Radiological properties of high energy x-rays inside the in house developed heterogeneous thorax phantom

摘要

Phantoms are the devices which have been used in medical physics since the beginning. Soon after the discovery of x-rays physicist develop phantoms for diagnostic as well as for dosimetric purpose. Earlier it was thought that in order to quantify the dose delivered to a tissue of interest; the measurement should be made on the tissue itself. When the harmful effects of radiation were realized, the need for tissue substitute became clear, and thus the concept of phantom came in to the picture. As human body consists of almost 60 percent of water thus the dose measurement is done on the homogeneous phantom which is water equivalent. Whereas advance diagnostic techniques clearly suggest that the human body is complex heterogeneous medium comprises of varied densities. To study the radiological properties of photon inside heterogeneous medium thus necessitate developing a heterogeneous phantom for dosimetric purposes. However, the inconsistency of tissue equivalent materials still presents a large obstacle. The selection of the appropriate materials is critical to the design and function of any type of phantom. Though the anthromorphic phantoms are commercially available but they are not cost effective and as accurate as human tissues. Present study focuses on designing of in house cost effective thorax phantom (TP) and to study the isodose distribution of 15 mega voltage (MV) x-ray beam inside it. The TP was built by replicating the actual heteroginties of the human thorax. The lung proto-type was made with the porus wooden dust of pine, the rib cage with bone equivalent material and the soft tissue part by bee's wax. The density distribution on CT images of thoracic region for 20 patients and inside the TP was calculated using Hounsfield number. The depths of different isodose curves were measured on CT images of both the mediums for 10×10 field size using 15 MV photon beam. Density of lung, chest wall, and soft tissues were found to be similar in actual patient a nd in design TP. The isodose depths for patient and for TP are found to be almost same. As the radiological properties of developed in house phantom are found to be equivalent to that of the actual thoracic region of human in terms of heterogeneities and isodepth dose profiles thus it can be use as a quality assurance tool for pre-treatment plan verification of the actual patients.