During all radiotherapy treatments, there is always a small unavoidable fraction of the delivered dose that is absorbed by radiosensitive tissues/organs outside the irradiated volume, known as peripheral dose (PD). PD is due to radiation that is scattered from the patients' body, the linac head components, the treatment room walls and lastly, radiation leakage from the linac head. Stereotactic radiosurgery/radiotherapy (SRS/SRT) procedures main aim is to deliver a very high dose per fraction to the target, and thus the corresponding PD outside the treatment volume is an important issue, especially for the long term surviving patients.
The main concern in a treatment plan is on how to apply the maximum dose to the target, without exceeding the dose constraints of the surrounding organs at risk. These dose constraints, which are based on clinical studies, aim at minimizing side effects (normal tissue complications), which could even be the induction of secondary cancer . The risk for secondary cancer is of a main concern especially in long term surviving patients, who are treated for benign diseases or for curatively non metastatic malignancies. Epidemiological evidence from human populations demonstrate that organ doses above 5-10 cGy for protracted exposures, or 1-5 cGy, for acute exposures, could increase the risk of some types of cancer .
The thyroid gland is a very radiosensitive organ that, although is not the target during intracranial treatments, it can be affected by scattered radiation . Especially, in young patients, it has been shown that there is a significantly increased risk of cancer in the thyroid gland, after exposure to radiation, as part of therapy in childhood cancers . The breast is also one of the sensitive organs regarding the carcinogenic effects of radiation, and there is an excessive risk of secondary cancers being induced for the breast even at doses as low as 1-9 cGy .
The Cyberknife is a frameless, image-guided, stereotactic radiosurgery system with sub-millimeter clinical accuracy . The system comprises of a 6 MV linear accelerator mounted on a robotic arm, along with an image guided system. Through the image guidance cameras (which are composed of a pair of orthogonal diagnostic x-ray tubes and corresponding image detectors), specialized software, which uses x-ray images obtained throughout the treatment, verifies the patient position, based on radiographic landmarks, such as fiducials , skull anatomy  or spine anatomy landmarks . After the initial setup of the patient, when the tumor is localized and aligned, the radiation is delivered. The treatment is modified in real time to compensate for tumor movements. Several hundred treatment beams are chosen out of a repertoire of more than one thousand possible beam directions, using inverse treatment planning. These beams are delivered in a non-isocentric manner via small circular fields of varying size and weighted with different monitor units (MU) .
To our knowledge, there have not been any other reports of PD in patients undergoing intracranial treatment with Cyberknife using Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters in the literature. However, there have been previous reports that have studied PD in stereotactic radiosurgery and radiotherapy treatments [10–14].
MobileMOSFET seems to be an appropriate dosimetry system for in vivo measurements of low peripheral doses during stereotactic radiosurgery/radiotherapy due to its extremely small size (active area 0.04 mm2), and its simple and immediate read out, compared to the thermoluminescent dosimeter, and its accuracy at low doses [15–21].
The aim of this study was to evaluate PD at preselected anatomical areas on the patient skin, corresponding to radiosensitive organs using the MOSFET dosimeters and to investigate the influence of the supplemental shielding, the number of MU and the collimator size on the PD.