Evaluation methods for detecting changes in beam output and energy in radiation beams from high-energy linear accelerators

Abstract

There is need for simple methods for checking consistency of beam outputs and energy in linear accelerators used for radiotherapy. A method was designed by the department using perspex phantom with which the dosimetric data of two medical linear accelerators (Clinac 600 CD, Clinac 2300 CD) were evaluated over a period of 30 months. The efficacy of methods followed was checked. Routine beam consistency checks were designed for photon beams with 15 cm/ 5 cm depth ionizations in perspex phantom and variable depth combinations for electron beams. Calculated ionization ratios were compared with measured values to show their significance. The dose/MU for all radiation beams was maintained within 2% accuracy over the period of 30 months. Clinac 600 CD machine showed decreasing trend of cGy/MU, while Clinac 2300 CD showed increasing trend of cGy/MU over a period, which needed tuning of monitor chamber two times each. Tuning of output to achieve standard value was carried out once, for all electron energies when the output dose/MU exceeded 3%. During one week (June 2005), there were slight changes in electron energy detected using the ratio method, which did not recur anytime afterwards. The methods designed are adequate to find the consistency in the beam output and energies in the radiotherapy linacs.

Beam configuration and physical parameters of clinical high energy photon beam for total body irradiation (TBI)

“…An earlier report [14] highlighted the efficacy of using ratios of ionization in perspex as a measure of photon beam qualities for 6 and 15 MV. In Table 3 it is observed that as the FSD changed from 2 m to 4 m, the ionization ratios for depths 20/15 and 25/15 in phantom showed slight variations in a progressively decreasing manner.…”

Estimation of absorbed dose in clinical radiotherapy linear accelerator beams: Effect of ion chamber calibration and long-term stability

“…Therefore in the year 2008 only first TL postal dose intercomparison was possible. However, beam output and energy stability have been evaluated since patient treatment with linacs began in 2005, and have been documented in a previous report[ 2 ].…”

“…[ 6 ] The method of maintaining a constant cGy/MU in our linac photon and electron beams was explained in a previous report. [ 2 ] When the daily check of output/MU showed drift beyond ±2% limits retuning is carried out by the local service personnel. [ 2 ]…”

“…[ 2 ] When the daily check of output/MU showed drift beyond ±2% limits retuning is carried out by the local service personnel. [ 2 ]…”

“…With the experience gained in maintaining an accredited laboratory elsewhere[ 1 ] in the past, an attempt has been made to scrutinize the database in terms of surveillance of dosimetry standards at our radiotherapy clinic in the present center. Objective of our present study, is referred to our earlier report[ 2 ] on status of beam level dose delivery, and correlate to realization of true absorbed dose in field conditions, in the user institution. As there is no telecobalt machine available for surveillance of dosimetry methods, an attempt was earlier made to study the efficacy of an extrapolation chamber as a ‘secondary standard’ for institutional use.…”