Incidental detection of superior sternal cleft on Technetium-99m methylene diphosphonate bone scan

INTRODUCTION

A congenital midline cleft of the sternum is a rare developmental anomaly, usually occurring as a part of a defined syndrome, e.g., Cantrell's pentalogy.[1] It results from failed ventral midline fusion of the sternal bars, which normally occurs during the first 3 months of embryonic life.[12] Isolated sternal cleft without any associated anomaly is very uncommon and presents in early infancy because of the obvious cosmetic deformity.[1] The anomaly is more common in females.[3] We report an extremely rare case of isolated upper sternal cleft in an adult male patient, found incidentally on technetium-99m (Tc-99m) methylene diphosphonate (MDP) bone scan.

CASE REPORT

A 45-years old male patient, who was a recently diagnosed case of moderately differentiated adenocarcinoma of prostate with Gleason's score of 3 + 4 = 7, was referred for Tc-99m MDP bone scan to rule out skeletal metastases. He complained of upper backache for 2 months and had a history of trauma to the left lower leg 1 week prior to performing the bone scan. The serum prostate specific antigen level was >100ng/ml. On clinical examination, there was a midline bony defect in the region of the manubrium sterni with no apparent bulge or paradoxical respiratory movements.

A whole body planar Tc-99m MDP bone scan and single-photon emission computed tomography-computed tomography (SPECT-CT) of the thoracic region was performed 3 h after intravenous injection of 20 mCi of Tc-99m MDP on a dual head gamma camera (Infinia: single slice SPECT-CT; GE, USA). The study was performed with a urinary catheter in situ.

The planar images of the bone scan revealed focal MDP uptake in the D4 vertebra [Figure 1 (red arrow)], focal uptake in the mid shaft of the left tibia [Figure 1 (blue arrow)] and a U-shaped photopenia in the region of the sternum [Figure 1 (black arrow)]. Corresponding SPECT-CT images of the thorax revealed localization of the focal tracer uptake in the sclerotic metastasis in the body of the D4 vertebra [Figure 2 (red arrows)]. The U-shaped photopenia in the sternum corresponded to a bony cleft involving the manubrium and upper half of the sternum [Figure 2 (blue arrow)].

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Figure 1 The anterior (a) and posterior (b) planar technetium-99m methylene diphosphonate bone scan images show a U-shaped photopenic defect involving the manubrium and upper half of the sternum (black arrow). Focal tracer uptake in the mid shaft of the left tibia corresponds to known traumatic injury at the same site (blue arrow). Focal tracer uptake is seen at a metastatic site in the D4 vertebra (red arrow)

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Figure 2 Coronal, sagittal, and transaxial images of the single-photon emission computed tomography-computed tomography of the thorax demonstrates photopenia in the region of superior sternal cleft (blue arrow). The computed tomogram demonstrated bilateral paired sternal cartilaginous centers on either side of the wide gap in the anterior chest wall. The inferior most sternal segment and xiphoid were positioned at the midline and were covered by normal appearing cutaneous tissues. A sclerotic lesion is noted in the body of the D4 vertebra (red arrow)

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The Tc-99m MDP bone scan thus helped in diagnosing a congenital superior sternal cleft, metastasis in the body of the D4 vertebra and post-traumatic changes in the mid shaft of the left tibia.

DISCUSSION

A cleft sternum, also known as a bifid sternum or sternal fissure, is a partial or a complete separation of the two lateral sternal bars.[4] Sternal cleft is a rare anomaly with reported incidence <1% of all chest wall malformations.[5] Most isolated sternal defects are due to failure of the mesenchymal lateral processes to fuse during the 8^th week of gestation.[4] Although the embryologic development of the sternum has been studied in detail, the etiology of the cleft sternum is unknown. Congenital sternal clefts can be divided into three separate varieties: Superior, inferior and complete. The most frequently encountered form is the partial superior type, accounting for 67% of all patients, followed by the complete form (19.5%), the partial inferior form (11%) and the sternal foramen (2.5%).[6] The superior clefts have been described as U-shaped if the cleft extends to the last sternal body segment or V-shaped if it extends to the xiphoid process.[7] The superior defects are almost always isolated anomalies, occasionally associated with cervicofacial hemangiomas. Complete absence of the sternum as opposed to a sternal cleft, is usually seen in other fetal anomalies such as ectopia cordis or the pentalogy of Cantrell[4] and PHACES (Posterior fossa malformations, Hemangiomas, Arterial malformations, Coarctation of the aorta/Cardiac defects, Eye abnormalities, and Sternal defects) syndrome.[89]

Our patient had an isolated superior sternal cleft with a U-shaped photopenic defect in the manubrium sterni on the Tc-99m MDP bone scan. Only one bone scan image has been reported so far of sternal cleft.[10] The other differential diagnoses of photopenia in the sternum include sternal foramen, metal attenuation artefact and metastasis in a known case of malignancy.

CONCLUSION

To conclude, our case demonstrates that not only areas of increased tracer uptake, but areas of photopenia on a bone scan should also be carefully evaluated. Awareness about the sternal cleft and other differential diagnosis is necessary when reporting a photopenic defect in the sternum.