Atypical multivacuolated lipoblasts and atypical mitoses are not compatible with the diagnosis of spindle cell/pleomorphic lipoma-reply
Abstract
The authors disagree with our notion that atypical multivacuolated lipoblasts, mitotic figures (both atypical and typical) and infiltration of the underlying skeletal muscle are compatible with a histological diagnosis of classical pleomorphic lipoma (PL). For cases showing such features, they propose the term "atypical" pleomorphic lipomatous tumor (APLT) [3].
Typical mitotic figures are a very common feature of PL, found in about half of the lesions. In our study, atypical mitoses were detected in 16% of cases (4/25) of PL.
The PL with atypical mitoses and atypical spindle cells illustrated in our paper [2] occurred in the classical clinical setting, showed sharp circumscription, was of a relatively small size (3 cm) and demonstrated no recurrence in 2 years of follow-up (case 13 of our previous study [1]). Similarly, the remaining 3 PL with atypical mitoses (cases 8, 10, 16 [1]) were also relatively small, circumscribed tumors without recurrence with 3, 10 and 1 years of follow-up.
Besides the univacuolated lipoblasts (LPB), occasional PL also contain multivacuolated LPB, a feature with absolutely no impact on the clinical course and prognosis. For example, the PL depicted in figure 1B of our manuscript (Case 2 [1]) showed multivacuolated LPB.
However, it was again a sharply circumscribed tumor occurring in the classical setting with no recurrence in 17 years post-removal. Our study did not specifically focus on the presence of atypical spindle cells with hyperchromatic nuclei.
However, even such cells have no impact on the prognosis, as again illustrated by case 13 of our publication. Moreover, despite the fact that some of the abovementioned "atypical" features occurred in a considerable number of our cases, there have been only 2 recurrences in the 44 patients with available follow-up.
Interestingly, both recurrent cases (cases 6 and 25 [1]) represented SCL with none of the aforementioned "atypical" features (except univacuolated LPB). In fact, these 2 "recurrences" likely represent incomplete excisions, allowing the neoplasms to regrow.
It is also true that several cases showed infiltration into the underlying skeletal muscle. When reviewed, all such tumors (SCL cases 18, 20, and 35 [1]) were located on the face, where muscle "infiltration" is often encountered in various benign proliferations including SCL/PL [4] due to the lack of well-defined fascial planes and superficial location of the mimic muscles.
Taken together, if the stringent criteria proposed by Creytens et al were applied, up to about one-half of tumors currently classified as PL would be diagnosed as APLT. On the other hand, we agree with Creytens et al that "sarcomatous" transformation of PL may very rarely occur, but the diagnostic criteria for these neoplasms must be better defined.
In conclusion, the morphological spectrum of SCL/PL is broad with a minimal risk of aggressive behavior. On the other hand, true sarcomatous PL do very rarely occur and may have a higher risk of local recurrence.
Until more accurate criteria for the distinction between "classical" PL and APLT are defined, we advocate a very conservative use of the term APLT to avoid overtreatment. We believe it is better to treat a few sporadic recurrences than to trigger confusion among pathologists and surgeons and unnecessarily aggressive surgical treatments for benign tumors.