tate transaminase was elevated at 98 IU/L (normal < 35 IU/L), but the alanine transaminase level was 39 IU/L (normal < 35 IU/L). The gamma glutamyl transpeptidase was 75 IU/L, within the normal range. The blood urea nitrogen and creatinine levels were within the normal range. The serum lactate dehydrogenase was elevated at 643 IU/L (normal < 450 IU/L). The serum total protein was 5.6 g/dL with a serum albumin of 2.2 g/dL. The serum alkaline phosphatase was 252 IU/L (normal < 126 IU/L). The serum creatine phosphokinase and myoglobin were both elevated at 309 (normal < 100 IU/dL) and 149.0 ng/mL (normal < 70 ng/mL), respectively. The blood culture was negative.

On the initial contrast-enhanced chest CT scan (Brilliance 64; Philips Medical Systems, Cleveland, OH, USA), there was a cortical disruption in the sternum, and air bubbles were also seen in the sternal marrow and retrosternal space. Gas was bilaterally spread throughout the pectoralis muscles to the axilla (Fig. 1A). The soft tissue surrounding the sternum was diffusely enhanced from the level of the 2nd costosternal junction to the 5th. Especially, at the level of the 3rd costosternal junction, lobular enhancing soft tissue masses were suspected anterior to the sternum (Fig. 1B). We suggested sternal osteomyelitis with gas gangrene. Additionally, the CT images demonstrated peripheral consolidation in both lower lobes and a minimal amount of bilateral pleural effusion, thought to be due to pneumonia. However, the whole lung evaluation using the lung window settings revealed no definable lung masses or nodules.

Fine needle aspirations for the pectoralis muscle and sternal marrow were performed with Gram staining and both the aerobic and anaerobic culture. Empirical intravenous antibiotics were then initiated (ceftriaxone, clindamycin, and gentamicin). All Gram stains and cultures from the aspirates were negative. The polymerase chain reaction was negative for both tuberculosis and non-tuberculous mycobacteria. Fungal cultures were negative. Even after a 2 week-course of empirical antibiotics and analgesics, the soft tissue swelling on the anterior chest wall, especially around the sternum, did not regress. A follow-up CT scan was then obtained.

On the second contrast-enhanced CT scan, the previously observed extensive gas through the pectoralis muscles had resolved. However, the enhancement of soft tissue surrounding the sternum remained without change. An incisional biopsy for the patient's sternum and the suspicious soft tissue masses was performed. The histopathologic results were consistent with metastatic, poorly-differentiated carcinoma, most likely squamous cell carcinoma (Fig. 1C).

Then, the patient underwent a work-up looking for the primary malignancy. A whole body positron emission tomography (PET)-CT scan was performed and demonstrated multiple increased uptakes suggesting bone metastases to the axial skeleton, including the spine, rib, pelvis, and skull base, especially the clivus (Fig. 1D). Pelvic nodal metastases around the prostate were also revealed. In addition, brain MRI was performed to evaluate the skull base. A metastatic mass involved the clivus, but there was no evidence of a primary tumor in the nasopharynx.

\begin{figure}
\centering
\begin{subfigure}{0.24\textwidth}
\includegraphics[width=\linewidth]{figure_1_1.pdf}
\end{subfigure}
\begin{subfigure}{0.24\textwidth}
\includegraphics[width=\linewidth]{figure_1_2.pdf}
\end{subfigure}
\begin{subfigure}{0.24\textwidth}
\includegraphics[width=\linewidth]{figure_1_3.pdf}
\end{subfigure}
\begin{subfigure}{0.24\textwidth}
\includegraphics[width=\linewidth]{figure_1_4.pdf}
\end{subfigure}
\caption{A 69-year-old man who suffered from fever and pain in his anterior chest wall. (A) Soft tissue surrounding sternum is enhanced. Sternal marrow cavity is also enhanced, and cortical disruption was evident. Gas was spread through the pectoralis muscles to the axilla, bilaterally. These findings are interpreted as sternal osteomyelitis with gas gangrene. (B) At the level of the 3rd costosternal junction, lobular shape, hyperdense, soft tissue masses are suspected anterior to the sternum (arrow), and infiltrated into the marrow cavity of the sternum and retrosternal space. (C) High power field ($\times$ 400) of the specimen from parasternal soft tissue contains poorly-differentiated carcinoma, most likely squamous cell carcinoma. (D) Whole body positron emission tomography-CT demonstrates multiple bone metastasis to the axial skeleton, including the spine, rib, pelvis, and skull base, as well as intra-abdominal nodal metastasis around the prostate.}
\label{figure_1}
\end{figure}