72 many grading schemes, adenoid cystic carcinoma grading quick and simple in that it is based solely on growth pat- tern. As early as 1958, Patey and Thackray [12] noted that a solid growth pattern imparts a poor prognosis. Subse- quently, grading of this tumor has evolved into stratiﬁcation into three grades of increasing aggressiveness based on predominant growth pattern: [10, 11] grade 1: tubular, grade 2: cribriform, grade 3: solid (Fig. 2). Gen- erally, a tumor with a greater than 30% solid component belongs in the ‘grade 3’ category. However, it is suggested that any solid component imparts a poor prognosis, and that the relationship between solid growth pattern percentage and prognosis is somewhat linear and that assigning a cut- off may be arbitrary. The current WHO classiﬁcation refers to tumors by predominant pattern rather than actually assigning a numeric grade. One of the controversies surrounding grading of adenoid cystic carcinoma is its prognostic utility independent of tumor stage. Spiro et al. [8] suggest that this is not the case, however, more recently, da Cruz Perez et al. [13] show that grade is an independent prognosticator on multivariate analysis. One potential difference that the grading scheme used by Spiro et al. [8] differs from the typical scheme particularly with regards to a solid component cut-off of over [ 50%. The difﬁculties in conversion between the two grading schemes are illustrated in Table 3. Repro- ducibility of grading in adenoid cystic carcinoma has not been well addressed in the literature. In one study, [14] ironically, the grading system used by Spiro et al. [8] has less interobserver variability. While prognostically useful, it is unclear whether grading of adenoid cystic carcinoma is useful in patient management. Regardless of grade, all adenoid cystic carcinomas are treated with surgery plus irradiation because locally they are aggressive and ‘high risk’. With regard to the neck, most decisions on the neck dissection will not depend on grade, though there is some evidence to suggest that solid/grade 3 adenoid cystic carcinomas have a higher likelihood of lymph node metastasis [3]. With regards to this point, the rare phenomenon of dedifferentiation or high grade transformation may have Fig. 2 The various patterns/ grades of adenoid cystic carcinoma. a Tubular, b cribriform, c solid. All grades are cytologically monomorphic and retain small dark angulated nuclear features Head and Neck Pathol (2009) 3:69–77 Table 3 Comparison of common pattern grading schemes in adenoid cystic carcinoma Grade Perzin et al. [10], Szanto et al. [11] Grade Spiro et al. [8] 1 2 3 Predominantly tubular, no 1 solid component Predominantly cribriform, solid component \30% acceptable Solid component [30% 2 3 Mostly tubular or cribriform (no stipulations on minor solid components) 50% solid Mostly solid relevance. Adenoid cystic carcinoma with high grade transformation refers to the presence of a pleomorphic mitotically active high grade carcinoma component aris- ing in an otherwise conventional adenoid cystic carcinoma of any pattern/grade (Fig. 3a) [15]. The trans- formed component is typically of a purely ductal phenotype with a solid or cribriform appearance. Unlike conventional adenoid cystic carcinoma, which is charac- terized by small, hyperchromatic, monomorphic nuclei, and scant cytoplasm, transformed components show prominent nuclear size and chromatin variability. Com- mon features include ﬁbrocellular desmoplasia, abundant mitoses, necrosis, and microcalciﬁcations. Unique patterns in high grade transformation include micropapillary and squamoid growth. However, there is still morphologic overlap between solid conventional adenoid cystic carci- noma and high grade transformation and the transition from conventional adenoid cystic to high grade transfor- mation is often gradual. Table 4 delineates key distinguishing features between solid conventional ade- noid cystic carcinoma and adenoid cystic carcinoma with high grade transformation. Basically, the aggressive nuclear, stromal, architectural and immunohistochemical features common to both variants are more exaggerated in high grade transformation, while solid conventional ade- noid cystic carcinoma shows only slight deviation from tubular or cribriform conventional adenoid cystic carcinoma.