Background Osteosarcoma is a relatively common bone tumour; with an incidence of 0.2 to 3/100 000, it is an orphan disease. Mifamurtide has managed to increase survival without increasing side effects.

Purpose To evaluate the safety and efficacy of mifamurtide in two paediatric patients diagnosed with osteogenic sarcoma.

Materials and Methods We conducted a prospective study of two paediatric patients diagnosed with osteogenic sarcoma. Weekly, we attended the oncology sessions and we tracked them during the chemotherapy, and after that, through the electronic clinical history.

Mifamurtide is indicated in children, adolescents and young adults for the treatment of high-grade resectable non-metastatic osteosarcoma after surgical resection. It is used in combination with post-operative chemotherapy.

In the two cases, the treatment followed the SEOP-SO-2010 guidelines of the Spanish Society of Paediatric Oncology for 37 weeks.

After surgery (week 15) mifamurtide was started as adjuvant treatment: 2 mg/m² twice weekly for the first 12 weeks and followed by once-weekly for an additional 24 weeks, for a total of 48 infusions in 36 weeks.

Results Chemotherapy started according to protocol, the patients were aged 12 and 15 years (July and November 2010, respectively).

One patient had a flu-like reaction after the first dose of mifamurtide, so the following doses were administered with premedication (acetaminophen and dexchlorpheniramine). Other side effects: anaemia and thrombocytopenia, requiring human stimulating factors and platelet concentrates; vomiting was treated with aprepitant.

When chemotherapy finished, the patients were in complete remission, this situation continues today, 10 and 13 months later.

Conclusions The SEOP protocol plus mifamurtide achieved complete remission in both cases.

The use of mifamurtide can be considered safe and it did not increase side effects, we observed only a flu-like reaction attributed to mifamurtide which resolved with premedication.

The effectiveness of mifamurtide in osteogenic sarcoma treatment cannot be considered as assessed due to the small sample size.

No conflict of interest.