Diagnostic Imaging of Primary Cutaneous Osteoma of the Forehead

Osteoma cutis is a rare benign cutaneous tumor characterized by ectopic bone formation in the dermis or subcutaneous tissue [1]. Most cases of osteoma cutis are secondary to preceding inflammation, tumors, or scars [2]. However, there are also primary cases where no preceding lesion is evident. Primary cases are broadly classified into those with metabolic abnormalities, such as pseudohypoparathyroidism and Albright’s hereditary osteodystrophy [3], and those without underlying conditions. Herein, we report a case of a narrowly defined primary osteoma cutis located on a young woman’s forehead, along with various preoperative images, requiring differentiation from calcifying epithelioma or skull osteoma, accompanied by a discussion.

A 20-year-old Japanese woman presented to another hospital with a chief complaint of a slowly enlarging subcutaneous nodule on her left forehead, which she had noticed since the age of 15 years. Ultrasonography and magnetic resonance imaging (MRI) performed at the previous hospital suggested calcifying epithelioma, and she was referred to our hospital. On palpation, a firm, poorly mobile subcutaneous nodule was felt. Ultrasonography showed a well-defined, dome-like, solid subcutaneous nodule measuring about 7 mm, without any prominent blood vessels in Doppler mode (Fig. 1a). Ultrasonography revealed continuity with the frontal bone, with extension into the subcutaneous tissue that was difficult to discern (Fig. 1a). We considered benign tumors, such as calcifying epithelioma and osteoma, as potential differential diagnoses. If it was an osteoma, it was necessary to prepare a bone chisel, and if it was a calcifying epithelioma, no special instruments were required, but a spindle resection of the skin was necessary. In particular, spindle resection causes dog-ear deformity, so the difference in this surgical method is directly linked to cosmetic satisfaction. MRI displayed an approximately 7-mm mass with low signal on both T1- and T2-weighted images within the frontalis muscle, with no apparent connection to the frontal bone (Fig. 1b–e). Additional simple computed tomography (CT) scans performed at our hospital depicted a high-absorption area, measuring approximately 7 mm, beneath the frontalis muscle (Fig. 1f, g). A small area of decreased absorption was noted between the nodule and frontal bone. Because there was no obvious tumor adhesion to the skin, resection was performed by skin incision only under local anesthesia, and the nodule was dissected from the periosteum without any evident adhesion to the frontal bone. The border with the frontalis muscle was indistinct (Fig. 2). Upon histological examination, plentiful pale eosinophilic bone tissues were observed in the subcutaneous tissue and the dermis, confirming the diagnosis of osteoma cutis (Fig. 3). At the 6-month follow-up, the patient was satisfied with the postoperative wound and exhibited no neurological symptoms or signs of recurrence.

Osteoma cutis is a tumor characterized by pathologically eosinophilic bone tissue within the subcutaneous to adipose tissue [4]. Osteoma cutis can be categorized as secondary (85%) or primary (15%) [2]. Antecedent conditions such as calcifying epithelioma, scarring, burns, acne vulgaris, trauma, and nevocellular nevus are considered potential causes of secondary cases [1]. Primary cases are often associated with metabolic abnormalities, such as pseudohypoparathyroidism and Albright’s hereditary osteodystrophy, or those without underlying conditions [5].

The etiology of primary lesions remains unclear; however, potential factors may include residual mesenchymal tissue of embryonic origin, stray osteoblasts, or periosteal tissue. Lever categorizes narrowly defined primary osteoma cutis into four types (Table 1) [6]. Type 1 is a widespread osteoma present at birth or early postnatally throughout the body; type 2 is a large, single-plaque-like osteoma present at birth intradermally on the scalp, extremities, or in the subcutaneous tissue; type 3 is a single small osteoma appearing after adolescence at various sites; type 4 is a multi-plaque-like osteoma occurring on the face in women. Previous reports have demonstrated that CT is valuable and accurate for detecting osteoma cutis [7]. It can evaluate the localization of bony tissue beneath the skin. Similarly, ultrasonography can be beneficial for efficiently assessing subcutaneous bone tissue in an outpatient setting. Surgical excision is the primary treatment option for osteoma cutis. Additionally, exfoliative curettage, topical tretinoin [8], as well as yttrium aluminum garnet (YAG) [9] and CO₂ lasers [10] have been reported. If pathology following excision reveals osteoma cutis, the presence of a history of acne vulgaris, trauma, or tumors in the same area should be reconfirmed. If no such history exists, it is essential to consider screening for genetic disorders and hyperparathyroidism, such as Albright’s hereditary osteodystrophy, pseudohypoparathyroidism, or pseudo-pseudohypoparathyroidism [2]. Bone lesions manifest in the skin due to the lack of response of the target organ to parathyroid hormone. In addition to skin symptoms, some patients may show specific phenotypes such as growth retardation, cognitive impairment, dental abnormalities, and brachycephaly [11].

The patient was a young woman who had been aware of the nodule on her forehead since adolescence. She had no history of significant trauma or recurrent acne vulgaris and no evident nodular lesions or other abnormalities on the head CT scan. Her blood calcium, phosphorus, and parathyroid hormone levels were within the normal range. Therefore, she was finally diagnosed with the single small type of narrowly defined primary osteoma cutis based on Lever’s classification. Further genetic testing was not conducted. Table 2 presents the differentiated diseases that are important to recognize for surgical excision of small subcutaneous tumors in the forehead region [12‒17]. Preoperative assessment with ultrasonography, MRI, and CT assists in diagnosis by evaluating adhesions to the frontal bone, continuity with the intracranial cavity, and internal contrast effects. MRI revealed low and high signal intensities on T1- and T2-weighted images in extracranial meningiomas fibrous dysplasia [18]. Aneurysmal bone cysts exhibit variable signal intensity on T1-weighted images and high signal intensity on T2-weighted images [18]. Previous literature has reported osteosarcoma occurring in the frontal bone, similar to calcifying epithelioma, which exhibited an irregular signal pattern on MRI. Malignant tumors on the skull are exceedingly rare [19]; however, malignant and benign tumors should be considered. Ultrasonography, conducted in this study, offers a straightforward initial screening method for detecting a mass; however, its boundary with the frontal bone was unclear. This demarcation appeared unclear on MRI in both T1- and T2-weighted sequences. Nonetheless, MRI can be valuable for distinguishing it from other mass lesions. CT proved to be the most effective modality for determining the mass from osteoma in this study, surpassing ultrasound and MRI, as it provided the clearest visualization of the continuity between the mass, frontalis muscle, and bone within the high-absorption zone. MRI revealed low intensity on T1- and T2-weighted sequences, narrowing the preoperative differential diagnosis to calcifying epithelioma and skull osteoma. The diagnosis of osteoma cutis was confirmed based on the extent of intraoperative bone adhesion and pathological findings. The patient presented with a solitary, small tumor that had developed in adulthood, which was subsequently considered a single, small cutaneous osteoma. We performed preoperative ultrasonography, CT, and MRI for a rare case of narrowly defined primary osteoma cutis. Even for small subcutaneous forehead tumors, performing preoperative imaging evaluations is essential, and the aggregation of imaging findings is imperative. The CARE Checklist has been completed by the authors for this case report, attached as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000543125).

All procedures adopted in the present study respect the ethical standards of the World Medical Association Declaration of Helsinki. Ethical approval was not required for this study, following local guidelines. Written informed consent was obtained from the patient to publish this case report and any accompanying images.

The authors have no conflicts of interest to declare.

The authors received no funding for any aspect of this manuscript.

Soma Nakaso: acquisition of data and writing of the paper. Nobuaki Ishii, Masataka Akimoto, and Rei Ogawa: critically revising the work. Rei Ogawa: acquisition of data and critically revising the work. All authors have given the final approval of the version to be published and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

The data that support the findings of this study are not publicly available due to their containing information that could compromise the privacy of research participants but are available from the corresponding author upon reasonable request.