Abstract
It is possible to obtain a quality assurance (QA) of the dosimetry within a short time by using the new mathematical tools for a water phantom where dose measurements were made at two points only for a few square field sizes of the linear accelerator beam. The human body is not homogeneous. Water phantom makes it possible to create inhomogeneous phantoms by introducing blocks within it at suitable position to simulate body organs that may affect the dosage significantly. Two low cost inhomogeneous phantoms were developed using cork sheets and acrylic blocks to simulate the effects of normal lungs and cancerous lungs respectively using finite geometry and layer geometry. Monte Carlo Simulation was performed for each of these phantoms and detailed vertical and horizontal dose measurements were carried out. Percentage Depth Dose (PDD) measurements performed for the two point formalisms fixed at 100 cm Source to Surface Distance for both the homogeneous and inhomogeneous mediums and were compared with the doses generated by a Treatment Planning System. The quality of the methodology has ascertained firstly for a homogeneous medium. The formulated formalism of Tissue phantom ratio (TPR) was employed for inhomogeneous media particularly for finite and layer geometry using scattering factors obtained initially from detailed depth dose measurements. TPR conversion factors from homogeneous to inhomogeneous geometry were determined. The scattering factor was determined as a ratio of the depth dose in inhomogeneous medium and homogeneous medium. The quality factors of TPR values of homogeneous to inhomogeneous TPR conversion factor were also calculated. For all cases, the present results gave values which agreed very well to either actually measured values or with values calculated using TPS and these were also less than the international standard of deviation of 5%. The low cost inhomogeneous phantoms through modifications of the water phantom deliver better information on QA consuming less time than before and offering better QA than a detector array. The present work will have an impact on the quality assurance of dosimetry and safety of radiotherapy.
Highlights
It is estimated that approximately 40% of all cancer patients undergo radiation therapy with curative or palliative intent
The present study described the conversion of the measured data values into a comprehensive and consistent data set by the simulated formula that gives the Tissue phantom ratio (TPR) from Percentage Depth Dose (PDD) with depth as a function of field sizes
The comparative study of measured depth dose DD(M)H and TPS generated depth dose DD(TPS)H in homogeneous medium are shown in Figure 3 for different field sizes
Summary
It is estimated that approximately 40% of all cancer patients undergo radiation therapy with curative or palliative intent. The main objective of radiotherapy is to enhance the tumor control by delivering maximum dose to the target, while reducing dose to the normal tissues [4]. It is, generally, International Journal of Clinical and Experimental Medical Sciences 2017; 3(2): 23-29 given with the external beams of photons or electron produced by a medical linear accelerator. One is overall QA done in details initially during commissioning of the system, and periodically, typically once each year
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