岳鹏举,田伯乐
(四川大学华西医院,四川成都 610041)
胰腺囊性占位性疾病(pancreatic cystic lesion,PCL)是一种罕见疾病,学术界对此进行了广泛的研究,现综述如下。
根据20年前统计,腹部彩超对PCL的发现率仅为0.2%[1]。随着影像学技术的进步,PCL发现率越来越高。在过去10年中,对于胰腺囊性肿瘤(pancreatic cystic neoplasm,PCN)的发现量就增加了10倍[2]。近年来有报道指出,成人腹部 CT检查中2.6%存在 PCL,80 岁以上人群中 8% 存在 PCL[3-4]。MRI对 PCL 发现率更高,为2.4% ~13.5%[5-6]。
PCL具有生物多样性,疾病种类可以由完全良性到高度恶性。胰腺手术难度高,术后并发症发生率高(尤其是胰头手术)[7],为避免不必要的手术,明确疾病的术前诊断非常重要。目前PCL的术前诊断,对胰腺外科医生仍然是一项挑战[8]。
近年来,对此类疾病的术前诊断有了显著进步。Valsangkar等[7]调查了美国马萨诸塞州总院33年内因PCL手术的851例患者,发现33年来对PCL的手术趋势有了显著变化。1978—1989年间浆液性囊腺瘤占总切除率的27%,而近年来这一数据下降至12%,表明因术前诊断技术的进步,减少了良性PCN的手术率。此外,因癌前病变的诊断及手术增加,恶性PCN的手术率由1978—1989年的41%下降至2005—2011年的12%。
PCL可以分为炎症性和非炎症性。炎症性囊肿又称胰腺假性囊肿,由纤维及肉芽组织包裹而成,是急慢性胰腺炎的后遗症[9]。非炎症性囊肿由上皮细胞包裹,分为肿瘤性和非肿瘤性,肿瘤性囊肿又可因囊液性质分为黏液性和非黏液性。10年前有研究发现,有40%的PCN被误诊为假性囊肿[10]。在早期研究统计中,超过90%的胰腺囊肿为假性囊肿[11],而最新统计报道中,这一数据下降至50%以下[12-14]。
PCN占胰腺肿瘤的10% ~20%[2]。根据世界卫生组织(WHO)的分类,将PCN分为浆液性囊性肿瘤(serous cystic neoplasm,SCN)、黏液性囊性肿瘤(mucinous cystic neoplasm,MCN)、导管内乳头状黏液性肿瘤(intraductal papillary mucinous neoplasm,IPMN)和实性假乳头状肿瘤(solid pseudopapillary neoplasm,SPN)[15]。此外,有些胰腺实体肿瘤可能发生囊性变性,表现为囊实性包块,如胰腺内分泌肿瘤和胰腺导管腺癌。
在一项大型PCL手术调查中发现,IPMN最为常见,占总数的23.6%;其次为MCN,占23.4%;神经内分泌肿瘤占7.3%;SPN占3.4%;胰腺导管腺癌囊性变占 0.8%[7]。
随着腹部断层影像技术的进步,多数患者无症状而体检偶然发现PCL[7]。有症状的患者其症状通常不典型,包括腹痛、恶心、呕吐等[16-17,7],由 PCL 直接引起的症状通常包括黄疸、急性胰腺炎、腹痛、背痛、腹泻和体质量减轻[18]。其中黄疸、体质量明显减轻和疼痛在特定肿瘤类型中提示肿瘤恶性可能。
PCL通常因症状或体检时依靠影像学检查发现,对疾病的初步诊断靠患者的临床特点,如年龄、性别、囊肿的影像学特征和囊液分析等。这种初步诊断不可能完全精确,但大多数情况下是正确的,对决定患者下一步的治疗有所帮助。
胰腺囊性占位性病变的影像学诊断方式主要包括CT、MRI 和 超 声 内 镜 (endoscopic ultrasonography,EUS)。EUS的优势在于可以通过细针穿刺抽取囊液进行分析。Canto等[19]的研究比较了不同的影像学检查方式对诊断PCL的作用,指出在检查PCL时MRI和EUS优于CT。
有研究表明薄层CT(MDCT)扫描和MRI对PCL的诊断准确率为40% ~60%[20-22],且这两种检查区分黏液性和非黏液性病变的准确率为70%~85%[22-23],此外它们鉴别病变良恶性的准确率高达70% ~80%[24]。恶性病变在CT及MRI的主要特征性表现包括:包块直径>3 cm,主胰管扩张,胆总管扩张,囊壁不规则增厚呈结节样,肿瘤含有固体成分,相关淋巴结肿大等[25-26,22]。也有报道指出 MRI在诊断IPMN以及对于假性囊肿和PCN的鉴别方面比MDCT更有优势[27-28]。
EUS对PCL有良好的成像,但仅靠EUS无法鉴别黏液性囊肿和非黏液性囊肿[29]。Ahmad等研究发现EUS对肿瘤性囊肿和非肿瘤性囊肿的鉴别准确率为40%~93%,但此研究中经验丰富的不同的内镜医师,对同一患者的诊断经常不同[30]。在一项对145名PCL患者的研究中发现,EUS对病灶的发现率大于CT和MRI,并通过细针穿刺细胞学检查,从CT及MRI诊断为良性病变的患者中确诊了3例为恶性[31]。
囊液通常在EUS引导下细针穿刺抽取,目前有多项研究对囊液的细胞学、肿瘤标志物、胰酶和分子生物学等方面进行分析,来评估囊液分析在诊断PCL中的价值。
细胞学检查可以通过识别囊液中的黏液生成细胞来鉴别黏液性囊肿与非黏液性囊肿,并可以通过发现恶性细胞或重度不典型增生细胞确诊恶性病变(如囊腺癌)[32]。此种方法特异性高而敏感性较低,Brugge等[29]曾报道细针穿刺细胞学检查在鉴别黏液性与非黏液性病变中,特异性为83%,敏感性为34.5%;而在鉴别良恶性时,敏感性只有22%。Genevay等[32]对112例病理诊断为胰腺黏液性肿瘤的患者的细胞切片进行了回顾性研究,发现上皮细胞重度不典型增生对预测患者患恶性肿瘤的特异性为85%,敏感性为72%。
在EUS引导下细针穿刺检查中,刮拭囊壁可以提高诊断准确率,一项对37例PCL患者的研究中指出,细针穿刺活检刮拭囊肿壁后,对黏液蛋白的检测敏感度由23%上升至62%[33]。
囊液癌胚抗原(carcino-embryonic antigen,CEA)的测定是区分黏液性与非黏液性病变的最精确的检查方法[34],当使用192 ng/mL为界限时,通过CEA含量鉴别黏液性与非黏液性病变的特异性为84%,敏感度为73%[29]。但因CEA在所有黏液性肿瘤的囊液中含量均升高,所以不能鉴别MCN与IPNM[35]。此外囊液CEA含量也不能准确鉴别病变的良恶性[36]。
囊液淀粉酶升高表明囊腔与胰管相通。Van Der Waaij等[37]的研究指出,囊液淀粉酶 <250 U/L 几乎可以排除胰腺假性囊肿。但并非淀粉酶升高就能诊断为假性囊肿或排除胰腺黏液性囊性占位,因为淀粉酶升高也见于IPNM。
囊液中的分子标志物近年来正在被越来越多地研究,Pathfinder TG是一种生物标志物平台,它的检测包括k-ras基因突变、DNA组分以及与肿瘤抑制基因连锁的微卫星杂合性的缺失[38]。
Khalid等[39]对胰腺囊液进行了分子标志物检测,并发现DNA组分含量增高以及k-ras基因突变的幅度增高预示着病变为恶性肿瘤,同时k-ras基因突变表明病变为黏液性。但另有研究质疑这一结论,Panarelli等[38]认为检测 Pathfinder TG对鉴别 PCL有一定帮助,但这种检查方法往往不准确,不能取代细胞学检查。
Chai等人研究了胰腺囊液中CEA含量、细胞学检查以及k-ras基因突变对诊断胰腺黏液性病变的作用,发现囊液CEA含量升高是最敏感的检查方法,但在CEA与细胞学检查都未诊断出的25例病例中,kras基因突变检测出了其中2例(8%)[40]。
根据以上研究可以得出结论,CEA和细胞学检查是囊液分析中最有用的诊断方法。与以上2种方法相比,分子生物学检测(特别是k-ras突变)也有少许帮助作用。考虑到k-ras基因检测的敏感性较低,所以阴性结果不能排除黏液性病变,但当k-ras基因检测阳性时,即使CEA值没有升高,仍可以支持黏液性囊性病变的诊断[4]。所以,一般情况下,建议分析胰腺囊液的细胞学、CEA以及淀粉酶含量,当临床怀疑胰腺囊性占位为黏液性病变而CEA值不高时,可以进一步对囊液k-ras基因进行检查。
胰腺假性囊肿男性多见,患者常有急性或慢性胰腺炎病史。囊腔一般为单腔或寡腔,内部很少出现分隔。在增强CT下,囊壁一般薄层均匀,不会出现增强结节影[41],胰周脂肪可出现炎症性改变,胰腺实质可出现钙化[42]。在MRI下,囊腔为均匀的T1低信号、T2高信号,若图像不均匀则表明有出血或坏死组织碎屑,ERCP和 MRCP下通常可见囊腔与胰管相通[2]。在EUS下,有些表现为均匀无回声,有些则可见坏死组织碎屑沉积,表现为絮状回声[43]。
假性囊肿囊液颜色一般为黄色或棕色,黏液性低,CEA含量低而淀粉酶含量较高。存在感染时,可含有脓液。胰腺假性囊肿患者60%可自愈[2],若无临床症状可观察随访,当囊肿较大且存在症状时,需临床干预。干预措施一般为引流而非切除,引流方法包括内镜下引流、透视下经皮穿刺引流以及手术引流[44]。EUS引导下穿刺引流是一种新兴的安全有效的治疗方式[45-46],对89%的患者有效,复发率为 12%,并发症(出血、感染、轻度胰腺炎)发生率为11%[47]。与之相比经皮穿刺引流成功率只有21%,手术引流成功率较高,但并发症发生率较高(12% ~35%)[48-50]。
SCN 占 PCL 的 1/3[51],患者平均年龄 60 岁,75%为女性[52]。恶性 SCN 仅占 SCN 的1% ~3%[53-54],因此SCN被认为是一种胰腺良性病变。肿瘤细胞起源于腺泡细胞,其病理特征为囊壁内衬单层柱状上皮,外有少量纤维组织[55]。有研究表明,SCN约44%位于胰腺头颈或钩突部,其余56%位于胰体尾[52]。SCN的EUS典型表现为海绵状或蜂窝状——含有多个微囊肿的囊肿(囊肿由多个直径3~5 mm微小囊肿通过薄膜相隔形成)。而在CT下,这些小囊肿聚集在一起,可能被误认为实性占位。SCN患者中约30%在CT图像上可见特征性中央钙化瘢,呈太阳放射状[51]。10%的患者表现为寡腔囊肿或巨大囊肿,被误诊 MCN[51]。MRI对SCN的成像通常也呈蜂窝状,T1低信号,T2高信号[56,10]。T2加权成像时,微囊肿及分隔呈节段性高信号,呈“葡萄征”。肿瘤有时会对外分叶,但边界清楚,不会侵及周围脂肪组织及其他器官。MRI成像与CT不同之处在于很少能看到特征性的中央钙化瘢[10]。
SCN囊液一般无色透明,CEA及淀粉酶含量均低[9]。SCN生长速度缓慢,平均每年增长 0.5~0.6 cm[57]。肿瘤直径<4 cm时,仅22%的患者出现临床症状;而肿瘤直径>4 cm时,77%的患者存在临床症状,且肿瘤生长速度显著高于4 cm以下病灶[52]。因此,患者存在临床症状或肿瘤直径>4 cm时,应选择手术治疗;而无症状、肿瘤直径<4 cm的患者可选择定期复查。
MCN患者90%为女性,平均患病年龄48岁[58],90%的病灶位于胰体尾[16]。MCN的病理学特征为:囊壁内衬附单层黏液柱状上皮,上皮下间质为稍致密的卵巢样间质[16]。这种黏液性病变可以分为良性的黏液性囊腺瘤、良恶交界性囊腺瘤和恶性的黏液性囊腺癌。目前认为MCN是一种癌前病变,可以发展为原位癌或者浸润性黏液性囊腺癌[16]。
MCN在EUS下通常表现为单腔囊肿或少于6个子腔的寡腔囊肿,每个囊腔(包括子腔)直径>2 cm,平均肿瘤直径7~10 cm[59]。囊腔与胰管不通,囊壁厚约1~2 mm,约25%出现钙化[60-61]。CT 下 MCN 与水的密度几乎相同,当囊壁增厚(>2 mm),出现附壁结节或肿块影时,应考虑浸润性癌变可能[62]。区分MCN的囊壁突起为附壁结节或黏蛋白是困难的。根据最新研究,EUS对附壁结节检测特异性为83%,敏感度为75%,这一数据在CT下分别为100%和24%[63]。EUS下,附壁结节通常有血流信号且不随体位改变移动;黏蛋白则常表现边缘高回声中间低回声,边缘光滑,随体位的变化改变位置[63]。MRI成像下,MCN在T1加权下的信号强度与液体含量相关,单纯液体含量高时为低信号,当囊腔出血或囊液黏蛋白含量高时则表现为高信号;T2加权成像时肿瘤常表现为高信号囊腔含有低信号的分隔[64]。
囊液分析对术前诊断MCN有较大帮助,通常囊液为无色透明液体,黏度高,CEA含量高而淀粉酶含量低。据统计,10%的 MCN在手术切除术时存在癌变[7]。MCN为癌前病变且有可能已发生癌变,故诊断后需手术切除治疗,良性MCN术后一般不会复发[16],国内有报道,恶性MCN若能根治切除,5年生存率可达60%,远高于胰腺导管腺癌[65]。诊断不明确时,对病灶<3 cm的患者可选择密切观察。近年来研究表明,病灶<3 cm时,恶性变风险约为3%,与手术切除死亡率相同[66-68]。
近年来,兴起了一些通过非侵入方式消除MCN的研究,一项随机对照试验表明,EUS下乙醇灌洗囊腔,可以明显减小囊腔,1/3的患者囊腔完全消融[69]。乙醇灌洗囊腔后注射紫杉醇可使62%的患者囊腔完全消融[70],且局部注射紫杉醇后,血药质量分数低至几乎不可检测,很少引起全身副作用[71]。此种方法还处于临床实验阶段,目前尚不能推广。
IPMN最初发现于1982年[72]。目前对IPMN的诊断越来越多,占胰腺肿瘤的1%,以及PCN的25%[2],近年来更成为手术切除 PCN中最常见的一种[7]。IPMN由胰腺导管内黏蛋白生成细胞呈乳头状增殖形成,可产生大量黏蛋白堵塞胰管,造成胰管扩张[2]。根据肿瘤位置可以分为主胰管型(MD-IPMN)、分支胰管型(BD-IPMN)和混合型[73]。IPMN组织学分类为良性、交界性和恶性(原位癌和浸润癌)。MD-IPMN男女患病几率相同,BD-IPMN女性多见。IPMN患者平均年龄69岁[7],恶性MD-IPMN患者平均年龄比交界性 MD-IPMN患者平均年龄大6.4岁,证明IPMN随时间变化向恶性转化。MD-IPMN中70%以及BD-IPMN中60%分布于胰头颈及钩突部[7]。MD-IPMN约33%切除时伴有浸润性癌变,BD-IPMN为14%[7]。
EUS下MD-IPMN表现为:主胰管弥散性扩张伴附壁结节,管腔内充盈缺损。BD-IPMN表现为多个胰腺囊性病变与胰管相通,主胰管大小正常或稍增粗。IPMN囊液一般无色透明,CEA和淀粉酶含量高。
CT和MRI下MD-IPMN主要表现为分叶状囊性病变,胰管扩张。BD-IPMN多见于钩突部位,壁薄不规则,边缘增强。MRI对于胰管增粗的成像更明显,能更清楚地表明分支胰管与肿瘤囊腔相通[64]。提示IPMN癌变的特征包括:囊壁出现结节或肿块影,肿瘤直径>3 cm,主胰管直径>5 mm,囊液细胞学检查阳性或可疑阳性,出现梗阻性黄疸[74]。
国际胰腺病学协会对IPMN的治疗指南指出,所有MD-IPMN都应手术切除[74]。与 MD-IPMN相比,BD-IPMN的治疗更具有挑战性,主要有以下原因:①BD-IPMN比MD-IPMN恶性变率显著减低(BD-IPMN为6%~46%,MD-IPMN为49%~92%)[75-79];②BD - IPMN 术后有复发风险;③BD -IPMN通常多发,若病变累及全胰,需行全胰切除术,手术风险高,患者生存质量差。因此,对于BD-IPMN患者,肿瘤较小且无症状时,可以选择定期通过MRI或超声内镜观察随访,若发现上述提示癌变的高危特征,则需手术治疗[74]。
SPN常见于年轻女性,约70%位于胰体尾[7]。影像学特点为边界清楚的囊实性包块。有研究对28例SPN患者进行EUS检查,发现50%为实性,39%为囊实混合性,11%为囊性[80]。此研究中,EUS引导下细针穿刺检查对SPN的诊断率为75%。细针穿刺细胞学检查显示肿瘤细胞紧密结合,形成柱状或乳头状结构,免疫组化检查波形蛋白和CD10阳性[27]。
SPN患者被诊断出时一般无症状,病变较大时可出现腹痛,SPN为低度恶性肿瘤,通常不会远处转移,但若不进行治疗,会侵犯邻近器官及大血管[81],其治疗方式主要为手术切除。
胰腺神经内分泌肿瘤的EUS成像通常为密度均匀、边界清楚的实性包块[36]。约10%的胰腺内分泌肿瘤为囊性[82]。在一项对9例囊性胰腺神经内分泌肿瘤的研究中,4例为囊实性,5例为纯囊性[82]。
囊性神经内分泌肿瘤囊液中CEA及淀粉酶含量均较低[83]。细针穿刺囊液或固体成分细胞学检查显示浆细胞凝聚成圆形或椭圆形,核仁中度增大,免疫组化示突触素和嗜铬粒蛋白阳性[84]。手术切除是治疗神经内分泌肿瘤最有效的手段。
胰腺淋巴囊肿是很罕见的良性非肿瘤性囊肿,均匀分布于整个胰腺,男性较女性常见,患者多无症状,而因体检偶然发现。囊肿含大量组织碎屑沉积,EUS成像通常为界限清楚的不均匀低回声实性占位[85]。囊液为乳白色,细胞学检查显示含有鳞状上皮细胞、角化上皮细胞碎屑和淋巴细胞[86]。患者无症状可保守治疗;若有症状或不能排除肿瘤时,可手术治疗。
良性上皮囊肿又称单纯性囊肿,也是胰腺罕见的非肿瘤性囊肿之一。影像学常表现为单腔囊肿,囊壁薄,无附壁结节。该病与常染色体显性遗传病、多囊肾病相关。当无法排除肿瘤性病变时,可以手术治疗。
棘球蚴病是感染棘球绦虫的幼虫(棘球幼)所致的慢性寄生虫病,多见于肝脏,单独胰腺受累非常罕见[87]。在腹部超声或EUS下表现多变,取决于疾病阶段。早期表现为单腔囊肿,囊壁常表现为双强回声线间隔一层低回声层,内部可见数个棘球蚴砂形成的强回声灶。中期表现为多腔多隔囊肿,分隔为子腔的囊壁。晚期囊壁为一层厚钙化壁[88]。诊断此种疾病需要患者有疫区接触史,血清学包虫测试阳性,以及影像学表现。治疗方法包括阿苯达唑药物治疗和手术治疗。
总之,近年来胰腺囊性占位性疾病发现率越来越高,术前诊断随着MRI和EUS细针穿刺检查的普及取得了显著进步,减少了许多不必要的手术。在治疗方面,涉及消化内科、胰腺外科、放射科的多学科综合治疗,起到了非常重要的作用。
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