Gastric Carcinoma

Todd M. Blodgett, MD

Alex Ryan, MD

Hesham Amr, MD

Key Facts

Terminology

Adenocarcinoma arising from gastric mucosa

Imaging Findings

Hypermetabolic FDG activity in primary gastric tumor, regional lymph nodes (LN), peritoneum, distant metastases
Regional LN > 8 mm: Suspicious for metastatic disease
Peritoneal dissemination: Peritoneal caking, nodularity, beaded thickening, malignant ascites
No method currently available capable of staging N disease accurately enough to enable decision making based on the extent of surgical lymph node dissection
PET/CT for staging, recurrence, response to therapy
CECT: Focal gastric wall thickening or mass lesion with enhancement
PET sensitivity for primary tumor depends on extent
PET and CT similar sensitivity for primary (94% vs. 93%), PET higher specificity (92% vs. 62%)
Lymphadenopathy: PET insensitive for N1 disease (56%), equally sensitive for N2-3 disease as CT (78%)
Mucinous and signet-ring cell adenocarcinoma: Low FDG avidity 2° to high mucus, lack of glut-1 transporters

Top Differential Diagnoses

Gastric Ulcer
Gastrointestinal Stromal Tumor (GIST)
Gastric Lymphoma
- Variable PET activity
Gastritis
- Usually mild activity; nonfocal
Physiologic FDG Activity

Axial graphic shows focal thickening of the gastric mucosa

, compatible with early gastric adenocarcinoma.

Graphic shows a large mass

arising from the lesser curvature of the stomach, compatible with a large adenocarcinoma of the stomach.

TERMINOLOGY

Abbreviations and Synonyms

Gastric adenocarcinoma, early/advanced gastric carcinoma (EGC/AGC), gastric cancer, stomach cancer

Definitions

Adenocarcinoma arising from gastric mucosa
- Malignant pathway: Gastritis → gastric atrophy → metaplasia → dysplasia → cancer
- Most common primary tumor to metastasize to ovaries, usually bilaterally (Krukenberg tumors)

IMAGING FINDINGS

General Features

Best diagnostic clue
- FDG PET
  - Hypermetabolic FDG activity in primary gastric tumor, regional lymph nodes (LN), peritoneum, distant metastases
- CT
  - Polypoid mass ± ulceration
  - Focal wall thickening with mucosal irregularity, ulceration
  - Regional LN > 8 mm: Suspicious for metastatic disease
  - Peritoneal dissemination: Peritoneal caking, nodularity, beaded thickening, malignant ascites
Location
- Fundus and cardia: 40%
- Body: 30%
- Antrum: 30%
- Most likely sites of recurrence post-gastrectomy: Gastric bed, peritoneal dissemination, liver
- Large proportion of non-FDG-avid, non-intestinal subtype tumors found in distal 2/3 of stomach
- Tumor commonly invades directly into
  - Pancreas via lesser sac
  - Transverse colon via gastrocolic ligament
  - Liver via gastrohepatic ligament
- 60% of carcinoma of cardia will spread to distal esophagus
- 5-20% of antral carcinoma will involve duodenum
Size: Often very large before tumor becomes symptomatic
Morphology
- Type I: Elevated, protrude > 5 mm into lumen
- Type II: Superficial lesions that are elevated (IIa), flat (IIb), depressed (IIc)
- Type III: Early gastric cancers that are shallow, irregular ulcers surrounded by nodular, clubbed mucosal folds

Imaging Recommendations

Best imaging tool
- PET/CT for staging, recurrence, response to therapy
  - No modality is currently sensitive enough to confidently guide surgical planning
- CECT to determine whether tumor is resectable
- Barium X-ray screening is effective but has low sensitivity
  - Side effects include constipation and mis-swallowing of barium into trachea
Protocol advice
- PET sensitivity for recurrent disease postgastrectomy may be ↑ by water ingestion (˜ 300 cc) to distend remnant stomach
- FDG uptake secondary to insulin release associated with food intake can be avoided by feeding at 120 minute time point after FDG administration
- CT evaluation also aided by negative contrast ingestion (water or gas)

CT Findings

General features
- Mucosal irregularity
- Enhancing focal gastric wall thickening
  - Gastric distention important for accurate assessment
- Enhancing mass lesion
  - Normal gastroesophageal junction may be mistaken for mass
- Polypoid mass ± ulceration
- Gas-filled ulcer crater within mass
- Tumor depth difficult to assess
Primary by type
- Infiltrating type: Loss of normal rugal folds over thickened wall
- Scirrhous type: Thickened wall with marked contrast enhancement
- Mucinous type: Wall thickening and calcification with decreased attenuation due to mucin content
- Cardia tumor: Lobulated mass with irregular soft tissue thickening
Extension/metastasis
- CT can delineate fat pad between tumor and organ to determine resectability
  - May normally be absent between stomach and left lobe of liver
  - Inflammation can obscure fat plane between tumor and pancreas
  - Cachexia results in loss of fat planes, producing potential false positive for organ invasion
- Can often detect peritoneal carcinomatosis
  - Valuable for avoiding futile laparotomies
  - Tiny deposits may be overlooked
- Extension into perigastric fat appears as wisp-like perigastric soft tissue stranding
Lymph nodes
- Subcentimeter lymph nodes may harbor malignancy
- Enlarged lymph nodes may be nonmalignant (inflammation, infection)
- Perigastric nodes are best visualized when stomach is fully distended

Nuclear Medicine Findings

General
- False positives
  - Normal stomach uptake
  - Gastritis
  - Inflammatory regional lymph nodes
  - Cholecystitis can produce false positives in liver
- Water or food ingestion may decrease false positive from 31% to 8%
  - Food has advantage of slower emptying from stomach
  - Misregistration artifacts can occur because of shifting gas/fluid volumes between CT and PET acquisition
  - Additional single-field PET/CT acquisition can avoid misinterpretation
Initial diagnosis
- FDG PET sensitivity for primary tumor
  - Early GC: 47%
  - Advanced GC: 98%
  - In general, better sensitivity for more advanced disease
- PET and CT similar sensitivity for primary (94% vs. 93%)
  - PET has higher specificity (92% vs. 62%)
- In general, well-differentiated gastric adenocarcinomas tend to take up less FDG than poorly differentiated ones
- Exceptions
  - Poorly differentiated tubular adenocarcinomas show an especially wide spectrum of FDG uptake, from low to intense
  - Mucinous and signet-ring cell adenocarcinoma: Low FDG avidity 2° to high mucus, lack of glut-1 transporters
Staging
- Pre-treatment staging is essential to determine potential curability and to plan optimal therapy
- Lymphadenopathy
  - PET and CT have similar sensitivity for detection of local and distant lymphadenopathy
  - Overall lymph node evaluation: PET less sensitive than CT (56% vs. 78%) but more specific (92% vs. 62%)
  - PET alone has low sensitivity for N1 disease (56%)
  - N1 insensitivity less important because all patients with AGC will undergo at least D1 dissection
  - PET and CT have high specificity for N1 disease
  - Positive findings may change endoscopic mucosal resection to more aggressive surgical approach in patients with EGC
  - Almost all PET-positive lymph nodes prove to be involved in patients with AGC
  - PET and CT equally sensitive for N2-3 disease (about 80%)
  - N2/N3 status determination important: Can change extent of lymph node dissection and, in the case of N3 disease, curative potential
- Peritoneal dissemination: PET has low sensitivity and CT low specificity, but similar accuracy overall
- FDG avidity of primary tumor predictive of long-term survival in patients undergoing complete resection of primary
Response to therapy
- Post-therapy change in tumor SUV predicts both response to therapy and long-term survival with 77% sensitivity, 86% specificity
- Decreased tumor uptake by > 35% of baseline allowed accurate prediction of response 14 days after initiation of cisplatin-based polychemotherapy
  - Overall accuracy of 83%
  - 1/3 of patients initially have insufficient FDG uptake for quantification
  - Initially low FDG uptake probably correlates with tumors in which chemotherapy has limited effectiveness
Other radiotracers
- 18F-fluorothymidine sensitive for locally advanced gastric cancers with improved detection of tumors with signet ring cells or mucinous contents

DIFFERENTIAL DIAGNOSIS

Gastric Ulcer

Benign ulcer
Crater margin sharply defined and smooth en face, symmetric, confluent with healthy mucosa, mucosal folds radiate from ulcer edge
Most located in lesser curve or posterior wall of antrum, body of stomach

Gastrointestinal Stromal Tumor (GIST)

Well-demarcated, spherical, intramural masses that arise from muscularis propria; often project exophytically, intraluminally
May have overlying mucosal ulceration
Larger GISTs often outgrow vascular supply → necrosis and hemorrhage

Gastric Lymphoma