Instrumental Quality Control of Therapeutic Linear Accelerator Performance

Mohammed Ahmed Ali Omer

doi:10.11648/j.ajpa.20170505.12

Abstract

The objective of the article was to assess therapeutic linear accelerator performance. Material & method used were quality control tools, direct measurement & theoretical calculation methods. The analysis of results showed that: shift of machine isocenter was 1 mm then increases up to 2 mm through the gantry angles 0 to 300° and 300 to 360 respectively. The diaphragm rotation isocenter clock & anti-clock wise was 1mm. the light and radiation fields showed concise matching up to 9×9 cm, then for 10×10, 14×14 and 16×16 cm there were incongruence by 0.25, 0.3 and 0.41 cm respectively. The increment of the field sizes (2×2, 4×4 - 20×20) cm following SSD increment fitted with the inverse square law significantly (R2 = 1). The theoretical (calculation method) field size was greater than the measured (practical) field size relative to SSD by 0.2 cm. The system output in Gy/Mu increases significantly (R2 = 0.9) as the field size increases in logarithmic equation; while it decreases as SSD increases. The measured output on phantom surface was greater (0.8Gy/MU) than that calculated theoretically which was (0.5 Gy/MU). A significant (R2 = 0.8) reduction in output reading following the increment of temperature for Linac 10 MV and 6 MV respectively, while the pressure lead to significant (0.6) increment of system output reading. TLD showed narrow penumbra extension as 0.32 and 0.2 cm for lianc 6MV and 10MV respectively compared with 0.5 and 0.3 cm at maximum depth dose when obtained from dose histogram.

Highlights

The linear accelerator (Linac) has been one of the most favorable and commonly used in developed countries as radiation teletherapy machine offering dual function as electron and x-ray photons, leading for eclipsing of 60Co-teletherapy machine among these countries, while 60Co-teletherapy machine commonly utilized in developing countries
Linear Accelerator Performance, laser beams of patient setup, machine isocenter and the beam distribution with relative penumbra profile which are all in turn will be under focus and as a trend of this study
Method of Theoretical and Practical Field Size. To carry out such correlation, different field sizes of 20×20, 14×14, 12×12, 10×10, 8×8, 6×6, 4×4, 2×2 cm2 with different SSD = 90, 95, 100, 105, 110, 115, 120 cm have been adjusted and the relevant light fields have been calculated based on the inverse square law and from the measurement of light field projected over the graphic paper and the SSD decreased from 100 cm to 95, 90 cm from 100 to 105, 110, 115, 120 and the relevant field sizes have been measured and calculated

Summary

Introduction

The linear accelerator (Linac) has been one of the most favorable and commonly used in developed countries as radiation teletherapy machine offering dual function as electron and x-ray photons, leading for eclipsing of 60Co-teletherapy machine among these countries, while 60Co-teletherapy machine commonly utilized in developing countries. The source of errors have been increases as the high technology and complexity of the system being adapted, the need for QC and routine inspection are inevitable to trigger and classify the tolerance and action levels before errors have an impact on patient care [4], as it has been noted that implementing of QA program in radiotherapy will contribute in prevention of systematic errors as well as reducing the random errors [5,6,7] In relation to such trend; many parameters could influence the accuracy of radiotherapy plan, for instance: variation in radiation field size (FS), source to surface distance SSD, system output, environmental factors affecting radiation dosimetry. A change in temperature of 10°C causes a change in air density due to its moisture content of only about one-half per cent, the air density changes are due to variations in temperature and pressure and in turns it is density (unconfined air) varies inversely as its absolute temperature and directly as its pressure based in equation (1) [8], the Rontgen is uniquely defined when these conditions are given and to fulfill this requirement the definition states that the ionization shall be measured in air at a temperature of 0°C and a pressure of 760 mm mercury and the correction of density d at 0°C (273°, absolute) and 760 mm mercury may be calculated from the density dt at temperature t°C (273 + t, absolute) and pressure p by the equation (2):

Objectives

Methods

Conclusion