A Study of Intratumoral Fibrous Interstitium in the Growth and Proliferation Process in Rectal Carcinoid Tumors

The aim of the study was to investigate the relationship between the fibrous interstitium found in the spaces between tumor nests and the growth and proliferation process in 33 rectal carcinoid lesions (26 resected endoscopically and 7 resected surgically). The proportion of the tumor occupied by fibrous interstitium (F/T ratio) was measured in tissue specimens using an image analyzer, and the composition and distribution pattern of the fibrous interstitium were classified based on 21 lesions in which special staining was performed successfully. The results were then compared with tumor nest malignancy and preoperative endoscopic ultrasonography (EUS) findings for 23 lsions. The F/T ratio was significantly higher in the 16 lesions smaller than 5 mm and composed of low malignancy tumor nests (0.49 ± 0.14) than in the 17 lesions 5 mm or larger (0.25 ± 0.10). However, the F/T ratio was high (mean 0.41) for 2 of the 17 lesions 5 mm or larger comprised of highly malignant tumor nests. The fibrous interstitium was composed of part of the smooth muscle resulting from destruction of the muscularis mucosae and collagenous fibers. The 9 lesions with mixed interstitium in which smooth muscle predominated were comprised of low malignancy tumor nests with a significantly higher F/T ratio (0.55 ± 0.09). In contrast, of the 12 lesions with separated interstitium, the 4 lesions in which collagenous fibers predominated had a significantly higher F/T ratio (0.34 ± 0.05) than the 8 lesions in which smooth muscle predominated (0.18 ± 0.05). Three of these 4 lesions had infiltrated as far as the deeper sections of the submucosal layer on EUS, and 1 of the lesions was a highly malignant tumor with an invasion depth of adventitia. The EUS internal echo images of the tumors were classifiable into: (1) homogenous low-echoic; (2) honeycomb; (3) multiple high-echoic dots; and (4) unvisualized. These classifications reflected the tumor structure (structure of the dominant nest, F/T ratio, and fibrous interstitium distribution pattern) and depth of invasion. At the immature stage, there were large amounts of destroyed muscularis mucosae present in the spaces between the dispersed tumor nests. At the mature stage, the tumor nests formed a single solid mass, and in lesions that had infiltrated to the deeper sections of the submucosal layer or were highly malignant, the number of collagenous fibers inside the tumor increased, suggesting that fibrous interstitium is closely related to rectal carcinoid tumor growth and proliferation. These histologic findings can be predicted from EUS, and may therefore be useful in assisting preoperative diagnosis.