has the potential to be a single-step modality for staging(9).

However, comparative studies of PET/CT and PET/MR based on the reported NPC invasion map and under different disease scenarios are limited. In this study, a head-to-head comparison was conducted to determine if simultaneous PET/MR is superior to PET/CT for evaluation of primary tumor extension, nodal involvement, distant metastasis, and post-treatment status in NPC patients in a high-incidence area.

Patients

From May 2018 to March 2022, 46 patients with NPC were included in the study. Inclusion criteria were: i) biopsy-proven NPC, ii) no contraindications to MRI, iii) serum glucose level < 10 mmol/L before PET/CT scan, and iv) the ability to provide written informed consent. Patients with a history of previous head and neck malignancies, or concomitant cancers in different anatomical locations were excluded. All patients were staged according to the 8th edition of the American Joint Committee on Cancer (AJCC)/Union for International Cancer Control (UICC) staging system.

The participants signed an informed consent form after completing routine PET/CT and then undergoing PET/MR. The medical records and imaging studies were reviewed retrospectively and additional informed consent was waived. The study protocol was approved by the Institutional Review Board of XX University XX Hospital (IRB no. B2023-269).

2-[18F]FDG PET/CT and 2-[18F]FDG PET/MR

All patients were required to fast for at least 6 hours to maintain serum glucose levels below 10 mmol/L before the injection of 2-[18F]FDG (approximately 3.6 MBq/kg). PET/CT imaging was acquired approximately 60 min after the injection using mostly uMI550/780/uEXPLORER scanner (United Imaging Healthcare) (Supplementary Table 1). Four cases were scanned using the Discovery VCT 64 scanner (GE Healthcare) (Supplementary Table 2). The parameters used for the CT scan were: Current 120–270 mAs; Voltage 120–140 kV; Pitch 0.516–0.625. Various CT windows (soft-tissue, lung, liver, bone window, etc) were used to optimize visualization of specific tissues or abnormalities. PET images were acquired for 2–3 min per bed position in 3D mode. CT scans were reconstructed using a matrix of 512×512. PET scans were reconstructed with a matrix of 128×128 using the ordered subset expectation maximization (OSEM) algorithm and CT-based attenuation correction.

After the PET/CT acquisition, a whole-body PET/MR scan was performed using the uPMR790 HD TOF PET/MR containing a 3.0-Tesla MR imager and an integrated PET detector (United Imaging Healthcare). The median delay for PET/MR was 1.3 hours. Further details regarding PET/MR scan have been described in previous studies(10, 11). Briefly, PET images and MRI sequences, including T1 weighted images (T1WI), T2 weighted images (T2WI) with and without fat saturation (T2-fs), and diffusion-weighted images (DWI), were acquired at axial, sagittal, and coronal plane positions. The PET images were reconstructed using a standard process provided by the vendor, which included TOF-OSEM with 20 subsets and 3 iterations, an image matrix of 256×256, a voxel size of 2.4×2.4×2.85mm³, and a 3 mm post-Gaussian filter.

Image Interpretation and Diagnostic Criteria

The image interpretation was performed independently by two nuclear medicine physicians with a background in medical imaging including MRI for 8 years and 3 years as well as extensive experience in the field of nuclear medicine for over 20 and 10 years respectively. They were blind to the clinical information and diagnostics. Any discordant cases were discussed to reach a consensus.

T staging assessment

The reference standard for T staging of NPC included nasopharyngoscopy to detect mucosal extension and ceMRI to identify submucosal extension(6). Following the NCCN guidelines(12), ceMRI was considered the gold standard for evaluating primary tumor extension. In this cohort, six patients underwent head-and-neck ceMRI concurrently, and the consistency of diagnosis among PET/CT, PET/MR, and ceMRI was compared. Detailed diagnostic criteria for anatomic structure invasion surrounding the nasopharynx were reported in a previous study(7).

For MRI, the primary tumor was assessed primarily on the axial ceMRI and T2WI that showed clear tumor borders. In the case of pathological diagnosis, any submucosal abnormalities observed on MRI were considered to be the result of tumor infiltration since current clinical practice is to encompass all abnormal areas in the irradiation field, thereby ensuring the inclusion of all regions with potential microscopic disease(13).

On PET/CT images, lesions with 2-[18F]FDG uptake that exceeded those of surrounding tissues were identified as having tumor infiltration. The maximum of standardized uptake value (SUVmax) of the region of interest (ROI) was measured on PET image, and corresponding CT image helped identify the morphology and localization of the lesions.

N staging assessment

The 2013 updated consensus guideline of the neck node levels was used to determine cervical lymph node (CLN) levels(14). On MR or CT image, metastatic CLN was defined based on morphological criteria, which included: i) the presence of necrosis or extracapsular spread, ii) the shortest axial diameter was ≥ 5mm in the retropharyngeal region or ≥ 10mm in other regions of the neck, iii) a cluster of two or more lymph nodes of borderline size(15). On PET images, metastatic CLN was defined as: i) 2-[18F]FDG uptake exceeded the surrounding normal tissue, ii) asymmetric metabolic activity greater than that of normal lymph nodes at the same level in the contralateral neck(16).

M staging assessment

The presence of distant metastasis was determined based on the 2-[18F]FDG uptake and the lesion’s morphology displayed on MRI or CT, as mentioned above.

Measurement of metabolic and MRI parameters

SUV was calculated as activity in a volume of interest (VOI) [Bq/mL]/(injected dose [Bq]/body weight [g]). SUVmax was obtained from the single highest voxel within the VOI. SUVmean of background was determined by drawing 1.0 cm-sized VOI in the right temporalis muscle and was finally obtained by averaging the three measurements. SUVmax of the lesions was divided by the SUVmean of the temporal muscle. Separating RLN from the primary tumor is challenging when using PET/CT imaging. As a result, the analysis involved combining the SUVmax of both the RLN and primary tumor (named SUVmax_T + RLN), which were then compared between the two modalities. For those with positive CLNs, the SUVmax were calculated as the sum of bilateral cervical nodes. For PET/MR, an ADC sequence was generated using two b-values (b-0, b-1000s/mm2).

Statistical Analysis

Statistical analysis was conducted using SPSS Statistics 26.0. Data were tested for normality. Mean ± SD and median (range) were used to present normal-distributed variables and skewed variables, respectively. Reproducibility of each measure was assessed from the difference between the two scans, and coefficients of variation (CVs) were calculated to assess variability. Bland-Altman plots were generated to visualize the limits of agreement obtained from the Bland-Altman analysis. A two-tailed P-value < 0.05 was considered statistically significant.

Clinical characteristics

The patient characteristics are shown in Table 1. A total of 37 male and 9 female patients with pathologically confirmed NPC, aged from 28 to 74 years old (median, 54.5 years), were enrolled. Among them, 25 had newly diagnosed nonmetastatic NPC, while four were recurrent NPC. Two and eight patients developed distant metastasis at the initial diagnosis and after curative treatment, respectively. Two patients developed simultaneous primary tumor recurrence and distant metastasis. The remaining five patients had sequela or negative results after curative treatment. The median blood glucose level was 5.6 mmol/L. The radiotracer average dose was 3.6 ± 0.8 MBq/kg. The median delay of PET/MR was 1.3 hours (range: 0.3-3 hours).

Table 1

Clinical information and patient characteristics (n = 46)
Characteristics	Patients (%)
Disease state
Primary nonmetastatic (untreated)	25 (54.4%)
Primary metastatic (untreated)	2 (4.3%)
Recurrence after treatment	4 (8.7%)
Metastasis after treatment	8 (17.4)
Simultaneous recurrence and metastasis after treatment	2 (4.3%)
Follow-up (developed sequela or normal)	5 (10.9%)
Sex
Male	37 (80.4%)
Female	9 (19.6%)
Age, Median (range), years	54.5 (28–74)
Height, (Mean ± SD), cm	167.7 ± 6.6
Body weight, (Mean ± SD), kg	65.7 ± 10.6
Dose, Mbq/kg (Mean ± SD)	3.6 ± 0.8
Blood glucose, Median (range), mmol/L	5.6 (4.4–9.9)
PET/MR delay, Median (range), hours	1.3 (0.3, 3.0)
Abbreviations: SD = standard deviation

Comparison of PET/CT and PET/MR in evaluating primary tumor invasion (T staging) into risk-stratified anatomic structures

The 36 cases analyzed including 25 patients with primary nonmetastatic NPC, four with recurrent NPC, two with primary metastatic NPC, three with metastasis after treatment, and two with simultaneous recurrence and metastasis after treatment. As shown in Table 2, PET/MR was superior to PET/CT in identifying tensor/levator veli palatine muscle involvement (Fig. 1a) and clivus invasion (Fig. 2a) in the high-risk structures. PET/MR ruled out false-positive results in 12 sides of prevertebral muscle on PET/CT (Fig. 1c). Additionally, PET/MR detected three more sides of medial pterygoid muscle involvement (Fig. 1b) in the medium-risk structures, and one left massa lateralis atlantis involvement (Fig. 2b, Supplementary Fig. 1) in the low-risk structures.

Table 2

Incidence of tumor invasion into risk-stratified anatomic sites surrounding the nasopharynx for analyzable cases*
Anatomic sites	No. of cases or sides		Anatomic sites	No. of cases or sides		Anatomic sites	No. of cases or sides
Anatomic sites	PET/CT	PET/MR	Anatomic sites	PET/CT	PET/MR	Anatomic sites	PET/CT	PET/MR
High risk			Medium risk			Low risk
Tensor/Levator veli palatine muscle^#	21	23	Medial pterygoid muscle^#	11	15	Cervical vertebrae^※	0	1
Prevertebral muscle^#	40	28	Sphenoidal sinus	4	4	Temporal lobe^#	1	1
Clivus	14	15	Pterygopalatine fossa^#	3	3	Orbital apex^#	1	1
Basis of sphenoid bone	14	14	Cavernous sinus^#	4	4	Maxillary sinus^#	1	1
Nasal cavity	4	4	Foramen ovale^#	5	5	Pituitary fossa	1	1
Pterygoid process^#	7	7	Hypoglossal canal^#	1	1	Meninges	0	0
Medial pterygoid plate^#	12	12	Lateral pterygoid muscle^#	1	1	Superior orbital fissure	0	0
Foramen lacerum^#	8	8	Foramen rotundum^#	1	1	Inferior orbital fissure	0	0
Vomer bone	7	7	Ethmoid sinus	1	1	Infratemporal fossa	0	0
Petrous apex^#	3	3	Great wing of sphenoid bone	0	0	Cerebral cistern	0	0
Occipital bone	3	3	Jugular foramen	0	0	Hypopharynx	0	0
Lateral pterygoid plate^#	1	1	Oropharynx	0	0	Frontal sinus	0	0
* 36 patients who have primary nasopharyngeal lesions: primary nonmetastatic (n = 26), primary metastatic (n = 2), recurrence after treatment (n = 4), metastasis after treatment (n = 2), Simultaneous recurrence and metastasis after treatment (n = 2). Sides and cases were used for symmetrical structures and nonsymmetrical structures, respectively.
^# For symmetrical structures, bilateral structural involvement were calculated as two.
^※ left massa lateralis atlantis.

Comparison of PET/CT and PET/MR in the evaluation of regional lymph node involvement (N staging)

PET/MR identified 49 positive retropharyngeal lymph nodes (RLNs) in 30 patients, while PET/CT identified 35 positive RLNs in 25 patients (Table 3, Fig. 3a). PET/MR and PET/CT were identical in identifying the involved regions of CLN, with a total of 111 regions detected. Level IIb (40, 36.0%) was the most frequently involved level, followed by level IIa (36, 32.4%), level III (16, 13.8%), level Va (9, 8.1%), level IV (7, 6.3%) and level Ib (3, 2.7%). No nodal metastasis was observed in level Ia and Vb, and skip metastasis was not found (Table 3). Additionally, PET/MR clearly outlined the borders of positive CLNs (Fig. 3b).

Table 3

Incidence of nodal spread for analyzable cases*
	PET/CT	PET/MR
RLN
Total number	35	49
Left side	15	21
Right side	20	28
CLN
Total regions	111	111
L-Ia	0	0
R-Ia	0	0
L-Ib	0	0
R-Ib	3	3
L-IIa	15	15
R-IIa	21	21
L-IIb	18	18
R-IIb	22	22
L-III	6	6
R-III	10	10
L-IV	3	3
R-IV	4	4
L-Va	4	4
R-Va	5	5
L-Vb	0	0
R-Vb	0	0
*patients who have RLN and/or CLN metastasis

Comparison of PET/CT and PET/MR in the evaluation of distant metastasis (M staging)

Two patients had primary metastatic NPC (de novo metastasis) and 10 developed distant metastases after curative treatment were analyzed. Specifically, in four cases, PET/MR detected the same number of relatively certain lung and bone metastases as PET/CT (Fig. 4b-c, Supplementary Table 3), while PET/MR fails to display small (≤ 5mm) and ground-grass lung nodules. PET/MR identified more liver micrometastases compared to PET/CT in three cases. These liver micrometastases were either indistinct or undetected on PET/CT (one scanned by GE, one by uMI-550, the other one by uMI-780) (Fig. 4a, Supplementary Fig. 2 and Table 2).

Changes in the overall staging

One patient was down-staged to Stage I by PET/MR due to a misjudgment of prevertebral muscle involvement on PET/CT. Another patient with recurrent NPC was up-staged to Stage II due to the detection of medial pterygoid muscle involvement on PET/MR.

Quantitative analysis

Totally, 36 and 28 paired cases were analyzed for SUVmax_T + RLN and SUVmax_CLN, respectively. SUVmax_T + RLN and SUVmax_CLN were both significantly higher on PET/MR than those on PET/CT (both P < 0.001). A Bland-Altman plot was used to illustrate the disparity between SUV measurements. For SUVmax_T + RLN and SUVmax_CLN, mean differences are 4.6 (95% confidence interval, 0.3 and 9.0) and 23.8 (95% confidence interval, -7.3 and 54.9), respectively. All the scatter ordinates in the Bland-Altman plot are within the 95% confidence interval for the difference, indicating a satisfactory quantitative agreement between the two modalities (Fig. 5, Table 4).

Table 4

Bland-Altman plots results of SUVmax_T + RLN and SUVmax_CLN between PET/CT and PET/MR.
	SUVmax_T + RLN	SUVmax_CLN
Sample size	36	28
Mean difference	4.6	23.8
95% Confidence interval of mean difference	0.3, 9.0	-7.3, 54.9
P-value	< 0.001	< 0.001
Abbreviations: T = primary tumor; RLN = retropharyngeal lymph node; CLN = cervical lymph node.

Comparison of PET/CT and PET/MR in other disease settings

For patients who had received radical treatment and scanned for follow-up, PET/MR clearly display the range of temporal lobe necrosis and the internal morphological characteristics in one case (Supplementary Fig. 3).

Diagnosis consistency among PET/CT, PET/MR, and ceMRI

Six patients underwent head-and-neck ceMRI concurrently. The consistency rate was determined by dividing the number of same structures detected (bilateral structure was counted as two) by the total number of structures detected by ceMRI. PET/MR achieved 100% consistency with ceMRI in five patients, while only three patients by PET/CT. For RLN, four patients in PET/MR and two patients in PET/CT were identical to ceMRI. PET/MR further detected a positive RLN in one case. For CLN evaluation, three cases were excluded due to the insufficient lower limit of the neck scan. The involved CLN regions revealed by PET/MR were equivalent to ceMRI, whereas PET/CT was inferior to PET/MR and ceMRI in two cases (Supplementary Table 4).

This study showed that integrated PET/MR had better diagnostic performance than PET/CT for assessing local tumor invasion and RLN metastasis. PET/MR was identical to PET/CT in determining the involved CLN regions. Additionally, PET/MR detected more liver micrometastases than PET/CT in the limited cases. Furthermore, PET/MR provided a better anatomic reference that facilitated outlining the border of lesions, which was crucial for target delineation of NPC for radiotherapy.

Typically, the weakness of PET/CT lies in the inferior soft tissue resolution compared to MRI(17–19). MRI has the advantage of assessing parapharyngeal spaces, intracranial invasion, and retropharyngeal and supraclavicular lymph nodes in NPC(20). In this cohort, the superiority of integrated PET/MR lies in recognizing muscle involvement and skull base invasion. Clear soft tissue resolution of PET/MR is apparent. Controversy exists in the identification of bony involvement. Karsten et al. conducted a prospective study involving 67 patients with solid tumors who underwent both PET/CT and PET/MR scans; Similarly, PET/MR offers superior lesion conspicuity compared to PET/CT when evaluating bone metastases(21). According to NCCN guideline, MRI is generally preferred over CT to evaluate tumors that encroach on the skull base. CT, conversely, is a complementary to MRI for evaluation of bony erosion or cartilage invasion(3). We observed bony involvement in two patients with untreated primary NPC, thus ruling out osteoradionecrosis. As discussed in Lofgren’s article(22), the pathology of bone metastasis was hard to be obtained due to the impracticality of obtaining multiple biopsy specimens from one patient, particularly for skull base bone involvement. It can be argued that not all abnormalities detected by MRI can be solely attributed to tumor infiltration. Nevertheless, the current standard practice is to be cautious and include all areas of abnormality within the radiation field in order to eradicate microscopic disease(13). Nasopharyngeal cancers are generally not resected; therefore, primary tumor extention is mainly evaluated through nasopharyngeal biopsy and ceMRI. A study indicated that the sensitivity of PET/MR (99.5%) was higher than that of head and neck MRI (94.2%) and was even more accurate than the combination of MRI and PET/CT in the staging of NPC(6).

NPC has a tendency to metastasize to cervical lymph node, and RLN is considered as the sentinel node (1). In our cohort, PET/MR detected more suspicious metastatic RLNs than PET/CT. The discrepancy was attributed to the merging of some RLNs with primary tumors in PET/CT. Conversely, MRI, especially T2-fs, accurately delineates the margins of lymph nodes, and is sensitive in detecting necrotic or cystic lymph nodes. The rates of lymph node metastasis from the upper neck to the lower neck decreased orderly, and skip metastasis was not found, which was consistent with the findings from previous studies(2, 7, 23). PET/MR and PET/CT were identical in determining the involved cervical levels. However, PET/MR still provided clearer outlines of lesions, which may limit or accurately match the irradiation field, thus reducing radiotherapy-induced sequelae.

It should be noted that patients in our cohort did not maintain ultrasound-guided fine needle puncture (FNA) for CLN. According to the Chinese Society of Clinical Oncology (CSCO) clinical guidelines, the recommend level of FNA for regional lymph node is III (low-level)(24). NCCN guideline recommends a biopsy of the primary site or FNA of the neck (alternatively) for the clinical workup of NPC. Lymph node metastasis was mainly determined non-invasively based on imaging modalities(3). PET/CT can assess T, N, and M stages of NPC simultaneously and false-negative micrometastatic neck nodes could be treated with prophylactic whole-neck irradiation(5).

For overall staging, one patient was down-staged to Stage I and one was up-staged to Stage II on PET/MR. This may lead to management changes, patients with stage I NPC typically receive radiotherapy alone, while stage II NPC may require additional concurrent chemotherapy alone with radiotherapy (1). Furthermore, the crucial aspect is the impact on the external beam radiation therapy gross tumor volume delineation. By accurately defining tumor margin with PET/MR, the radiation field can be tailored to reduce radiation volume.

Despite recent advancement in treatment modalities, locoregional recurrence still occurs in 10–20% of cases in early-stage disease and up to 30% of cases in locally advanced disease after definitive treatment(25, 26). In this cohort, only six patients with recurrent NPC were analyzed, and PET/MR detected left medial pterygoid muscle involvement in one patient and ruled out a false-positive prevertebral muscle involvement in another. Accurate detection and early diagnosis of recurrent and/or residual tumor may improve patient’s prognosis since localized disease is potentially salvageable(26). PET/CT is still challenging for early detection of recurrent NPC due to treatment-induced tissue alterations and inflammatory conditions that can mimic disease activation(27). Queiroz et al. compared cePET/MR with cePET/CT in 87 patients with suspected recurrence of head and neck cancer and suggested that cePET/MR may be better at specifying possible tumor recurrence with obscure FDG uptake than cePET/CT(28). PET/MR has been recommended as an alternative modality to PET/CT in post-chemoradiotherapy evaluation (29).

Apart from tumor recurrence, distant metastasis is the primary cause of death in NPC. Zhou et al. reported that the diagnostic performance of PET/MR is superior to that of PET/CT in detecting liver metastasis(30). Similarly, PET/MR resulted in more liver lesions detected in our study. The improved diagnostic accuracy may have an impact on treatment decisions. A typical example is resectable colorectal cancer with liver metastasis, where the number of liver lesions determines the possibility of surgical resection and the need for neoadjuvant therapy(30). However, one patient with liver metastasis was scanned by GE Discovery VCT 64 that is showing its age, and may not offer the same capabilities for PET imaging. The finding that PET/MR detects more liver metastases is partially due to a generation of improvement in device technology. Therefore, the results still need further confirmation.

In addition to morphologically identifying lesions, theoretically, the metabolic information of PET can quantitatively reflect the heterogeneity and activity of the tumor. Our results demonstrated that the SUVmax of the primary tumor and regional lymph nodes were significantly higher for PET/MR than for PET/CT. Similarly, in a study that included 121 patients with 241 lung lesions, PET/MR was performed after PET/CT (113.9 ± 28.5 min post-injection), and SUVmax and SUVmean were shown to be significantly higher for PET/MR than for PET/CT (P < 0.001 each)(31). In another study, PET/MR and PET/CT acquisitions were started at 85 ± 20 min and 146.2 ± 20 min after injection of 2-[18F]FDG, respectively. SUVmax and SUVmean measured on PET/MR were significantly lower than those on PET/CT for lesions (P < 0.05)(32). However, the Bland-Altman plot suggests a satisfactory quantitative agreement between SUVs from the two modalities, indicating consistent diagnostic trends.

In conclusion, PET/MR provides valuable information on primary tumor extension, nodal involvement, distant metastasis, and post-treatment status, which may serve as a single-step staging modality for NPC. Further studies are needed to determine which patients would benefit most from PET/MR, and to investigate the survival benefit of PET/MR-guided dose painting and involved neck level radiotherapy for NPC patients.

Limitations

Firstly, due to the single injection and double examination pattern, the improved lesion detection ability of PET/MR may partly attribute to the delayed PET acquisition. Secondly, the study cohort was relatively small, especially under certain disease settings such as recurrent or metastatic cases. Last but not the least, pathological confirmation of cervical lymph nodes and metastatic lesions were not performed.

Conflicts of Interest

None declared.

Acknowledgments

This study was supported by grants from the National Key Research and Development Program of China (2022YFC2406902), the Shanghai Municipal Key Clinical Specialty Project (SHSLCZDZK03401), Shanghai Science and Technology Project (19DZ1930700), the Shanghai Science and Technology Committee Program (20DZ2201800), the Three-year Action Plan of Clinical Skills and Innovation of Shanghai Hospital Development Center (SHDC2020CR3079B).

Data Availability Statement

The raw data utilized in this study cannot be made publicly available due to ethical reasons. The authors may be contacted at [email protected] or the Department of Nuclear Medicine, Zhongshan Hospital, Fudan University in Shanghai.

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A head-to-head comparison of 2-[18F]FDG PET/CT and 2-[18F]FDG PET/MR in patients with nasopharyngeal carcinoma under different disease settings

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Abstract

Objectives

Methods

Results

Conclusions

Figures

Introduction

Materials and methods

Patients

2-[18F]FDG PET/CT and 2-[18F]FDG PET/MR

Image Interpretation and Diagnostic Criteria

T staging assessment

N staging assessment

M staging assessment

Measurement of metabolic and MRI parameters

Statistical Analysis

Results

Clinical characteristics

Comparison of PET/CT and PET/MR in the evaluation of regional lymph node involvement (N staging)

Comparison of PET/CT and PET/MR in the evaluation of distant metastasis (M staging)

Changes in the overall staging

Quantitative analysis

Comparison of PET/CT and PET/MR in other disease settings

Diagnosis consistency among PET/CT, PET/MR, and ceMRI

Discussion

Limitations

Declarations

Conflicts of Interest

Acknowledgments

Data Availability Statement

References

Supplementary Files

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