Design and Fabrication of Water Phantom for Treatment Verification in High Dose Rate (HDR) Brachytherapy of the Cervix

Abstract

The aim of this study is to design and fabricate a local anatomical water phantom for dose distribution measurements during high dose rate brachytherapy of the cervix. Phantoms for dosimetry are used when there is a need to simulate the conditions of a procedure to measure dose at certain points of interest. It is dangerous to place a human being in a beam for dose measurements. Perspex (PMMA) sheets of thickness 6 mm and 10 mm were used in fabricating the phantom. The phantom designed is cuboid in shape with predominantly 6mm thickness of the perspex material. It has a height of 41 cm and breadth 31 cm. The pieces of perspex material were glued to each other using Trichloromethane (chloroform) at room temperature. It is an organic compound with the formula CHCI3. A perspex sheet of 10 mm was used to make one end of the phantom thicker than the other. This was done to support the reconstruction box. Two thin film holder slabs of dimensions 31.5 × 2.8 cm were fabricated by joining two sheets of 6 mm perspex together with the chloroform. A small cavity of 2.8 × 2.5 cm was created in the slab to represent the bladder and the rectum (organs at risk). These same cavities are meant to hold the detectors (films) in place for the measurement of dose to the organs at risk. The film holders with the cavities were then positioned vertically, but anterior and posterior to each other. Another holder, rectangular in shape was fabricated to hold the film holders. This holder was made from perspex sheets of thickness 6 mm and 10 mm with dimensions of 6.8 × 6.8cm. This is to allow for the distances between the bladder and the rectum to be varied during the dose measurements. It must be noted that the anatomical distance between the bladder and the rectum which are posterior and anterior to the cervix vary from patients to patients. Special clamping devices were fabricated to hold the applicators in a firm position during the intracavitary brachytherapy insertions. These special clamps were made by joining perspex of 10 mm thickness in a cuboid shape; a hole was drilled in them to enable a plastic screw to lock the applicators. The relative electron density of the phantom designed was determined to be 1.069 which is comparable to water. This makes the phantom suitable for dose distribution measurements.