Using Stereotactic Radiation Systems to Irradiate and Re-Irradiate Head and Neck Cancers
Rex Cheung*, MD, PhD
Keywords: Re-irradiation; Robotic; Stereotactic; CyberKnife; Head and Neck Cancers
Head and neck cancers are challenging to treat with conventional radiation therapy. There are many critical structures that include optic chiasm, optic nerves, cochlea, eyes, temporal lobes, brain stem, spinal cord, parotid glands, mandible, esophagus and larynx . Among the head and neck cancers, nasopharyngeal cancers, skull base cancers, and recurrent head and neck cancers are the most challenging to irradiate. Recent advance in Intensity Modulated Radiation Therapy (IMRT) has allowed treating head and neck cancers and spare these critical structures without compromising the tumor control . Stereotactic radiation systems have been used to boost the tumor to further improve the outcome . This paper, a part of series of commentaries on stereotactic radiation treatment systems  use on selected challenging clinical situations, discusses the use of a robotic stereotactic machine as an example in boosting head and neck cancers and also for treating recurrent head and neck cancers [2,4].
Stereotaxy of CyberKnife
CyberKnife (Accuray Inc., Sunnyvale, CA) robotic stereotactic radiation system  uses skull tracking, Xsight spine tracking, Xsight lung tracking for peripheral lesions, implanted fiducial tracking, and Synchrony real-time tracking system. With the many degrees of freedom of the robotic arm, it could deliver very high dose accurately with active tracking. Other advanced systems may also have similar capacities [6-8]. The CyberKnife system uses robotic arm to go through a large number of nodes (positions) to optimize the dose distribution. To limit damage to the surrounding critical normal tissues, the treatment margins could be optimized. The accuracy of CyberKnife has been found to be < 1mm .
Treatment planning for stereotactic radiation and re-irradiation
The definition of the salvage target volumes are different from those used in the initial treatment, usually the margins are tighter. PET scan may be useful in defining the gross tumor volume (GTV) and clinical target volume(CTV) . For example, the CyberKnife treatment planning computer system allows multi-modal image registration including CT to MRI and CT to PET image registration. This would allow more accurate target delineation. PET and MRI may also be useful in planning initial radiation treatment.
The outcome and toxicity are generally better in irradiation of head and neck cancers with more advanced stereotactic machines [11,12]. For example, in boosting nasopharyngeal cancers, the tolerance of the optic apparatus, brain stem and temporal lobes are usually reached at about 70 Gy in 2 Gy fractions . Conventional radiotherapy is limited by these dose constraints. Stereotactic radiosurgery (SRS) has been used to boost the nasopharyngeal primary of a median dose of 66 Gy with another median dose of 12 Gy SRS (EQD2  of 22 Gy) to a total equivalent dose of 78 Gy using frame based LINAC. There was no complication related to the added SRS boost .
Not surprisingly, stereotactic radiation treatment systems have been known to be useful in treating challenging cases in head and neck cancers [6-8]. For example, CyberKnife has become well recognized to be useful in radiation re-treatment of recurrent head and neck cancers because of its robotic stereotaxy. The results of CyberKnife re-treatment are encouraging in both salvage rates and toxicity rates. CyberKnife’s robotic stereotactic system could also be used for image guidance for precise radiation delivery in conventional fractionations when the tumor is large, close to critical structures or in re-irradiation .
Selecting patients with recurrent head and neck cancers for re-irradiation may need to consider cancer control elsewhere, expected life expectancy, the proximity and condition of the critical structures, and time to radiation [1, 14].The normal tissue tolerance may be different than that of initial course of radiotherapy [15-17]. To find the combined effects of different doses and fractions between the initial radiotherapy and re-irradiation, Biologic Effective Dose (BED) and Biologic Equivalent Dose at 2
Gy per fraction (EQD2)  may be calculated, and the dosimetric guidelines may be applied. However, tumor control and normal tissue tolerance data based on actual re-irradiation clinical experience are scant [6,15,18,19]. The advantages of hypofractionated stereotactic body radiation therapy (SBRT, usually means 1-5 fractions) are that it is more effective and convenient for palliating symptoms. It has been postulated the large fractions of SRS and SBRT may overcome the radiation resistance of recurrent nasopharyngeal cancers .
A hypothetical case for clinical decision making discussion
Suppose a healthy 40 years old Asian man presented with a recurrent 2cm nasopharyngeal carcinoma and bilateral lymph nodes. He was treated with a course of chemoradiation to 70 Gy in 2 Gy fractions for a stage III nasopharyngeal carcinoma. Should this patient be re-treated in this hypothetical case?
The prognostic value of performance status has consistently been found to be important in selecting which patients will benefit from re-irradiation [15,20], this patient would have a high probability of benefitting from the re-irradiation. For rT1-T2 nasopharyngeal cancers, SRS of 12. 5 Gy (BED of about 28 Gy) has been found to be quite effective and safe (12). Larger doses have been used when the tolerance for risk of serious normal tissue complication is higher e.g. for this young healthy patient . Unlike initial treatment, there are no well accepted dosimetric guidelines for normal tissue tolerance in re-irradiation . However, the Quantec guidelines  and Task Group 101 report  may be useful in helping to select the optimal dose for the treatment. A list of the Biologic Equivalent Dose at 2 Gy (EQD2)  could be calculated for each of these critical organs, the tolerance of these structures could be estimated in the literature [15,19]. The tolerance dose levels could then be adjusted according to the toxicity tolerance based on the performance status and desire of the patient. For example, a 5-10% may be acceptable to some patients in the re-irradiation setting as opposed to the 0-5% risk in the initial treatment setting.
Stereotactic radiation systems may provide excellent capacity for irradiating and re-irradiating challenging head and neck cancers [6-8].
Cite this article: Cheung R. Using Stereotactic Radiation Systems to Irradiate and Re-Irradiate Head and Neck Cancers.
J J Rad Oncol. 2014, 1(1): 007.