Re: Intramuscular lipomas: Large and deep benign lumps not to be underestimated. Review of a series of 51 cases

Dear Sir,

We have read with great interest the article by Dr. Ramos-Pascua et al. entitled “Intramuscular lipomas: Large and deep benign lumps not to be underestimated. Review of a series of 51 cases” published in Revista Española de Cirugía Ortopédica y Traumatología.¹ The article reviews a large number of intra- and intermuscular lipomas, a relatively rare pathology. In this letter we would like to highlight some points in regard to the article in particular, and to intramuscular lipomas in general.

Firstly, we believe that intra- and intermuscular lipomas should not be included within the same group. While it is true that these lipomas have similar histopathological characteristics, they also have their own clinical, histological and imaging features. Although the authors clearly distinguish between both entities in some parts of the text, they use the term “intramuscular lipoma” to refer to the entire sample, thus generating confusion for readers. Perhaps “deep-seated lipomas” would be a more suitable term to refer to the variety of different subtypes of lipomas located under the fascia. Furthermore, intramuscular lipomas can be subclassified into infiltrative types (non-capsulated), well-defined (capsulated) and mixed (with areas of infiltration and capsulation). Except for capsulated intramuscular lipomas, which are the least common, in terms of histology, intramuscular lipomas present mature adipocytes which infiltrate surrounding muscles. This corresponds to the characteristic images showing streaks of varying width (representing muscle fibers and fibrous tissue), with occasional interruptions.² These are different from intermuscular lipomas, in which there are normally no muscular fibers within the mass and the streaks are thin and continuous, representing intermuscular fibrous tissue.

Secondly, an interesting finding of the sample is a greater prevalence of males compared to females. This is in contrast to other studies, which report intramuscular lipomas only or mostly among females.^2–4 Additionally, Nishida et al. reported a higher prevalence of males in a group of 27 intermuscular lipomas. One could ask if the combination of intramuscular and intermuscular lipomas in the study by Ramos-Pascua et al. may have altered the prevalence of males. It would be interesting to know the epidemiological data of gender of patients in each subtype of deep-seated lipomas.

Thirdly, the percentage of recurrence of intra- and intermuscular lipomas may depend on different factors. For example, recurring intramuscular lipomas may be due to inadequate tumor resection caused by its infiltrative nature and the common absence of a well-defined capsule. Su et al. suggested carrying out a detailed preoperative planning and frozen sections in order to ensure healthy surgical margins.³ On the other hand, intermuscular lipomas are usually well-defined and without local infiltration of the surrounding tissue, from which they can be easily separated. Moreover, defining the correct percentage of recurrences requires a longer follow-up period, as some of these lesions tend to recur more than 10 years after the initial resection. We agree with Ramos-Pascua et al. in that their follow-up period was too short to extract precise conclusions about recurrence.

Fourthly, we believe that cytogenetic tests would be a common approach in the diagnosis of intramuscular lipomas.⁵ Their usefulness has been proven and would include lipoma-like lesions which were previously considered as intramuscular lipomas. Perhaps a greater knowledge of the cytogenetics of intramuscular lipomas will allow us to better understand and distinguish this pathology.

In conclusion, we believe that intra- and intermuscular lipomas should be approached as two different subtypes within the group of deep-seated lipomas.

Level of evidence

Level of evidence V.