[2033] A Practical Approach To Differentiate between WHO Types A and B3 Thymomas and Thymic Carcinomas

Anja C Roden, Eunhee S Yi, Jakub Pecl, Stephen D Cassivi, Yolanda I Garces, Marie Christine Aubry. Mayo Clinic, Rochester

Background: Interobserver agreements for the WHO classification of thymic epithelial neoplasms (TEN) is only moderate. Common disagreements occur for type B3 vs thymic carcinoma (TCa), type A vs B3, and A vs TCa. However, the prognosis of TCa is worse with 10-yr survival of 20-67% vs 36-83% for B3 and 76-100% for A. No single marker is available to facilitate the histologic assessment. We studied whether a combination of TdT, Glut-1, and CD205 might be used to facilitate the distinction between type A and B3 thymomas and TCa.
Design: Specimens from 50 pts treated with TEN were included. Three pathologists independently classified all cases according to WHO and agreed upon type A, B3 or TCa. Sections were stained for CD205, TdT, and Glut-1. Membranous (CD205, Glut-1) or nuclear (TdT) staining were considered positive. Staining distribution was assessed for CD205 and Glut-1. Number of + cells was expressed as (100 * number + epithelial cells) / number total epithelial cells (mean of 5 Hpf counted).
Results: Expression of TdT, CD205, and Glut-1 and distribution of CD205+ and Glut-1+ cells are shown in Table 1.

Table 1: Expression and staining distribution of TdT, CD205, and Glut-1

WHO Type	TdT	CD205			Glut-1
	# + cases¹	# + cases¹	Diffuse	Patchy	# + cases¹	Diffuse	Patchy
A (n=16)	11 (68.7)	14 (87.5)	1	13	6 (37.5)	0	6
B3 (n=15)	14 (93.3)	15 (100)	8	7	10 (66.7)	0	10
TCa (n=19)	0	10 (52.3)	3	7	17 (89.5)	10	7

¹Absolute number of + cases (%)

Counts of Glut-1 and CD205+ cells are in Table 2.

Table 2: Counts of CD205+ and Glut-1+ cells

% + cells	CD205			Glut-1
	Type A	Type B3	TCa	Type A	Type B3	TCa
<5	2	0	5	3	3	4
5-25	3	4	2	3	6	1
25-50	5	0	2	0	0	3
50-75	2	4	0	0	1	2
75-100	2	7	2	0	0	9

TENs that do not contain TdT+ lymphocytes and show either diffuse expression of Glut-1 and/or Glut-1 in >25% cells (vs. type A) or > 5% (vs. type B3) are TCa.

Some type A thymomas can be differentiated from B3 based on Glut-1 expression (>25% cells, B3). B3 thymomas usually have TdT+ lymphocytes while some A thymomas have not. CD205 staining was patchy to diffuse in most TEN (TCa 52.3%, type A 87.5%, B3 100%).
Conclusions: A panel of Glut-1 and TdT might be used as adjunct to distinguish type A from TCa and type B3 from TCa. In a few cases, these markers might also help to differentiate type A from B3. CD205 appears not useful in this context.
Category: Pulmonary

Wednesday, March 21, 2012 9:30 AM

Poster Session V # 290, Wednesday Morning