Far above: Radiation coverage with proton therapy; no exit dose. Above: Radiation coverage with photon therapy; exit dose exists
Skull base chondrosarcomas are rare malignant tumors typically arising from bone. They are a diverse group of neoplasms defined by transformed cells forming a cartilaginous matrix. Although typically slow growing, these extra-axial tumors are associated with high morbidity due to their location and local aggressiveness. Without effective treatment, progressive tumor enlargement can lead to compression or invasion of local structures including cranial nerves, brain stem and the optic pathways. Patients typically present with headaches and diplopia (double vision).
A 40-year-old right-handed male, initially presenting with newly onset double vision, was evaluated by Johns Hopkins Skull Base Neurosurgery for a second opinion. Fine resolution MR imaging revealed a skull-based intracranial lesion involving the clivus and the left cavernous sinus suggestive of a chondrosarcoma.The patient underwent a surgical resection, and pathologyconfirmed a diagnosis of primary chondrosarcoma.
After case presentation and review at the Johns Hopkins multi-disciplinary tumor board, the patient was referred to the John Hopkins Proton Center for consultation.
Post-operative adjuvant radiation therapy significantly lowers the incidence of recurrence compared to surgery alone (44% to 9%). Bloch OG, Jian BJ,Yang I, et al. Cranial chondrosarcoma and recurrence. Skull Base. 2010;20(3):149- 156. doi:10.1055/s-0029-1246218. Given the critical location, overall favorable outcomes and high radiation doses necessary to treat skull-based chondrosarcomas, every effort is made to reduce the radiation dose to nearby critical structures including brainstem and optic pathways.
Proton therapy is the preferred form of adjuvant radiation for skull base tumors including chordomas and chondrosarcomas. Based on the radiation resistant histology and the challenging tumor location, proton therapy is uniquely suited to provide improved local control and survival. Improved treatment planning and delivery provides careful controlled depth of the radiation beam and sharp dose fall-off near critical structures. Optimized proton dose distribution can achieve increased conformality with the use of multi-field optimization (MFO) pencil beam scanning techniques.These techniques modulate the proximal aspect of the individual beam and allow for increased sparing of normal tissue.
ASSESSING THE BENEFITS OF PROTON THERAPY
Christina Tsien, MD, Johns Hopkins Proton Therapy Center Professor (PAR) of Radiation Oncology and Molecular Radiation Sciences
A team of clinicians, physicists and dosimetrists carefully undertook comparative planning with proton and photon radiation treatment. A plan comparison was assessed for both target coverage and the ability to safely limit radiation dose to critical structures and healthy tissues.The comparison of dose distributions between proton and photon therapies revealed significant differences, highlighting the benefits of highly conformal intensity modulated pencil beam proton therapy, advanced image guidance with CBCT and robust planning strategies.
In the present case, the proton plan was able to achieve significantly better tumor coverage of approximately 99 percent (prescribed dose of 72 Gy) compared to 91 percent for the photon plan.The maximum dose to the brainstem was 57 Gy for the proton plan compared to 63 Gy for the photon treatment plan.The mean dose of 4.8 Gy to the brainstem achieved for the proton plan was significantly lower than the 24 Gy of the photon plan.
The patient received a biological equivalent dose of 72 cobalt Gy delivered over 40 treatment fractions of proton radiation over 8 weeks. No additional systemic therapy was given. Patient tolerated treatment well with limited toxicities and is now being seen for routine follow-up visits.
A recent study of 159 patients with skull base chondrosarcomas, confirmed the efficacy of systemic high dose adjuvant proton therapy to achieve high local control rate with minimal toxicity.The authors noted that “at present, surgery designed to achieve removal of the tumor with a relatively low risk of disability, followed by high-dose proton beam therapy, appears to achieve the best long-term results in skull base chondrosarcoma.” Feuvret L, Bracci S, Calugaru V, et al. Efficacy and Safety of Adjuvant Proton Therapy Combined with Surgery for Chondrosarcoma of the Skull Base: A Retrospective, Population-Based Study. Int J Radiat Oncol Biol Phys. 2016;95(1):312-321. doi:10.1016/j.ijrobp.2015.12.016.
The lower red arc with the arrows is the radiation dose with proton therapy and that the higher arcs are the radiation dose with photon therapy.
To refer a patient or find out more about the Johns Hopkins Proton Therapy Center, visit hopkinsproton.org.