Measurement of surface dose in external radiotherapy of brain frontal lobe: A study on patient and phantom

Introduction: Radiotherapy plays an important role in the treatment of most malignant and also benign primary CNS tumors. Radiotherapy affects both tumor cells and normal cells. The aim of this study was to determine the absorbed dose of radiation by the skin surfacing organs at risk (lens, parotid, thyroid and submandibular gland) during radiotherapy of tumors at the frontal lobe of the brain via direct measurement in patients and phantom. Materials and methods: There are two conventional techniques for external radiotherapy of patients with tumors of the right frontal lobe. These techniques were used to estimate the amount of surface dose absorption by organs at risk (lens of eye, parotid, thyroid, submandibular glands). Technique 1 was performed for 10 patients who referred to Reza Radiation Oncology Center. Technique 1 is the radiation of frontal field with an extension to the lateral field portion. Technique I and 2, which included radiation of one frontal field along with a bilateral field were performed for a phantom model with an anthropomorphic head and neck constructed from natural human bone with paraffin wax and NaCl as surrounding soft tissues. The surface dose for each organ was measured using TLD chips. Results: The measured radiation dose within all organs was within the safe limits in phantom and most patients, however, the surface dose were higher in patients in compare to phantom. In some patients, the right lens was not protected and therefore there would be some risk for cataracts. The findings provide estimation for probable complications along with frontal lobe radiotherapy. Conclusion: Our findings showed that regarding these two techniques, technique I impose less harm to normal tissue, since the right lobe of brain is only irradiated. Besides, the higher values of surface dose in patients compared to phantom might be due to larger tumor field in patients, because the tumor dimensions in phantom was considered 4 cm, while in some patients it was larger. © 2015, Singapore Medical Association. All rights reserved

Evaluation of organs at risk�s dose in external radiotherapy of brain tumors

Background: Radiotherapy plays an important role in the management of most malignant and many benign primary central nervous system (CNS) tumors. Radiotherapy affects both tumor cells and uninvolved normal cells; so, it is important to estimate absorbed dose to organs at risk in this kind of treatment. The aim of this study was to determine the absorbed dose to chiasma, lens, optic nerve, retina, parotid, thyroid and submandibular gland in frontal lobe brain tumors radiotherapy based on treatment planning system (TPS) calculation and direct measurement on the phantom. Methods: A head and neck phantom was constructed using natural human bone and combination of paraffin wax and Sodium Chloride (NaCl) as tissue-equivalent material. Six cylinders were made of phantom material which had cavities to insert Thermoluminescent Dosimeters (TLDs) at several depths in order to measure absorbed dose to chiasma, lens, optic nerve, retina, parotid, thyroid and submandibular gland. Three routine conventional plans associated with tumors of this region and a new purposed technique were performed on the phantom and dose distribution and absorbed dose to critical organs were compared using treatment planning system (TPS) calculation and direct measurement on the phantom. Results: Absorbed doses were measured with calibrated TLDs and are expressed in centigray (cGy). In all techniques absorbed dose to all organs except the lenses were at their tolerance dose levels and in the new purposed technique, absorbed dose to chiasma was significantly reduced. Conclusion: Our findings showed differences in the range of 1-5 in all techniques between TPS calculation and direct measurements for all organs except submandibular glands and thyroid. Because submandibular glands and thyroid are far from primary radiation field, TLD reading in these regions although small but differs from TPS calculation which shows very smaller doses. This might be due to scattered radiation which is not well considered in the TPS. In the new technique, because the chiasma is out of the radiation field, absorbed dose was reduced significantly. © 2015, Cancer Research Center. All rights reserved