Keywords
skull base - chondrosarcoma - radiation therapy - proton therapy
A 69-year-old gentleman presented with sudden vision loss in his left eye. Imaging
revealed a large sinonasal mass involving the left sinuses, pterygopalatine fossa,
orbit, and middle cranial fossa. Biopsy was consistent with a grade II chondrosarcoma.
The multidisciplinary tumor board recommended proton beam radiotherapy, but there
were concerns that the tumor's proximity to the optic chiasm would limit the ability
to deliver therapeutic doses. Therefore, the patient underwent an endoscopic endonasal
debulking of the tumor to decompress the optic chiasm. He subsequently underwent proton
radiotherapy to a total dose of 7000cGy over 8 weeks. Twelve months post-treatment,
his imaging demonstrated no progression of his cancer.
Chondrosarcomas are a heterogeneous group of endochondral bone neoplasms comprising
approximately 6% of skull base tumors.[1] Chondrosarcomas are classified based on histology, with conventional/classic being
the most common (80%).[2] Other subtypes include clear cell, mesenchymal, and dedifferentiated. Chondrosarcomas
are graded from I to III based on histopathologic differentiation.[1]
[3]
[4]
The recommended primary treatment modality is complete surgical resection when possible.[5] However, gross total resection cannot be achieved in approximately 60 to 80% of
chondrosarcomas given frequent involvement of critical neurovascular structures.[6]
[7] Radiation therapy can be used as primary treatment for unresectable cases or adjuvantly
for positive margins in high-grade tumors. Chondrosarcomas are historically considered
radioresistant due to low mitotic activity and poor vascularity.[8]
[9] Given this, relatively high radiation doses are recommended. However, proximity
to critical structures can limit the ability to deliver doses in this range for photon-based
radiotherapy.
Proton radiotherapy (PT) was developed as an alternative to photon-based radiotherapy.
Photon radiotherapy beams have significant exit doses due to the massless, chargeless
nature of X-rays.[10] PT has markedly decreased exit dose due to its Bragg peak, a physical property of
proton beams that results in the radiation dose being deposited at a specific depth,
with rapid fall-off beyond the target. This lack of exit dose can help minimizing
damage to adjacent critical structures.[10] Doses can also be escalated while maintaining similar doses to normal organs at
risk.[11]
[12]
[13]
There is little randomized data available for PT for chondrosarcoma. Local control
rates at 5 years after PT are between 75 and 99%.[14]
[15]
[16]
[17] A recent analysis of the National Cancer Database demonstrated that patients receiving
surgery with adjuvant PT had significantly better overall survival at 5 years (95.4
vs. 82.3%) and 10 years (85.1 vs. 72.8%) compared with surgery with adjuvant photon-based
radiotherapy.[18] In a systematic review of proton versus photon radiotherapy for paranasal sinus
cancers, PT was associated with superior 5 years disease-free survival and locoregional
control.[19] Complications after PT are similar to traditional radiotherapy including fatigue,
pituitary insufficiency, hearing impairment, and visual disorders; however, they potentially
may occur less frequently or with less severity.[16]
[17]
[20]
[21] Although there is strong rationale for PT in the treatment of radioresistant skull
base tumors like chondrosarcoma, optimal treatment regimen remains controversial.
A systematic review of 33 studies demonstrated variations in radiotherapy-protocols
did not yield differences between post-treatment symptom improvement or tumor volumes.[5] Moreover, widespread utilization is limited by the relative scarcity of centers
offering PT.
