Image-guided, intensity-modulated radiation therapy (IG-IMRT) for skull base chordoma and chondrosarcoma: preliminary outcomes

Skull base chondrosarcoma arises from endochondral bone in the cerebellopontine angle, paranasal sinuses, parasellar area, and clivus. Chordoma arises from ectopic notochord remnants, and approximately a third are within the skull base.1 Although both tumors are generally indolent and considered low grade, they are locally destructive, invasive, and fatal if not properly managed. The focus of this report is on skull base chordoma and chondrosarcoma. Surgery is the first line of therapy, with the aim to maximally debulk gross tumor.2 Optimally, a gross total resection (GTR) is performed; however, in the skull base, the location is such that critical structures are inevitably intricate to the tumor position, making an en bloc, or GTR, often not possible. In fact, residual tumor is often left intentionally as a consequence of preserving function depending on where the tumor is, and subtotal resection (STR) is often the postoperative result. Regardless of the extent of resection, local failure rates with surgery alone can be significant, and adjuvant radiation is considered a standard therapeutic option postoperatively. Within the radiation oncology department at the University of Toronto, our practice has been to offer all patients with skull base chordoma and chondrosarcoma adjuvant radiotherapy. As the therapeutic intent of radiation is to reduce the risk of local recurrence and increase survival, high doses of radiation are required, as these tumors are considered relatively radioresistant.3,4 However, similar to surgery, the inherent proximity to several organs at risk (OAR) presents a major challenge with respect to covering the target volume with the prescribed high dose. While we often make sacrifices to respect the OAR radiation tolerance, we also frequently allow greater dose exposure than traditionally deemed acceptable. Hence, radiation is also a high-risk procedure and requires treatment in centers with experienced practitioners. The historical inability of previous linear accelerator technology to shape and modulate the radiation beam is why largely ineffective low doses of radiation (ranging from 40 to 54 Gy in 1.8–2.0 Gy/d fraction sizes) were delivered, and yielded suboptimal local control (LC) rates.5 As a result, some even questioned the role of radiation altogether, in particular for chordoma. As proton therapy became available for medical use, chordoma and chondrosarcoma were amongst the first to be considered for this technology. The advantage for protons compared with conventional radiation lies in the dose profile, which allows a predictable edge in the dose fall-off with minimal exit dose.6 As a result, protons allow for higher doses to be deposited adjacent to the OAR, hence protons have become a standard of care. Over the last decade, photon-based linear accelerator technology has undergone a transformation. Multileaf collimators (MLCs) allow for intensity modulated radiotherapy (IMRT), while on-board image-guidance (IG) systems permit near-real-time tracking during delivery, and robotic technology has been incorporated to ensure millimeter precision in dose delivery. Ultimately, this translates into the ability to create sophisticated dose distributions with steep dose gradients that differentially dose the OAR and target, a reduction in the planning target volume margin, and excellent precision in the delivery, such that several centers have adopted IG-IMRT for skull base chordoma and chondrosarcoma, delivering equivalent doses to those typical of proton therapy.7–10 We report initial outcomes using high-dose IG-IMRT.