DISCUSSION :
Primary tumors of the sternum are rare, accounting less than 1% of all primary bone tumors[3]. Ala-Kulju and al. reported only 6 cases of primary sternal tumor in 20 years [4].Most of these tumors are malignant and the incidence of metastatic lesions, mainly from thyroid, kidney, lung, breast or prostate gland cancers, is equal to primary ones [4]. Despite of predominance of chondrosarcomas, other tumor may be presented as a bone mass, like lymphomas. Primary bone lymphoma (PBL) are infrequent and represents 5-7% of primary bone tumors, and 4-5% of extra-nodal non-Hodgkin lymphomas (NHLs)[1, 2]. Extremities are the most common sites of PBL (femur 27%, pelvis 15%, tibia and fibula 13%), and so sternal localization is very rare[1].
The absence of specific signs of sternal lymphomas makes the diagnostic difficult. In fact, clinical signs are usually a firm sternal painful swelling, progressing since several weeks[3]. General symptoms like night sweating and loss of weight can be absent[1]. Radiological explorations often show a lytic mass with effraction of the bonny cortex[5]. Extension can be found to both deep and superficial layers and lymph node involvement is less frequent in bone non-Hodgkin lymphomas [5]. The local extension is better explored by an MRI than by a scanner [6], and comparing to the rest of radiological tools, Positron Emission Tomography coupled with computed tomography is more sensible in the assessment of extension, especially in multifocal forms, and helps in the follow up after treatment[1]. All these elements explain why the diagnostic represent a challenge on itself.
Taking into consideration the frequency of malignant nature of sternal tumors, a histopathological confirmation is often necessary before any treatment, especially that in some cases, sternal resection with a curative oncological intent may be large with a complex reparation[2]. Needle biopsy is considered as insufficient to make the diagnosis, and a surgical biopsy is often needed[4]. In our case, and because of the special conditions with the COVID-19 epidemic and the rapid progression of the mass, we did not perform a surgical biopsy, and we erroneously considered the sternal mass as a chondrosarcoma. Indeed, there is no radiological signs that are specific to a particular type of tumor. Furthermore, malignant degeneration of untreated benign tumors was reported, justifying an aggressive approach in some doubtful cases[3]. In the case reported by Faries and al, surgical resection was maintained even after an open biopsy with frozen examination in an antalgic intent and to reduce the size of the tumoral process [5].
The diagnostic of DLBCL is based on histological examination with immunohistochemical testing. Referred to World Health Organization Classification of Soft Tissue and Bone Tumours, the group of a single skeletal site with or without regional lymph node involvement is considered primary lymphoma of bone. Expression of CD20 represents a therapeutic target. Indeed, anthracycline-containing chemotherapy coupled to Rituximab is considered currently as the first line treatment of CD20-positive DLBCLs [1]. Radiotherapy at a dose of 30 Gy may be indicated after chemotherapy for the treatment of non-Hodgkin lymphomas including extra-nodal sites, as it was revealed on a British randomized trial. Higher doses may be reserved in cases of suboptimal response to chemotherapy [1]. Surgical resection, actually, do not have a place in the treatment of DLBCLs instead of surgical biopsy.
Conclusion : The diagnosis and treatment of sternal tumors may represent a real challenge. Even if chondrosarcomas are the most common, rare tumors such as DLBCLs must be kept in mind, especially that treatment strategies are different.