Low and intermediate grade chondrosarcomas (9-13% of all malignant bone tumours) are relative rare bone tumours. In 5-12% of all cases the chondrosarcomas are localized in head-and-neck region . The typical sites of skull base lesions are temporo-occipital junction, parasellar area, spheno-ethmoid region and clivus [2, 3]. Due to hystopathological type chondrosarcomas are divided into Grade 1 to 3 tumours according to mitotic rates (WHO classification) with 3 histological subgroups: classic, mesenchymal and myxoid . The mesenchymal type has more aggressive growth behaviour and is associated with a poorer prognosis. Histological differentiation from chordomas is often difficult and must contain immunhistochemical staining [2, 4]. Chordoma is immunopositive for epithelial markers like cytokeratin and endothelial membrane antigen (EMA), whereas chondrosarcoma is negative for both. Both chordomas and chondrosarcomas can be positive for S-100 and vimentin .
Most patients diagnosed are over the age of 40 years. Low grade chondrosarcoma has a low incidence of distant metastasis but is potentially lethal disease. Thus, local therapy has a crucial importance in the treatment of skull base chondrosarcomas. Surgical resection is the primary treatment standard. Unfortunately the late diagnosis and diagnosis at the extensive size of the tumours are common due to the slow growth kinetics; most patients are asymptomatic, or develop symptoms at a late stage of the disease. Consequently, complete resection is hindered due to close proximity to critical and hence dose limiting organs for radiation therapy i.e. optic nerves, chiasm and brainstem. Adjuvant or additional radiation therapy is very important for the improvement of local control rates in the primary treatment even after complete resection (no compartment resection possible). Anyhow, chondrosarcoma have a better outcome and prognosis compared to chordoma [6, 7].
Chondrosarcomas are commonly radioresistant , and high local doses are required for long-term local control. Image guidance in conformal precision radiation therapy provides a safe technique in the treatment of base of skull tumours . The highest dose conformality is possible using particle therapy with heavy ions and protons due to inverted dose profile allowing steep dose gradients and therefore providing further benefit in reducing toxicity and irradiation safety.
It is brightly accepted, that proton therapy can be considered the gold standard for treatment of rare skull-base tumours like chordoma and low grade chondrosarcoma . Loma Linda University Medical Center (LLUMC)  the Massachusetts General Hospital (MGH) in Boston  have the longest experience in proton therapy for these entities. 3-year local control for chondrosarcomas after fractionated proton radiation therapy in 25 patients at LLUMC was 94% and the actuarial 5 year survival rate was 100% respectively .
The outcome in 229 chondrosarcomas treated with a combination of proton and photon therapy at MGH/HCL shows 5-and 10-year local progression free survival rate of 98% and 94% for chondrosarcomas respectively. The results are significantly better than in chordomas treated at the same institution .
Proton therapy results from PSI in Villingen, Switzerland were published by Weber et al. and Ares et. al. The data showed 3- and 5-year local control rates of 100% and 94%, respectively. 64 patients, among them 22 patients with chondrosarcoma were treated to a median target dose of 68.4 GyE. The 3-year actuarial overall survival rate for the chondrosarcoma patients was 91% [12, 13].
Carbon ions though, have a higher biological effectiveness than either protons or photons, which is important in case of radioresistant tumours .
Carbon ion therapy is available at the National Institute of Radiological Sciences (NIRS) in Japan and at Heidelberger Ionenstrahl-Therapie (HIT) centre. The report of our Japanese colleagues experience is limited to 40 patients with chordoma and chondrosarcoma of the skull base. The patients could be treated effectively and without serious side effects . NIRS beam delivery technique relies on passive scanning necessitating various modulators to adjust for treatment depth and tissue inhomogeneities within the beam path.
Our experience is based on the clinical work at the Gesellschaft für Schwerionenforschung (GSI) Darmstadt in Germany up to July 2008. In comparison to the Japanese centres, the facility at GSI as well as the HIT relies on active beam delivery using the raster-scan technique. About 300 patients with base of skull chordomas and chondrosarcomas have been treated so far. Initially, these patients were treated within a clinical Phase I/II study. After the study was able to demonstrate successful treatment, carbon ion therapy became approved as the best therapy available in Germany. The actuarial local control rates for the chondrosarcoma patients after 3 and 5 years was 96.2% and 89.8% respectively, the 5 year overall survival rates was 98.2% .