Endoscopic submucosal dissection (ESD) has become a standard minimally invasive treatment for early gastric cancer (EGC), offering organ preservation, reduced costs, and improved quality of life. However, its application in undifferentiated or submucosal (SM)-invading cancers remains debated due to the higher risk of lymph node metastasis. According to the Japanese Gastric Cancer Treatment Guidelines 2021 (6th edition), ESD is typically indicated for differentiated mucosal-type cancers (elevated <2 cm or depressed <1 cm without ulceration).
This study aimed to evaluate the clinical outcomes of ESD for EGC with confirmed submucosal invasion (T1b) at a single tertiary center in Mongolia.

Between July 2021 and December 2024, a total of 248 ESD procedures for early gastric cancer were performed at Brilliant Hospital, Ulaanbaatar.
Patient and Lesion Characteristics

• Histology: mucosal, submucosal, and undifferentiated cancers
• Confirmed SM invasion: 33 cases (15 differentiated (45.5%); 18 undifferentiated (54.5%))
• Complete resection rate: 100%
• Submucosal depth: all lesions SM1–3 (SM1 <500 μm; SM2 >500 μm; SM3 = deeper invasion)

Extended Indications
Patients unfit for surgery or who refused surgery were included, even if lesions were undifferentiated or suspected of SM invasion endoscopically.
Follow-up Protocol
Endoscopic and histologic follow-up was conducted at 3–6 months post-ESD with biopsy sampling at the resection site.

Clinicopathological Findings

• Total SM invasion cases: 33
• Histology: 15 differentiated (45.5%), 18 undifferentiated (54.5%)
• Average lesion size: 8.1 × 6.5 mm
• All SM-invasive lesions: SM1–SM3 (predominantly SM3 >800 μm)
• Complete resection rate: 100%

Undifferentiated tumors were significantly associated with larger size and deeper SM invasion.
The mean follow-up duration was 3.4 ± 0.5 months, during which 1 local recurrence was observed. No cancer-related deaths occurred during the observation period (September 2022 – April 2025). Due to the short follow-up period, 5-year survival data are not yet available.
Endoscopic Follow-up Outcomes
Among 33 patients with documented follow-up:

• Mean duration: 5.45 ± 1.2 months
• Recurrent cancer detection rate: 1/33 (3%)
• Repeat biopsy performed: 8 cases (Positive for malignancy: 1 case (3%))
• Common endoscopic findings: Post-ESD scar (± inflammation): 95% | Stenosis requiring balloon dilation: 1 case (cardia) | No new or suspicious lesions observed

Discussion
This study highlights that most SM-invading EGCs were undifferentiated and larger than 20 mm, consistent with international data indicating higher invasiveness and lymphovascular risk.
Although these lesions fall outside traditional ESD criteria, complete resection was achieved in all cases, and no mortality occurred during short-term follow-up. Nevertheless, undifferentiated histology and deeper (SM2/SM3) invasion are strongly correlated with lymph node metastasis rates of 10–25%, underscoring the need for cautious case selection.
ESD may be a viable alternative for elderly or surgically unfit patients, provided that strict histologic assessment and close endoscopic monitoring are performed.

ESD achieved high en bloc and complete resection rates in patients with SM-invasive early gastric cancer, including undifferentiated types.
While short-term outcomes are favorable, the long-term oncologic safety remains uncertain.
ESD may be an acceptable treatment option for carefully selected patients, especially those unsuitable for surgery, with the requirement of diligent follow-up and multidisciplinary management.