Hypofractionated Stereotactic Radiotherapy Using CyberKnife as a Boost Treatment for Head and Neck Cancer, a Multi-institutional Survey: Impact of Planning Target Volume

Aim: To evaluate the role of hypofractionated stereotactic radiotherapy (hSRT) as a boost treatment for head and neck cancer. Patients and Methods: We conducted a multi-institutional retrospective review for the outcome of boost irradiation using CyberKnife for head and neck cancer patients from the charts of four Institutes. Twenty- five patients were treated with hSRT boost for primary site with a median follow-up of 28 months. Treatment sites were 11 nasopharynx, 7 oropharynx, one hypopharynx, 3 nasal cavity or paranasal sinus and three oral cancers. All patients underwent preceding conventional radiotherapy of 35 to 72 Gy (median, 50 Gy) in 1.2- to 2Gy-fractions. The dose and fractionation scheme of the Cyberknife SRT boost was individualized and the prescribed dose ranged from 12 Gy to 35 Gy in 1 to 5 fractions (median, 15 Gy in 3 fractions). Results: There were 18 complete responses, 6 partial responses and one progressive disease, resulting in 96% (24/25) response rate. Local control (LC) rates at 2- and 5-years were 89% and 71%, respectively. Progression-free survival (PFS) and overall survival (OS) at 2- and 5-years were 70%/ 83% and 70%/ 70%, respectively. Planning target volume (PTV) at boost treatment planning and initial response were predisposing factors for PFS and OS. Patients with PTV ≤20 cm 3 showed better PFS (92%) and OS (100%) than those with a PTV > 20 cm 3 (PFS, 61% and OS, 47%). Good initial response predicts better outcome in LC, PFS and OS. Conclusion: The results of the present study showed potential benefits of the CyberKnife hSRT boost. Smaller PTV and good initial response predict good outcome. External-beam radiotherapy with or without concurrent chemotherapy is generally considered a standard treatment method for head and neck cancer (1). However, close proximity of several critical organs, such as optic pathways, brain stem and spinal cord, sometimes limit high-dose delivery from conventional radiotherapy techniques. Recently, development of the image-guided stereotactic radiotherapy devices make it possible to deliver highly conformal radiotherapy for head and neck cancers, as is the case in central nervous system tumors (2, 3). The CyberKnife system was specifically developed to perform frameless stereotactic radiosurgery for intracranial lesions and the technique can now be applied to deliver conformal doses of radiation to tumors throughout the entire body including the head and neck region (2, 3). Although the effects of normal tissue sparing can theoretically allow the use of hypofractionation, necessity of therapeutic and prophylactic nodal irradiation make it difficult to use large dose per fractionation for relatively large target volume in the head and neck region. Thus, at first, hypofractionated stereotactic radiotherapy (hSRT) is mainly used for salvage treatment of locally-recurrent tumors (4). It has been