Positron-emission tomography for hepatocellular carcinoma:Current status and future prospects

Hepatocellular carcinoma(HCC)is one of the leading causes of cancer mortality worldwide.Various imaging modalities provide important information about HCC for its clinical management.Since positron-emission tomography(PET)or PET-computed tomography was introduced to the oncologic setting,it has played crucial roles in detecting,distinguishing,accurately staging,and evaluating local,residual,and recurrent HCC.PET imaging visualizes tissue metabolic information that is closely associated with treatment.Dynamic PET imaging and dual-tracer have emerged as complementary techniques that aid in various aspects of HCC diagnosis.The advent of new radiotracers and the development of immuno-PET and PET-magnetic resonance imaging have improved the ability to detect lesions and have made great progress in treatment surveillance.The current PET diagnostic capabilities for HCC and the supplementary techniques are reviewed herein.

Is there utility for fluorine-18-fluorodeoxyglucose positron-emission tomography scan before surgery in breast cancer? A 15-year overall survival analysis

BACKGROUND The prognostic value of preoperative fluorine-18-fluorodeoxyglucose positronemission tomography(18 F-FDG PET)scan for determining overall survival(OS)in breast cancer(BC)patients is controversial.AIM To evaluate the OS predictive value of preoperative PET positivity after 15 years.METHODS We performed a retrospective search of the Universitair Ziekenhuis Brussel patient database for nonmetastatic patients who underwent preoperative PET between 2002-2008.PET positivity was determined by anatomical region of interest(AROI)findings for breast and axillary,sternal,and distant sites.The prognostic role of PET was examined as a qualitative binary factor(positive vs negative status)and as a continuous variable[maximum standard uptake value(SUVmax)]in multivariate survival analyses using Cox proportional hazards models.Among the 104 identified patients who received PET,36 were further analyzed for the SUVmax in the AROI.RESULTS Poor OS within the 15-year study period was predicted by PET-positive status for axillary(P=0.033),sternal(P=0.033),and combined PET-axillary/sternal(P=0.008)nodes.Poor disease-free survival was associated with PET-positive axillary status(P=0.040)and combined axillary/sternal status(P=0.023).Cox models confirmed the long-term prognostic value of combined PETaxillary/sternal status[hazard ratio(HR):3.08,95%confidence interval:1.42-6.69].SUVmax of ipsilateral breast and axilla as continuous covariates were significant predictors of long-term OS with HRs of 1.25(P=0.048)and 1.54(P=0.029),corresponding to relative increase in the risk of death of 25%and 54%per SUVmax unit,respectively.In addition,the ratio of the ipsilateral axillary SUVmax over the contralateral axillary SUVmax was the most significant OS predictor(P=0.027),with 1.94 HR,indicating a two-fold relative increase of mortality risk.CONCLUSION Preoperative PET is valuable for prediction of long-term survival.Ipsilateral axillary SUVmax ratio over the uninvolved side represents a new prognostic finding that warrants further investigation.

Findings of ¹⁸F-Fluorodeoxyglucose Positron-Emission Tomography in Methotrexate-Related Lymphoproliferative Disorder

Introduction: The use of methotrexate (MTX) for rheumatoid arthritis (RA) is increasing. However, the immune suppression state leads to the occurrence of lymphoproliferative disorder (MTX-LPD). The purpose of this study was to describe the findings of 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) in MTX-LPD patients, and compare it with non-MTX-related malignant lymphoma (ML). Materials and Methods: We retrospectively reviewed 11 MTX-LPD patients (9 female, mean age 68.3 years) and 21 ML patients (7 female, mean age 60.6 years) with a histopathological diagnosis. FDG-PET imaging was performed using a standard oncology procedure. We assessed the disease distribution based on FDG-PET images and measured the maximum standardized up take values (SUVmax) for each region. Results: Mean values of SUVmax in MTX-LPD and ML were 14.6 and 17.2, respectively (p = 0.49). In MTX-LPD, 55 lesions met the Cotswold classification, consisting of 37 nodal and 18 extranodal lesions. In ML, 82 lesions were found, consisting of 68 nodal and 14 extranodal lesions. MTX-LPD showed a higher incident of the involvement in extranodal lesions throughout the whole body (p < 0.001). Conclusion: Because this disease occurs widely throughout the whole body, we need to pay attention to the less frequent sites as well when performing PET imaging in patients with MTX-LPD.

A controlled study of positron-emission-tomography and positron-emission-tomography/computed tomography in differential diagnosis of solitary pulmonary nodules—report of 60 cases

The differential diagnosis of solitary pulmonary nodules （SPNs） remains a challenge. It is acknowledged that combining positron-emission tomography （PET） and computed tomography （CT） offers the most reliable noninvasive method for the diagnosis of SPNs. Since Townsend et al1 developed integrated PET/CT in 1999, this technique has increasingly been introduced into clinical practice. To date, nuclear medicine physicians have usually undertaken PET/CT diagnosis, but the question is surfacing as how to make full use of the information of CT image to improve the accuracy of SPN diagnosis. To answer this question, we performed a retrospective study on 60 patients with SPNs.

^(18)F-氟脱氧葡萄糖PET-CT诊断头颈肿瘤

正电子发射型计算机断层-计算机断层(positron emission tomography-computer tomgraphy,PET-CT)扫描是将PET的功能代谢影像和CT的解剖影像融合在一起,能更准确的显示肿瘤组织,目前广泛应用于恶性肿瘤的诊断中。18F-氟脱氧葡萄糖(18fluorine-fluorinedeoxyglucose,18F-FDG)PET-CT在头颈肿瘤早期明确原发病灶、淋巴结转移、临床分期、制定放疗计划、疗效的监测等方面有一定的临床应用价值。

Cross-modality PET/CT and contrast-enhanced CT imaging for pancreatic cancer

AIM:To explore the diagnostic value of the crossmodality fusion images provided by positron emission tomography/computed tomography(PET/CT) and contrast-enhanced CT(CECT) for pancreatic cancer(PC).METHODS:Data from 70 patients with pancreatic lesions who underwent CECT and PET/CT examinations at our hospital from August 2010 to October 2012were analyzed.PET/CECT for the cross-modality image fusion was obtained using Ture D software.The diagnostic efficiencies of PET/CT,CECT and PET/CECT were calculated and compared with each other using aχ2 test.P<0.05 was considered to indicate statistical significance.RESULTS:Of the total 70 patients,50 had PC and 20had benign lesions.The differences in the sensitivity,negative predictive value(NPV),and accuracy between CECT and PET/CECT in detecting PC were statistically significant(P<0.05 for each).In 15 of the 31patients with PC who underwent a surgical operation,peripancreatic vessel invasion was verified.The differences in the sensitivity,positive predictive value,NPV,and accuracy of CECT vs PET/CT and PET/CECT vs PET/CT in diagnosing peripancreatic vessel invasionwere statistically significant(P<0.05 for each).In 19of the 31 patients with PC who underwent a surgical operation,regional lymph node metastasis was verified by postsurgical histology.There was no statistically significant difference among the three methods in detecting regional lymph node metastasis(P>0.05for each).In 17 of the 50 patients with PC confirmed by histology or clinical follow-up,distant metastasis was confirmed.The differences in the sensitivity and NPV between CECT and PET/CECT in detecting distant metastasis were statistically significant(P<0.05 for each).CONCLUSION:Cross-modality image fusion of PET/CT and CECT is a convenient and effective method that can be used to diagnose and stage PC,compensating for the defects of PET/CT and CECT when they are conducted individually.

Usefulness of positron emission tomography in primary intestinal follicular lymphoma

Double-balloon enteroscopy (DBE) and video capsule endoscopy are useful for the diagnosis of lymphoma in the small intestine. However, DBE cannot be safely performed in cases with passage disturbance due to wall thickening and stenosis. Additionally, video capsule endoscopy cannot be performed in such cases because of the risk of retention. Here, we report 4 cases of primary follicular lymphoma of the gastrointestinal tract that could be detected using 18F-fluoro-deoxyglucose positron emission tomography combined with computed tomography (PET-CT). The endoscopic findings of these 4 cases included lesions with wall thickening, which comprised macroscopically clusters of nodules, dense clusters of whitish granules or small nodules, fold thickening and ulcers with irregular margins that occupied the whole lumen with edematous mucosa. All patients fulfilled the World Health Organization grade 1 criteria. 18 F-fluorodeoxyglucose PET-CT can help predict the risks that may result from certain endoscopic examinations, such as DBE and video capsule endoscopy.

The Use of CT Perfusion to Determine Microvessel Density in Lung Cancer: Comparison with FDG-PET and Pathology

Objective： To investigate the validity of CT perfusion in assessing angiogenic activity of lung cancer. Methods： Fifty-six patients with lung cancer scheduled for elective surgical resection received 16-slice helical CT perfusion imaging. Time-density curve （TDC）, blood flow （BF）, blood volume （BV）, mean transmit time （MTT） and permeability surface area product （PS） were calculated. 18F-deoxyglucose-positron emission tomography （FGD-PET） was carried out in 14 out of the 56 patients to calculate standardized uptake values （SUVs）. Tumor microvessel density （MVD） was examined using CD34 immunohistochemical staining of the resected tumor tissue. Pearson’s correlation analysis was used to evaluate potential correlation between CT perfusion parameters and MVD or SUV. Results： Average time to peak height （TPH） of the TDCs （including two types of TDC） was 24.38±5.69 seconds. Average BF, BV, MTT and PS were 93.42±53.45 ml/100g/min,93.42±53.45 ml/100g,6.83±4.51 s and 31.92±18.73 ml/100g/min, respectively. Average MVD was 62.04±29.06/HPF. The mean SUV was 6.33±3.26. BF was positively correlated with MVD （r=0.620,P0.01） and SUV （r=0.891, P0.01）. PS was also positively correlated with SUV （r=0.720, P0.05）. A positive correlation was also observed between tumor MVD and SUV （r=0.915, P0.01）. Conclusions： CT perfusion imaging is a reliable tool to evaluate the tumor neovascularity of lung cancer.

^(68)Ga-PSMA PET/CT versus CT and bone scan for investigation of PSA failure post radical prostatectomy

Objective:To evaluate the use of Gallium-68 prostate-specific membrane antigen positron emission tomography/computed tomography(^(68)Ga-PSMA PET/CT),compared with conventional CT abdomen/pelvis(CTAP)and whole body single photon emission CT bone scan(BS),for detection of local or distant metastasis following biochemical failure/recurrence in post-prostatectomy patients.Methods:We conducted a review of our prospectively maintained,institutional database to identify 384 patients with post-prostatectomy biochemical failure/recurrence who underwent PSMA PET/CT,CTAP and BS from February 2015 to August 2017 in Nepean Hospital,tertiary referral centre.The results of the three imaging modalities were analysed for their ability to detect local recurrence and distant metastases.PSMA PET/CT and CTAP imaging were separately performed on the same day and the BS was performed within several days(mostly in 24 h).Difference in detection rates was determined between the modalities and the Chi square test was used to determine significance.Results:A total of 384 patients were identified with a median prostate-specific antigen(PSA)of 0.465 ng/mL(interquartile range =0.19-2.00 ng/mL).Overall,PSMA PET/CT was positive for 245(63.8%)patients whereas CTAP and BS were positive in 174 patients(45.3%).A total of 98 patients(25.5%)had local or distant metastasis detected on PSMA only,while 20 patients(5.2%)had recurrences detected on CTAP but not on PSMA PET/CT.Conclusion:The use of PSMA PET/CT has a higher detection rate of predicted local or distant metastasis compared to CTAP and BS in the staging of patients with biochemical recurrences after radical prostatectomy.

An unusual case of right-sided colon cancer with isolated lateral pelvic side wall lymph node metastases

Lymphatic drainage is typically in the direction of arterial supply,and lymphatic drainage for right-sided colon cancers typically travels to ileocolic lymph nodes.It is rare for right-sided colon cancers to metastasize to the lateral pelvic side wall lymph nodes in the absence of local invasion or other distant metastases.In this report,we present an unusual case of a young female with pT4a right-sided ascending colon cancer with isolated metastases to the pelvic iliac lymph nodes.The patient underwent minimally invasive laparoscopic right hemicolectomy and pelvic lymph node dissection with curative intent.She recovered well with no intraoperative complications and was referred for urgent adjuvant chemotherapy and radiotherapy.

Historical transition of management of sarcoidosis

Sarcoidosis is a systemic granulomatous disease of undetermined etiology, and it primarily affects the lungs and lymphatic system but may involve other organs. Recently, there have been several new insights in Japanese patients. The frequency of cardiac, ocular, and cutaneous sarcoidosis has increased in Japan, whereas, the proportion of patients with bilateral hilar lymphadenopathy decreased from 1960 to 2004.Propionibacterium acnes(P. acnes) has been studied extensively as one of the causative microorganism for granuloma formation, particularly in Japan. P. acnes-specific monoclonal antibodies are useful for diagnosing sarcoidosis. The potential association between smoking and sarcoidosis has been evaluated in a Japanese study, which found a higher prevalence of sarcoidosis among young smokers than that in previous reports. Recently, 18F-fluorodeoxyglucose positron-emission tomography, which permits visualization of activated inflammation, and endobronchial ultrasonography-guided transbronchial needle aspiration have been increasingly used to diagnose sarcoidosis. Cardiac sarcoidosis is found to be the main cause of death in Japan. The 2006 revised Japanese guidelines for diagnosing cardiac sarcoidosis are useful, particularly for subclinical cardiac sarcoidosis patients. Further studies and international communication and evaluation are needed to determine the causes of sarcoidosis, identify the risk factors for progressive disease, and develop new and effective treatments.

Primary hepatic neuroendocrine neoplasm diagnosed by somatostatin receptor scintigraphy:A case report

BACKGROUND Primary hepatic neuroendocrine neoplasm(NEN)is a rare condition,and it is difficult to differentiate between primary and metastatic hepatic NENs.Herein,we report a case of primary hepatic NEN that initially mimicked a hemangioma but showed a gradual increase in size on long-term careful observation.CASE SUMMARY A 47-year-old woman was incidentally diagnosed with a 12-mm liver mass,suspected to be a hemangioma.Since then,regular follow-up had been carried out.Ten years later,she was referred to our institute due to the tumor(located in segment 4)having increased to 20 mm.Several imaging studies depicted no apparent extrahepatic lesion.Positron emission tomography(PET)/computed tomography exhibited significant accumulation in the mass lesion,which made us consider the possibility of malignancy.Left hepatectomy was performed.The histopathological diagnosis was neuroendocrine tumor grade 2,with somatostatin receptor 2 a/5 positivity.Postoperative somatostatin receptor scintigraphy(SRS)showed no other site,leading to the diagnosis of NEN of primary hepatic origin.The gradual growth of the hepatic NEN over 10 years suggested that it was likely to be a primary liver tumor.CONCLUSION In this case,positivity on PET and postoperative SRS may have helped determine whether the tumor was primary or metastatic.

2-[18F]-fluoro-2-desoxy-D-glucose positron emission tomography initial staging impacts on survival in Hodgkin lymphoma

AIM:To assess the prognostic value and risk classification improvement of metabolic staging(MS)with Initial2-[18F]-fluoro-2-desoxy-D-glucose positron emission tomography(FDG-PET)in initial staging of Hodgkin’s Lymphoma(HL)patients to predict 5 years overall survival(5y-OS)and event free survival(EFS).METHODS:A total of 275 patients were included in this retrospective study,155 patients were staged with conventional anatomical staging(AS),and 120 also submitted to MS(FDG-PET).Prognostic analysis compared 5y-OS and 5y-EFS of patients staged with AS and MS.Risk-adjusted models incorporated clinical risk factors,computed tomography and FDG-PET staging.RESULTS:During the follow up of 267 evaluated patients,220(122 AS and 98 MS)achieved complete remission after first-line therapy(median follow-up:70±29 mo),treatment failure occurred in 79 patients and 34 died.The 5y-EFS for early vs advanced disease in AS patients was 79.3%and 66.7%,and 85.6%and53.6%in MS patients,respectively(P<0.01).The5y-OS for early and advanced disease with AS was91.3%and 81.5%,and 97.5%and 80.7%for patients staged with MS,respectively.Cox proportional hazards analysis demonstrated that FDG-PET added signifcant prognostic information and improved risk prediction(P=0.02).CONCLUSION:Initial staging FDG-PET could be used as an accurate and independent predictor of OS and EFS in HL,with impact in 5y-EFS and OS.

The possible value of ~18F-FDG positron emission tomography/computerized tomography imaging in detection of atherosclerotic plaque

Objective：To evaluate the clinical value with positron emission tomography/computerized tomography（PET/CT） imaging for the detection of vulnerable plaque in atherosclerotic lesions. Methods：Sixty people with a age of over 60[mean age （69.2 ± 7.1）years] underwent three dimension（3D） whole-body fluorine-18-2-fluoro-2-deoxy-D-glucose（^18F-FDG） PET/CT imaging and were evaluated retrospectively, including 6 cases assessed as normal and 54 cases with active atherosclerotic plaque. Fifty-four cases with SUVs and CT values in the aortic wall of high-FDG-uptake were measured retrospectively. These high-FDG-uptake cases in the aortic wall were divided into three groups according their CT value. Cases in group 1 had high uptake in atherosclerotic lesions of the aortic wall with CT value of less than 60 Hu（soft plaque）. Cases in group 2 had high uptake with CT value between 60-100 Hu （intermediate plaque）, Cases in group 3 had high uptake with CT value more than 100 Hu（calcified plaque）, Group 4 was normal. Results： In group 1, there were 42 high-FDG-uptake sites （average SUV 1.553 ± 0.486）. In group 2, there were 30 high-FDG-uptake sites（average SUV 1.393 ± 0.296）. In group 3, there were 36 high-FDG-uptake sites（average SUV 1.354 ± 0.189）. In group 4, there were 33 normal-FDG-uptake sites （average SUV was 1.102 ± 0.141）, The SUVs showed significant difference among the four groups（F = 678.909, P = 0.000）. There were also significant difference found between the normal-FDG-uptake group and the high-FDG-uptake groups（P = 0.000, 0.000, 0.001, respectively）. Conclusion：Different degrees of ^18F-FDG uptake in active large atherosclerotic plaque were shown in different stages of atherosclerotic plaque formation. The soft plaque had the highest FDG uptake in this study. This suggested that ^18F- FDG PET/CT imaging may be of great potential value in early diagnosis and monitoring of vulnerable soft plaque in atherosclerotic lesions.

Pre-Operative Perihepatic Lymph Node Assessment in Colorectal Cancer Liver Metastasis—A Review of Current Literature

Objective: Perihepatic lymph node involvement in colorectal cancer liver metastases is a negative prognostic factor. Resection of certain nodal stations around the liver has been shown to possibly improve survival. The aim of this review is to interrogate current literature on pre-operative investigations in diagnosing lymph node involvement. Method: A systematic review was conducted of articles published since 2006 to determine usefulness of pre-operative imaging in diagnosing lymph node involvement in colorectal cancer liver metastases. Results: Only 2 papers met the inclusion criteria for this study. Computed tomography (CT) scans were found to have sensitivities of 33% and 40%, specificities of 94% and 92%, positive predictive values (PPV) of 56% and 30%, and negative predictive values (NPV) of 85% and 95%. Positron emission tomography (PET) was studied in one of the paper and was found to have sensitivity, specificity, PPV and NPV of 57%, 100%, 100%, and 88% respectively. Conclusion: There is a significant lack of research on pre-operative investigations of perihepatic lymph node involvement in colorectal cancer liver metastases. Pre-operative CT and PET scans in assessing perihepatic lymph nodes were shown to be inaccurate. Newer pre-operative imaging modalities and research would be needed.

Role of the Reference Frame in Angular Photon Distribution at Electron-Positron Annihilation

The reasons of angular photon distribution occurrence at electron-positron annihilation are considered. It is shown that angular photon distribution is consequence of Doppler’s effect in the reference frame of the electron and positron mass center. In the reference frame bound with moving electron the angular photon distribution is absent. But it is replaced by the Doppler’s shift of photons frequencies. The received results are applied to the analysis of a positron-emission tomograph work.

肌骨影像学发展方向

1895年,德国物理学家伦琴(Roentgen)发现X线,当时照的第一张人体X线片就是他夫人的手。由于人体骨骼含钙多,密度高,与周围组织对比明显,因此非常适合X线成像。从常规X线摄片,到CT、MRI,再到PET-CT、PET-MRI以及分子影像学,100多年来,医学影像学已成为骨科必不可少的检查方法,在各种骨科疾病的诊断、治疗和随访中发挥着不可替代的作用。医学影像学科包括放射学和核医学。

^18氟-脱氧葡萄糖正电子发射断层扫描显像对鼻咽癌复发早期的诊断价值

核医学高端技术18氟-脱氧葡萄糖（18fluorine-fluorinedeoxyglucosepositron,^18FFDG）正电子发射断层摄影术（positron emission tomography/Computed Tomography,PET/CT）已逐渐应用于鼻咽癌复发的早期诊断,^18F-FDG PET结合CT显像及延迟显像在鼻咽癌复发的鉴别诊断中发挥独特优势,在发现远处转移灶方面也有较高敏感性.本文综合国内外研究对各种不同影像学检查在鼻咽癌复发早期诊断价值进行评价分析,并对18F-FDG PET/CT在早期诊断鼻咽癌复发及鼻咽癌放疗后随访等方面的应用价值做一综述.

^(18)F-FDG uptake as a biologic factor predicting outcome in patients with resected non-small-cell lung cancer

Background The outcome of surgical treatment of non-small-cell lung cancer （NSCLC） remains poor. In many patients the biological behavior of NSCLC does not follow a definite pattern, and can not be accurately predicted before treatment. ^18F-fluoro-2-deoxy-glucose （^18F-FDG） uptake on positron-emission tomography （PET） is associated with the aggressiveness of NSCLC. The present study focused on the role of ^18F-FDG uptake in predicting the outcome of surgically treated patients with NSCLC. Methods A retrospective analysis was made of 82 patients who underwent complete resection and preoperative FDG PET. The maximum standardized uptake value （SUVmax）, in addition to five clinicopathological factors and three biomolecular factors, which could possibly influence survival, was compared for possible association with patients＇ recurrence and survival, by the Log-rank test in univariate analysis and the Cox proportional hazards model in multivariate analysis. The association between SUVmax and other factors was also analyzed. Results Patients with SUVmax more than 11 had a disease-free survival and overall survival shorter than patients with SUVmax less than 11 in univariate analyses （P〈0.001, P=0.002）. In the multivariate analysis, SUVmax （dichotomized by 11） was the only significant predictor for tumor recurrence. TNM stage and SUVmax （dichotomized by 11） were independent predictors for the overall survival. Associations of SUVmax with p53 overexpression, proliferating cell nuclear antigen （PCNA） labeling index and microvascular density of the tumor were significant in the entire group. Conclusions ^18F-FDG uptake on PET may be used to noninvasively assess biological aggressiveness of NSCLC in vivo, identifying the surgically-treated patients with poor prognosis who could benefit from additional therapy.

PET/CT imaging of delayed radiation encephalopathy following radiotherapy for nasopharyngeal carcinoma

Background With the significant improvement in the survival of patients with nasopharyngeal carcinoma （NPC） undergoing radiotherapy and the growing availability of the sophisticated imaging modalities, the number of radiation encephalopathy （RE） cases relating to NPC radiotherapy is increasing. In this study, we investigated the metabolic and density changes of the compromised brain tissues during delayed RE using a positron-emission tomography-computed tomography （PET/CT） to provide clinical evidences for the diagnosis of delayed RE following radiotherapy for NPC. Methods The PET/CT manifestations and the clinical data of 53 pathologically confirmed NPC patients with delayed RE following radical radiotherapy and 15 healthy volunteers were investigated. The standardized uptake values （SUV） of the bilateral temporal lobes, the occipital lobe and the brain stem were measured respectively; and then the metabolic reduction rate of 88 temporal lobes and 13 brain stems were calculated for a statistical comparison between the two groups. Results The earliest case of delayed RE in the investigated patients occurred 1.5 years after radiotherapy. Delayed RE frequently involved the inferior temporal lobe. For patients with delayed RE confirmed by clinical symptoms and imaging findings, PET maintained a 100% coincidence rate with CT; however, in the 25 temporal lobes of the 35 delayed RE patients, PET revealed obvious hypometabolic changes whereas CT displayed normal density. The incidence of brain stem metabolic reductions was 24.5% （13/53） in the investigated patients, including 4 patients with hypometabolic changes shown by PET and negative finding shown by CT. The incidence of granuloma adjacent to the hypometabolic region in the temporal lobe was 12.5% （11/88）. Conclusion Delayed RE patients exhibit significant hypometabolic changes in the inferior temporal lobe, captured by PET much earlier than by CT. PET/CT offers a valuable means for the diagnosis of delayed RE in subacute stages and granuloma formation.