肺癌是世界上发病率和病死率最高的恶性肿瘤之一[1],其中非小细胞肺癌(non-small cell lung cancer,NSCLC)占肺癌患者总数的85%[2]。据美国癌症学会统计数据显示,NSCLC患者5年生存率仅为18%[3]。准确的预后评估,是选择个体化治疗方案的先决条件,也是提高患者生存率及生活质量的必备条件。目前,对于NSCLC的影像学诊断及评估主要基于传统显像模式,包括CT及MRI。传统显像模式一方面主要依赖于病灶形态学变化,无法提供代谢相关信息;另一方面,显像部位局限,无法同时提供全身其他转移病灶信息。而PET/CT可同时显示分子水平的全身代谢信息及肿瘤的形态特征,特别是18F-FDG PET/CT,在NSCLC的分期、治疗规划及疗效监测中均有重要价值[4],通过18F-FDG PET/CT半定量参数测定,包括SUV、肿瘤代谢体积(metabolic tumor volume,MTV)及病灶糖酵解总量(total lesion glycolysis,TLG),可为NSCLC提供良好的预后评估[5-7]。笔者就近年来上述代谢半定量参数在NSCLC的预后评估中的价值作一综述。
1 18F-FDG PET/CT显像基础18F-FDG是临床上最常用的放射性核素显像剂,其与葡萄糖的结构相似,可经细胞膜上的葡萄糖转运蛋白非选择性地进入细胞,胞内的18F-FDG在己糖激酶作用下磷酸化生成6-磷酸-18F-FDG(18F-FDG-PO4)。18F-FDG-PO4不能进一步被磷酸果糖激酶识别,可在细胞内滞留,其滞留量与细胞葡萄糖消耗量基本一致,因而能够反映体内葡萄糖的利用和摄取水平。
肿瘤组织中普遍存在细胞快速增生、细胞膜葡萄糖载体增多(主要为葡萄糖转运蛋白1和葡萄糖转运蛋白3)和细胞内磷酸化酶活性增高等生物学特征,使得肿瘤细胞内的糖酵解代谢明显增加[8],18F-FDG在肿瘤细胞内的摄取及浓聚程度与肿瘤细胞代谢活跃程度呈正相关;此外,也有研究证明18F-FDG的摄取与肿瘤侵袭性及细胞增殖活跃程度呈正相关[9-10]。因此,18F-FDG PET/CT在提供解剖学显像的同时,还可提供分子水平的功能显像,在肿瘤的诊疗过程中占据重要地位。
2 18F-FDG PET/CT代谢参数及其影响因素18F-FDG PET/CT代谢半定量参数主要包括SUV、MTV和TLG。其中,SUV为半定量指标,是18F-FDG PET/CT显像中最常用的代谢活性参数,其主要包括SUVmax和平均标准化摄取值(mean standardized uptake value, SUVmean);MTV和TLG是反映肿瘤负荷的代谢参数。关于18F-FDG PET/CT代谢半定量参数及其影响因素的具体情况详见表 1。
SUV是18F-FDG PET/CT诊断肿瘤病变时最常用的半定量指标,其中SUVmax的测量方法具有简易性、精确性和可重复性的特点,结果相对可靠。大量研究结果表明,对于早期诊断并接受手术治疗的NSCLC患者,原发肿瘤SUVmax对于患者的预后评估效果良好[16-19]。Yoo等[20]研究发现,未出现纵隔淋巴结转移的80例Ⅰ~Ⅱ期NSCLC患者在接受手术治疗后,原发肿瘤病灶高SUVmax组(SUVmax>4,P=0.004)更易出现肿瘤复发,作者提出对于早期NSCLC患者且SUVmax>4时可于术后行辅助性放化疗。
对于失去手术机会的晚期NSCLC患者,病灶SUVmax的预测效果仍不明确。有研究者报道,出现远处转移的NSCLC患者接受化疗前的血清胸苷激酶1水平与原发肿瘤SUVmax水平呈正相关,并且两者均是患者总体生存期(overall survival, OS)的独立危险因素[21]。另一研究者则提出,对初始治疗方案为放疗或放化疗的患者,通过单因素分析并未发现原发肿瘤高SUVmax与患者低生存率间存在显著联系(P=0.22)[22]。
近年来也有基于SUVmax相关参数评估NSCLC患者预后的研究。Jin等[23]利用双时相PET/CT计算115例患者的肿瘤△SUVmax(后时相SUVmax-前时相SUVmax)和△SUVmax变化率[(后时相SUVmax-前时相SUVmax)/前时相SUVmax],发现上述两参数均与晚期NSCLC患者的无进展生存期(progression-free survival, PFS)密切相关,提出肿瘤△SUVmax和△SUVmax变化率为晚期NSCLC患者疾病进展的独立危险因素。在另一项用SUVmax相关参数评估NSCLC患者预后的研究中,Billè等[24]收集并分析了413例接受手术治疗的患者(Ⅰ期222例,Ⅱ期95例,ⅢA期79例,ⅢB期及Ⅳ期8例),统计分析发现原发肿瘤与肝SUVmax的比值以及原发肿瘤与主动脉弓血池SUVmax的比值均与术后患者的PFS无显著相关性。
3.2 MTVMTV是一个体积代谢参数,反映肿瘤病灶中具有一定代谢活性的肿瘤体积。有研究者通过随访529例早期NSCLC并接受手术的患者(ⅠA期176例,ⅠB期197例,ⅡA期107例及ⅡB期49例),结果发现高MTV是患者术后复发及OS的独立危险因素[25]。
对于失去手术切除机会的晚期NSCLC患者,研究结果提示MTV较SUVmax对患者的预后评估更加准确[26-27]。Ohri等[6]提出对于接受放疗或放化疗的晚期NSCLC患者,MTV是患者预后OS的独立危险因素(MTV每增加10 cm3:HR = 1.04;95% CI = 1.03~1.06; P < 0.001),同时也发现高MTV可增加局部病灶衰竭的概率(MTV每增加10 cm3:HR = 1.16;95% CI = 1.08~1.23;P < 0.001)。该研究团队在另一项研究中也证实MTV与肿瘤侵袭性密切相关[28]。有研究者报道,对晚期NSCLC患者行治疗前后PET/CT对比显像,结果发现治疗前后的△MTV变化率[(治疗前MTV-治疗后MTV)/治疗前MTV]与患者预后OS存在显著相关性[29-30];Huang等[31]提出当△MTV>29.7%时,接受放化疗的晚期NSCLC患者将拥有更长的OS。
MTV作为同时结合肿瘤形态及功能代谢的复合参数,能够更加准确地反映肿瘤的代谢负荷,较SUVmax可为晚期NSCLC患者的预后评估提供更多的参考。
3.3 TLGTLG也是近年来PET/CT代谢参数评估预后的研究热点之一,是反映肿瘤病灶摄取程度和代谢体积的复合指标。针对可行手术切除的早期NSCLC患者,有研究者报道与SUVmax和MTV相比,TLG在术后肿瘤复发的预测及患者的OS评估上都更为准确[32-33]。
由于TLG可反映患者整体代谢负荷,故对于晚期NSCLC患者的预后评估具有优势。Moon等[34]对52例接受一线化疗的晚期NSCLC患者随访2年,结果发现△TLG变化率[(治疗前TLG-治疗后TLG)/治疗前TLG]对于患者的OS及PFS预后评估有重要参考价值,提出△TLG变化率=50%是区分高低危人群的最佳分界值。Soussan等[35]和Usmanij等[36]发现△TLG变化率的等级是评估晚期NSCLC患者PFS的重要分层因素。
在仅服用吉非替尼进行靶向治疗的患者中,TLG同样显示出良好的预后评估价值。Keam等[37]发现,虽然在低、中、高TLG组(高TLG组:> 455.1,中TLG组:102.8~455.1,低TLG组:< 102.8)中吉非替尼治疗的有效率差异无统计学意义(68.0%、76.0%和68.0%;P=0.274),但高TLG组患者的PFS较短(高TLG组:7.2个月;中TLG组:11.9个月;低TLG组:24.2个月; P<0.001),OS也较短(P=0.005)。
4 小结及展望18F-FDG PET/CT代谢半定量参数在NSCLC的预后评估中占据重要地位。SUV、MTV及TLG在早期NSCLC患者预后评估中的价值已得到肯定;MTV及TLG可反映全身代谢及所累及的肿瘤体积,可作为晚期NSCLC患者预后评估中的补充,为准确评估后治疗方案的选择提供参考依据;SUVmax及其他相关参数在晚期NSCLC患者的预后评估中的效果仍有待进一步的临床研究进行总结探讨。
利用18F-FDG PET/CT半定量参数评估预后尚存在几点不足:(1)MTV分界值尚无统一标准,需进一步大样本量临床试验探讨;(2)炎症病变及肿瘤性病变均可以表现为18F-FDG高摄取,从而影响MTV及TLG的测量;(3)少数类型的肿瘤,如肺泡细胞癌等,并不表现为18F-FDG高摄取,可能会导致预后评估的不准确。
核医学分子显像作为分子影像组学的重要组成部分,其代表性显像模式PET已在临床上得到广泛应用。目前,除临床应用最广泛的18F-FDG外,其他正电子显像剂如11C-蛋氨酸(11C-MET)、11C-乙酸(11C-Acetate)、11C-胆碱(11C-Choline)、18F-脱氧胸苷(18F-FLT)等也逐渐进入临床。将不同种类的正电子显像剂组合应用,可从不同角度反映肿瘤细胞的分子水平生物化学变化,从而进一步提高肿瘤诊断的灵敏度和特异度[38]。
随着核医学技术的发展及各种新型肿瘤分子探针的不断出现,PET在肿瘤诊疗过程中的应用将会得到越来越多地拓展及推广,对肿瘤的生物学特征及其在体行为也将有更加精准的描述和评估,PET/CT显像将在肿瘤的精准诊治方面发挥更大的指导作用。
利益冲突 本研究由署名作者按以下贡献声明独立开展,不涉及任何利益冲突。
作者贡献声明 陈挺负责资料整理、综述初稿撰写;张宏负责综述修改与审阅;田梅负责主题建立、审阅和定稿。
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