Benign vs Malignant Tumors

Which of the following histological features is most characteristic of malignant tumors and helps distinguish them from benign lesions?

A 52-year-old Indian woman presents with a 3-month history of progressive abdominal distension and early satiety. On examination, she has a firm, fixed, irregular mass palpable in the epigastrium. Laboratory investigations show hemoglobin 9.2 g/dL, albumin 2.8 g/dL, and elevated CA 19-9 (320 U/mL). Contrast-enhanced CT abdomen reveals a 6 cm heterogeneous mass in the gastric antrum with involvement of the serosa and ascites. What is the most likely diagnosis?

A 38-year-old Indian man undergoes upper endoscopy for chronic dyspepsia. A 2 cm submucosal nodule is identified in the gastric fundus. Biopsy shows spindle cells arranged in fascicles with rare mitoses (1 per 50 high-power fields). Immunohistochemistry is positive for c-KIT and CD34, negative for desmin. The lesion is completely excised endoscopically without perforation. What is the most likely diagnosis and expected clinical course?

Which of the following properties is LEAST helpful in distinguishing a benign tumor from a malignant tumor?