Measuring Depth of Invasion in Early Squamous Cell Carcinoma of the Oral Tongue: Positive Deep Margin, Extratumoral Perineural Invasion, and Other Challenges

Abstract

The 8th edition of American Joint Committee on Cancer (AJCC 8th) staging manual incorporated depth of invasion (DOI) into pT stage of oral cavity cancer. The aim of this study was to characterize several histological findings that may complicate measurement of DOI in early conventional squamous cell carcinomas (SCC) of the oral tongue: (1) lack of or minimal residual carcinoma following biopsy; (2) positive deep margin; (3) extratumoral perineural invasion (PNI); and (4) lymphatic or vascular invasion. Conventional SCC of the oral tongue (n = 407) with the largest dimension of ≤ 4 cm and with a negative elective cervical lymph node dissection (pN0) were reviewed. A clear plastic ruler was used to measure DOI by dropping a "plumb line" to the deepest point of the invasive tumor from the level of the basement membrane of the normal mucosa closest to the invasive tumor. Examples of identifying reference point on the mucosal surface of oral tongue from which to measure the DOI are illustrated. In the experience of one contributing institution, the residual carcinoma was absent in 14.2% of glossectomies (34/239), while in 4.8% of cases (10/205) there was only minimal residual carcinoma. In 11.5% (21/183) of pT2 cases the deep margin was positive and thus DOI and pT may be underestimated. Of all cases with PNI, extratumoral PNI was identified in 23.1% (31/134) of cases, but represented the deepest point of invasion in only two cases. In one case, lymphatic invasion represented the deepest point of invasion and could have led to upstaging from pT1 to pT2. In conclusion, DOI measurement for SCC of the oral tongue may require re-examination of the diagnostic biopsy in up to 20% of cases due to the absence or only minimal residual carcinoma in glossectomy specimens. In 11.5% of apparently pT2 cases, DOI may be underestimated due to the positive deep margin. Rarely, extratumoral PNI or lymphatic invasion may be the deepest point of invasion. Overall, two issues (absent or minimal residual disease and positive deep margin) may confound DOI measurement in early SCCs of oral tongue.

Keywords: AJCC staging; Carcinoma; Deep margin; Depth of invasion; Extratumoral perineural invasion; Oral tongue.

Fig. 1

An example of squamous cell carcinoma with extratumoral perineural invasion. The focus of extratumoral perineural invasion was 7 mm from the invasive tumor front; however, the carcinoma was tracking along the nerve running parallel to the overlying normal mucosa (“sideways”) and did not represent the deepest point of invasion. The black line illustrates how the distance between the invasive tumor front and focus of perineural invasion was measured. The blue line shows how the depth of invasion was measured. It may be challenging to select a reference point on the mucosal surface of oral tongue from where to measure the depth of invasion (especially if the tumor is ulcerated or not connected to the mucosa). Theoretically, the reference point may be chosen along the dorsal (superior), lateral, or ventral (inferior, towards floor of mouth) mucosa. Here, an arcuate, rather than straight, line (bright green) connecting normal mucosa on both sides of dysplastic mucosa overlying invasive squamous cell carcinoma was imagined. The “plumb line” was dropped through the middle one-third of the bulk of the tumor with deepest invasion. Higher magnification of the remote focus of perineural invasion is shown in the inset (hematoxylin and eosin, images taken from the scanned whole slide image with original magnification of × 1.2)

Fig. 2

Cross-section through left partial glossectomy with dorsal, lateral, and ventral (towards floor of mouth) mucosa (clockwise, starting from the top). Note, this is a permanent formalin fixed paraffin embedded section of the frozen section remnant shown in Fig. 3. On permanent section, an additional more superficial focus of residual invasive squamous cell carcinoma is seen closer to the mucosa (area #1 in a, b). Also, suture granulomas (area #2 in a, c) were identified. The deepest point of invasion was represented by foci of extratumoral perineural invasion (d). The final “plumb line” to measure the depth of invasion was drawn through areas #1 and #3. Hematoxylin and eosin, permanent formalin fixed paraffin embedded section, images taken from the scanned whole slide image with original magnification of × 1.2

Fig. 3

Cross-section through left partial glossectomy with dorsal, lateral, and ventral (towards floor of mouth) mucosa (clockwise, starting from the top). a The focus of residual squamous cell carcinoma (between the white and black asterisks) shows no connection to mucosa. Residual carcinoma is represented by foci of extensive perineural invasion (b). The potential reference points to measure the depth of invasion are along the dorsal, lateral, and ventral mucosa. Measuring depth of invasion from ventral mucosa would result in pT1, while depth of invasion measurement from the dorsal mucosa would result in pT3. On this section (see Fig. 2, also), depth of invasion was measured from the lateral mucosa (next to exclamation mark), because this area showed a focus of moderate to severe dysplasia. Hematoxylin and eosin, frozen section, images taken from the scanned whole slide image with original magnification of × 1.2. Note, this is the same case as in Fig. 2, which is a permanent formalin fixed paraffin embedded section of the frozen section remnant

Fig. 4

a, b An example of minimal residual squamous cell carcinoma of the oral tongue (hematoxylin and eosin (H&E), original magnification × 40). b A minute focus of residual squamous cell carcinoma, about 1 mm wide and 1 mm deep, with submucosal scar in the left lower corner (H&E, original magnification × 100). c The diagnostic biopsy was represented by five tissue fragments, all of which were smaller than 5 mm in greatest dimension and had invasive squamous cell carcinoma. The exact measurement of the depth of invasion in this case is difficult given the fragmented nature of diagnostic biopsy. Only one biopsy fragment had normal squamous mucosa allowing measurement of the depth of invasion (H&E, original magnification × 40)

Fig. 5

An example of a pT2 squamous cell carcinoma of the oral tongue with positive deep margin, indicating that the depth of invasion may be underestimated. a The apparent depth of invasion is 7 mm; however, the deep margin is involved by carcinoma (hematoxylin and eosin (H&E), original magnification × 20). b Carcinoma at deep margin. Hypothetically, if there is additional 4 mm (along the “plumb line”) of residual carcinoma in the tumor bed, this carcinoma is more appropriately staged as pT3 (H&E, original magnification × 100)

Fig. 6

a Two clusters of invasive squamous cell carcinoma (each with about 15 cells, by the black asterisk and in b, c) were 6.5 mm from the bulk of the tumor, suggestive of lymphatic invasion and representing the deepest point of invasion. The black line illustrates the way the distance between the invasive tumor front and remote foci of carcinoma was measured. The pT1 stage was assigned based on the depth of invasion by the bulk of the tumor which was 4.5 mm. b One of the small clusters of carcinoma is in the left upper corner and the second focus of carcinoma is in the right lower corner and in c. Hematoxylin and eosin, images taken from the scanned whole slide image with original magnification of × 1.2