Criteria of Folpe et al for Malignant Proliferating Trichilemmal Tumors (PTT)

 

Trichilemmal tumors show the histologic features of the follicular outer root sheath epithelium with

 

Parameters:

(1) location

(2) clinical rate of growth

(3) diameter in centimeters

(4) histologic growth pattern

(5) cytologic atypia in least differentiated area

(6) mitotic rate in 10 high powered (40x objective) field (HPF)

 

Parameters	Finding	Points
location	scalp	0
	non-scalp	1
clinical rate of growth	slow or moderate	0
	rapid	1
diameter	<= 5 cm	0
	> 5 cm	1
histologic growth pattern	noninfiltrative (circumscribed)	0
	infiltrative, with desmoplastic stromal response	1
cytologic atypia	bland to moderate	0
	marked	1
mitotic rate per 10 HPF	"low" (<= 2)	0
	"high" (>= 10)	1

 

where:

• Some of the features (rate of growth, histologic growth pattern, cytologic atypia and mitotic rate) seem to have intermediate levels. In the implementation I will adhere to a single cutoff point. An alternative is to award 0.5 points for intermediate level findings.

• Mitotic rate was counted in 30 high powered fields then averaged.

 

Other histologic features seen in malignant cases:

(1) tumor necrosis

(2) angiolymphatic space invasion

 

total number of malignant features =

= SUM(points for all 6 parameters)

 

Interpretation:

• minimum number of malignant features: 0

• maximum number of malignant features: 6

• If none of the malignant features are present, then the risk of malignant behavior is very low.

• The risk of metastatic disease and other malignant features increases with the number of malignant features.

• The authors do not feel that it is possible to reliably separate these into low and high grade tumors.