F.W. Wittkämper

Quality assurance of the dose delivered by small radiation segments

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.

Consistency and simplicity in the determination of absorbed dose to water in high-energy photon beams: A new code of practice

Recent revision of exposure and air kerma standards in Standards Laboratories require a simultaneous change in physical parameters at other positions in the dosimetry chain. Adoption of new data, recommended by international organizations, will introduce changes in absorbed dose determinations in high-energy photon beams using ionization chambers. A new code of practice has therefore been drafted using a consistent set of data. In this code of practice, single conversion factors are given to convert ionization chamber reading to absorbed dose to water for some types of reference ionization chamber as a function of radiation quality. Equations and recommended numerical data for the physical parameters and correction and conversion factors will be provided.

Dose intercomparison at the radiotherapy centres in The Netherlands. 1. Photon beams under reference conditions and for prostatic cancer treatment

In 1985, a dosimetry intercomparison was performed at all 20 radiotherapy centres in The Netherlands. Absorbed dose was determined with an ionization chamber under reference conditions in a water phantom for cobalt-60 gamma-ray and megavoltage X-ray beams. The mean difference between measured and stated dose values was 0.5% with a standard deviation of 1.9%, but up to 6% at maximum. As soon as all institutes apply a common dosimetry protocol, this maximum difference will reduce to about 2%. In addition, an anthropomorphic phantom was irradiated to simulate the treatment of a prostatic cancer. The dose, determined with an ionization chamber at the isocentre and thermoluminescent dosimeters (TLD) powder at several points situated in the target volume, the bladder and the rectum, was compared with the stated dose calculated with the local planning system. Only small differences were found between the measured and stated dose at the isocentre: on the average 1.5%, with a standard deviation of 1.5%. The difference between stated and measured dose at several points situated in the target volume was on the average 0.4%, with a standard deviation of 5.2%. Almost the same result was found for a point situated in the bladder. In the rectum, the average difference was about 4%, however, with a large standard deviation, 18%, due to the relatively steep dose gradient at these points.

Perturbation correction factors for some ionization chambers commonly applied in electron beams

Values for the fluence perturbation correction factor were determined in a number of electron beams for the commonly applied PTW/Markus plane-parallel chamber and the cylindrical NE 2571 Farmer chamber. These data were determined relative to the NACP plane-parallel chamber as a function of the mean energy at depth, Ez, calculated according to the method recommended in modern dosimetry protocols. For the cylindrical chamber the results are in agreement with the data recommended in these protocols. For the PTW/Markus chamber a small but significant fluence perturbation correction is necessary: up to about 3% for a value of Ez=2 MeV. In order to check the value of these fluence perturbation correction values, dose measurements were performed ionometrically and compared with values determined with the Fricke dosimetry system.

Experimental verification of the air kerma to absorbed doseconversion factor Cw,u

In a recently published code of practice for the dosimetry of high-energy photon beams, the absorbed dose to water is determined using an ionization chamber having an air kerma calibration factor and applying the air kerma to absorbed dose conversion factor Cw,u. The consistency of these Cw,u values has been determined for four commonly employed types of ionization chambers in photon beams with quality varying between 60Co gamma-rays and 25 MV X-rays. Using a graphite calorimeter, Cw,u has been determined for a graphite-walled ionization chamber (NE 2561) for the same qualities. The values of Cw,u determined with the calorimeter are within the experimental uncertainty equal to Cw,u values determined according to any of the recent dosimetry protocols.

Experimental determination of wall correction factors. Part II: NACP and Markus plane-parallel ionization chambers

For pt.I see ibid., vol.35, p.835-46 (1990). The formalism to derive the absorbed dose to water from ionization chamber measurements in a phantom includes several wall correction factors, katt, km and pwall, that depend on the shape, size and composition of the ionization chamber. Wall correction factors have been measured for a number of NACP and PTW/Markus chambers. Significant deviations from calculated values occur due to uncertainties in the contribution to the total ionization from the different materials of these inhomogeneous ionization chambers. Experimental values of pwall larger than one have been obtained for the NACP chamber indicating that the side and back wall material of this chamber reduce the ionization compared with the situation where all electrons originate from the water phantom.

Dose intercomparison at the radiotherapy centers in The Netherlands. 2. Accuracy of locally applied computer planning systems for external photon beams

As part of a quality assurance program in The Netherlands, the performance of computer planning systems was tested. Relative dose values, determined with an ionization chamber, were compared with dose values obtained from locally applied computer planning systems. Several clinically relevant situations were investigated: perpendicular incident beams, wedged beams, oblique incident beams, variable source-surface distances (SSD) and off-axis planes. The mean value of the ratios of calculated to measured dose values is 0.994, with an uncertainty of 2.4% (1 S.D.) and a maximum deviation of 9%, for all combinations of energies, planning systems and geometries investigated. The uncertainty for each situation separately was less than 2% (1 S.D.), except for the wedged beams and off-axis plane, which showed uncertainties of 2.6% (1 S.D.). Part of the additional uncertainty for the wedged beams originates from the value chosen for the wedge factor. Systematic deviations between calculated and measured dose values were investigated for three commercially available planning systems, separately. The mean deviation was smaller than 1% (1 S.D.), for most situations. Only for the wedged beams, larger deviations, up to a mean deviation of 2.6%, were observed.

Linking log files with dosimetric accuracy--A multi-institutional study on quality assurance of volumetric modulated arc therapy.

PURPOSE To systematically evaluate machine specific quality assurance (QA) for volumetric modulated arc therapy (VMAT) based on log files by applying a dynamic benchmark plan. METHODS AND MATERIALS A VMAT benchmark plan was created and tested on 18 Elekta linacs (13 MLCi or MLCi2, 5 Agility) at 4 different institutions. Linac log files were analyzed and a delivery robustness index was introduced. For dosimetric measurements an ionization chamber array was used. Relative dose deviations were assessed by mean gamma for each control point and compared to the log file evaluation. RESULTS Fourteen linacs delivered the VMAT benchmark plan, while 4 linacs failed by consistently terminating the delivery. The mean leaf error (±1SD) was 0.3±0.2 mm for all linacs. Large MLC maximum errors up to 6.5 mm were observed at reversal positions. Delivery robustness index accounting for MLC position correction (0.8-1.0) correlated with delivery time (80-128 s) and depended on dose rate performance. Dosimetric evaluation indicated in general accurate plan reproducibility with γ(mean)(±1 SD)=0.4±0.2 for 1 mm/1%. However single control point analysis revealed larger deviations and attributed well to log file analysis. CONCLUSION The designed benchmark plan helped identify linac related malfunctions in dynamic mode for VMAT. Log files serve as an important additional QA measure to understand and visualize dynamic linac parameters.

Comparison of recent codes of practice for high-energy photon dosimetry

Absorbed dose values were determined under the reference conditions in a phantom irradiated by high-energy photon beams with quality varying between 60Co gamma rays and 25 MV X-rays, using four commonly employed types of ionisation chamber. The ionisation chamber readings were converted to absorbed dose values applying the recent NACP, AAPM and SEFM Protocols and the revised HPA Code of Practice. The AAPM and SEFM Protocols gave consistent results for the four types of chamber whereas the NACP Protocol should be adapted to take the differences in chamber wall material and chamber dimensions into account. Absorbed dose values determined with the standard chamber and procedure recommended in the HPA Code of Practice show good agreement, within 0.8%, with absorbed dose values obtained using the AAPM and SEFM Protocols.

Dose intercomparison at the radiotherapy centers in The Netherlands. 3. Characteristics of electron beams

Dosimetric characteristics of a number of clinically applied electron beams were analyzed as part of a dosimetry intercomparison program performed at the radiotherapy centers in The Netherlands. Absorbed dose values, determined under reference conditions, were compared during site visits with stated dose values. The mean deviation was 0.2% with a standard deviation (SD) of 2.5%. The maximum deviation was 5.7%. The largest differences were due to differences in the calibration procedures and differences in the numerical values of conversion factors adopted from different dosimetry protocols. In addition, a number of clinically relevant parameters of the dose distribution on the central beam axis were analyzed including the depth of the 85% and 50% relative dose, the dose reduction at the depth of the 50% relative dose and the surface dose. The average difference between the stated and measured therapeutic depth (85% dose level) was -0.4 mm with an SD = 1.2 mm. Due to this dosimetric uncertainty observed a safety margin of about 3 mm at the therapeutic depth is recommended. The maximum difference between stated and observed mean energy of the electron beams had only a small influence, < 1%, on the absorbed dose determination. The normalized dose gradient is not an adequate parameter to describe the dose reduction beyond the therapeutic depth. The depth of a low dose level is a better parameter. The relative dose at the surface showed differences up to 10% between scanning electron beams and beams from accelerators with a single scattering foil and closed wall collimating system.