Cyber Knife – A proposal of dosimetric audit with shane phantom

Dosimetry audit is one of the major element of quality assurance in radiotherapy. IAEA (The International Atomic Energy Agency), and WHO (The World Health Organization), for a number of years, have been active in the efforts to enhance the quality and effectiveness of radiotherapy all over the world. Most of the audit methods are designed to be carried out on classical linear accelerators. At present, there is no commonly accepted and used scheme of dosimetry audit for such unconventional accelerators as the CyberKnife. The aim of this study is a proposal for the dosimetry audit scheme for the CyberKnife system. For this purpose, an anthropomorphic phantom SHANE by CIRS, designed for comprehensive testing of radiotherapy quality was used. The Phantom has inbuilt anatomical structures in the head and neck region which enables the verification of the patient position with the use of the 6D-Skull procedure, one of the techniques used for this purpose on the CyberKnife accelerator. The phantom also has inserts for placing the ionization chamber and dosimetric film. In order to verify the dosimetric and geometric parameters of the whole process of radiotherapy, a tomography of the SHANE phantom was performed. Five different PTVs (Planning Target Volume) of different volumes were drawn on the CT images (target volumes approximately 88cm³, 44cm³, 15cm³, 7cm³, 1.5cm³). For these target volumes, the treatment plans were prepared with the MultiPlan Ver. 4.6.1. system. The FIXED collimator system was used. In each plan number of collimators and their size were adapted to the contour of the PTV. Doses were calculated using the RayTracing algorithm. Each plan was verified with a QA (Quality Assurance) plan for slab phantom, according to the standard protocol used in the clinic. In case of the positive result of the QA test (acceptable dose difference up to 5%), the dose was measured with TM31010 Semiflex 0,125 cm³ ionization chamber (irradiation time ranged between 27 and 68 minutes). The percentage differences between the dose measured in the SHANE phantom and calculated with the TPS were: 4.29%, -1.42%, -0.7%, 1.37%, -1.88% (for PTV: 88cm³, 44 cm³, 15 cm³, 7cm³, 1.5cm³, respectively), were found to meet the criteria adapted in our center. Analyzing various methods of dosimetric audits for small fields to be found in the literature (percentage difference in the range 3 – 5%) we are of the opinion that the proposed dosimetric audit of the CyberKnife system, using SHANE phantom, is a correct and acceptable method of the verification of the process of radiotherapy. (author)