Discussion
Bony metastasis is a common manifestation in many disseminated malignancies [1]. However, metastases to the hands or feet are rare occurrences, only accounting for 0.1% of all bone metastases [2]. The presence of acrometastasis is an indicator of poor prognosis as it is usually found in patients with widespread malignancy, with shortened mean life expectancy and grave prognosis [3]. Acrometastasis is usually diagnosed in patients who have known cancer but there are some reports of it being the initial presentation of an occult malignancy [4]. Lung cancer has been reported as the most common primary malignancy associated with acrometastasis, followed by renal cell carcinoma, colorectal cancer and breast cancer [5,6].

These patients usually present with pain, soft tissue swelling, erythema and loss of function [7]. The pain is usually described as deep, and refractory to analgesia. Sometimes, ulceration and bleeding may develop. These signs and symptoms mimic an inflammatory process which may include several differential diagnoses such as osteomyelitis, gout, rheumatoid arthritis, tuberculous dactylitis, pyogenic granuloma and tenosynovitis [8].

It is known that bony metastasis usually involves bones rich in red marrow, such as the vertebrae. Small bones of the extremities such as the phalanges lack red marrow. This may be the reason why acrometastasis is so rare [3]. The exact pathophysiology of metastasis to hands or feet is unknown, but there have been several postulations. According to the latest literature review by Stomeo et al, the tumour cells spread to the extremities via blood circulation and not lymphatic spread. This could explain the reason why the majority of acrometastases are from primary lung cancers as they have direct access to the systemic arterial circulation, whereas tumour emboli from visceral malignancies such as renal cell carcinoma, breast cancers and colorectal cancers have to pass through the capillary beds of the pulmonary and hepatic system first, making them even more rare [3]. Healey et al have suggested that acrometastasis occur more frequently in dominant hands because they receive more blood supply compared to the non-dominant hands [9]. There was also an older hypothesis by Joll et al in 1923 that suggested trauma induced acrometastasis. He described that repeated injury may reduce the surrounding tissue’s resistance, allowing tumour emboli to seed and grow in the bones [10]. This theory is supported by another author who suggested that the release of chemotactic factors, such as prostaglandin, after trauma, may promote the migration and adherence of cancer cells to the bones [11].

There are no standard protocols pertaining to the treatment of acrometastasis, especially as these cases are so rare [2,3]. Due to their poor prognosis, most of the cases are usually evaluated on a case by case basis with treatment mainly aimed at palliation, adequate resection of tumour, relief of pain, for quick recovery and preservation of maximal hand function [2,12]. Amputation is the most common treatment option, especially for acrometastasis to the distal phalanges. Radiotherapy has also been shown to be able to achieve pain relief and gain local control of the disease. This is useful for acrometastasis involving areas such as metacarpals where amputation is not an option [6].

Conclusion
Acrometastases are rare and is usually linked to poor prognosis due to disseminated disease. As demonstrated in this case, an acrometastasis may mimic a benign disease such as an infection or inflammatory lesion. However, as the survival rate of cancer patients increases with the advancement of cancer therapy, we may see an increase in these cases in the future. Hence, physicians should have high index of suspicion for possible metastatic malignant lesion in patients presenting with painful swollen digit. A timely diagnosis and treatment of this condition may potentially improve patient’s quality of life.