Transmission measurements in air using the ESTRO mini-phantom

Control chart analysis of data from a multicenter monitor unit verification study

BACKGROUND AND PURPOSE

This study aims to investigate the process of monitor unit verification using control charts. Control charts is a key tool within statistical process control (SPC), through which process characteristics can be visualized, usually chronologically with statistically determined limits.

MATERIAL AND METHODS

Our group has developed a monitor unit verification software that has been adopted at several Swedish institutions for pre-treatment verification of radiotherapy treatments. Deviations between point dose calculations using the treatment planning systems and using the independent monitor unit verification software from 9219 treatment plans and five different institutions were included in this multicenter study. The process of monitor unit verification was divided into subprocesses. Each subprocess was analyzed using probability plots and control charts.

RESULTS

Differences in control chart parameters for the investigated subprocesses were found between different treatment sites and different institutions, as well as between different treatment techniques. 19 of 37 subprocesses met the clinical specification (± 5%), i.e. process capability index was equal to or above one.

CONCLUSIONS

Control charts were found to be a useful tool for continuous analysis of data from the monitor unit verification software for patient specific quality control, as well as for comparisons between different institutions and treatment sites. The derived control chart limits were in agreement with AAPM TG114 guidelines on action levels.

Prediction of stopping-power ratios in flattening-filter free beams

PURPOSE

In recent years, there has been an increasing interest in flattening-filter free (FFF) beams. However, since the removal of the flattening filter will affect both the mean and the variance of the energy spectrum, current beam-quality specifiers may not be adequate for reference dosimetry in such beams. The purpose of this work was to investigate an alternative, more general beam-quality specifier.

METHODS

The beam-quality specifier used in this work was a combination of the kerma-weighted mean and the coefficient of variation of the linear attenuation coefficient in water. These parameters can in theory be determined from narrow-beam transmission measurements using a miniphantom "in-air," which is a measurement condition well suited also to small and nonstandard fields. The relation between the Spencer-Attix stopping-power ratios and this novel beam-quality specifier was described by a simple polynomial. For reference, the authors used Monte Carlo calculated spectra and stopping-power data for nine different beams, with and without flattening filter.

RESULTS

The polynomial coefficients were obtained by least-squares optimization. For all beams included in this investigation, the average of the differences between the predicted and the Monte Carlo calculated stopping-power ratios was 0.02 +/- 0.17% (1 SD) (including TomoTherapy and CyberKnife example beams).

CONCLUSIONS

An alternative dual-parameter beam-quality specifier was investigated. The evaluation suggests that it can be used successfully to predict stopping-power ratios in FFF as well as conventional beams, regardless of filtration.

Design of mini phantom and measurement of cobalt-60 beam data parameters

Low cost mini phantoms were fabricated indigenously with different water equivalent material such as polymethyl methacrylate and Bee's wax of different shapes (with dome top surface and flat top surface). The beam parameters of the Co-60 machine, such as head scatter correction factor (S_h), phantom scatter correction factor (S_P), total scatter correction factor (S_C,P), collimator exchange effect were measured. Output ratio measurements were taken for both mini phantom and water phantom for different square and rectangular field sizes. Normalized output ratios were compared with ESTRO published values and (Storchi and Van Gasteren) S and G data. The percentage of variation between the measured and the literature values is about 0.7%. Collimator exchange effect were measured for water and mini phantom for different field size, were compared with ESTRO value. This was found to be 0.5% and 1.0% respectively. Phantom scatter correction factors were calculated for square and rectangular filed sizes; this was compared with ESTRO values, found to be 0.7% for square and 1.0% for rectangular filed size. It was also noted that there were no appreciable variation observed in ion chamber readings of different materials of mini phantoms for dome and flat surfaces. Mini phantom measurements were done for all types of phantoms and the measured values were compared with the existing data and they were in good agreement with the published values.

Measurement of in-air output ratios using different miniphantom materials

In-air output ratios, S(c), were measured using miniphantoms made of PMMA (thickness 2.4-24 g cm(-2)), graphite (1.8-26.5 g cm(-2)), copper (1.6-23.3 g cm(-2)) and lead (2.3-21.6 g cm(-2)), for collimator settings of 3 x 3 to 40 x 40 cm(2), and x-ray energies of 6 MV and 15 MV, respectively. The effects of the miniphantom on S(c) were quantified as correction factors as functions of collimator setting, material types and miniphantom thickness for each photon energy to correct the measured values. For miniphantoms with sufficient thickness to eliminate electron disequilibrium, the total correction factors can be expressed as multiplications of three factors: the attenuation correction factor, the mass energy absorption correction factor and the phantom scatter correction factor. This formalism implies that the collimator setting dependence of the correction factor is mainly caused by the energy spectrum shift. The narrow-beam attenuation coefficients in various phantom materials for different collimator settings were determined in narrow-beam geometries using a specially constructed collimator mounted on the tray holder of the accelerator. We have determined that the maximum total correction factor is approximately 1.01. For miniphantoms made of PMMA, graphite, copper and lead, at the miniphantom thickness of 10 g cm(-2), the maximum total correction factors are 1.002, 1.003, 1.005, 1.007, and 1.002, 1.003, 1.008 and 1.009 for 6 MV and 15 MV, respectively.

Influence of ion chamber response on in-air profile measurements in megavoltage photon beams

This article presents an investigation of the influence of the ion chamber response, including buildup caps, on the measurement of in-air off-axis ratio (OAR) profiles in megavoltage photon beams using Monte Carlo simulations with the EGSnrc system. Two new techniques for the calculation of OAR profiles are presented. Results of the Monte Carlo simulations are compared to measurements performed in 6, 10 and 25 MV photon beams produced by an Elekta Precise linac and shown to agree within the experimental and simulation uncertainties. Comparisons with calculated in-air kerma profiles demonstrate that using a plastic mini phantom gives more accurate air-kerma measurements than using high-Z material buildup caps and that the variation of chamber response with distance from the central axis must be taken into account.

Head scatter off-axis for megavoltage x rays

Our objective in this study has been to investigate how head scatter varies with the off-axis position in a 6 MV x-ray beam. We define the head-scatter off-axis ratio, HOA, as the ratio of the kerma due to head-scatter photons at the off-axis position x to the kerma from direct primary photons on the central axis. "Direct primary" are those photons that come from the source without interactions in the intervening structures. We determined HOA from measurements with an ionization chamber in a miniphantom. Head-scatter and direct primary photons contribute to a measurement of the ionization per mu Q(x) at the off-axis position x in the open field cx x cy. The ionization per mu QP(x), measured in the same position but with the field collimated to the smallest possible opening (cx x 3 cm), is intended to include only direct primary photons. Head-scatter photons cannot be completely eliminated, and the errors due to remaining head scatter and radiation back-scattered by the movable collimators into the monitor were estimated. For normalization of the final results, ionization due to direct primary photons was also measured on the central axis, QP(0). HOA was derived from these three measurements as HOA(cx,cy,x)=(Q(cx,cy,x) - QP(cx,cy,x))/QP(cx,cy,0). On the central axis (x=y=0), HOA represents the "scatter-to-primary ratio" between head scatter and the direct primary dose. Monte Carlo simulations were made to help with the interpretation and evaluation of the results. HOA could be fitted to a Gaussian model with two components corresponding to sources of widths 1.8 and 14 cm, projected on a plane 5 cm below the x-ray source. The narrow Gaussian component is interpreted as the source of photons scattered in the flattening filter and the primary collimator. The broad component is attributed to photons scattered in the secondary (variable) collimators. Conventional head-scatter models (e.g., a single Gaussian source model) do not fit the measured HOA data for large collimator settings (c>20 cm) or outside beam collimation. The full width at half-maximum (FWHM) of HOA(x) across the field increased with the field width (cx) in the direction of the measurements in a manner consistent with the field of view of the two sources. It was not sensitive to the field measure in the orthogonal direction (cy). Head scatter outside the field also increased with field size, reflecting an increased contribution of photons scattered at large angles. It exceeds the leakage through the collimator 2 cm outside the edge for square fields c>10 cm. Monte Carlo calculations showed considerably less head scatter outside the field than measurements.

A simplistic formalism for calculating entrance dose in high-energy x-ray beams

A calculation engine for independent checking of the delivered dose to the prescription point has been developed and tested in an earlier work by our group. One drawback with the present system is the inability to accurately predict the absorbed dose at the depth of dose maximum, d(max), where calculations may deviate by as much as 6-7%. Accurate dose values at dmax are necessary in order to make comparisons with in vivo dose measurements. The aim of this work is to extend the present model to predict dose values at dmax to within +/-2%. Depth dose measurements at different SSD (80, 90 and 100 cm) and field sizes (5 x 5 to 40 x 40 cm2) are made at photon energies in the range from 4 to 18 MV. The effect of an acrylic block tray present in the beam is also studied. Wedged beams are handled as separate beam qualities. An entrance dose factor is defined to correct the effect of electronic disequilibrium at dmax The entrance dose factor is found to be independent of SSD and tray, but it varies with beam quality and field size. After applying the entrance dose factor, the dose at dmax can be predicted to within 1.7% (2 SD).

Independent checking of the delivered dose for high-energy X-rays using a hand-held PC

BACKGROUND AND PURPOSE

The requirements on the delivered dose in radical radiation therapy are extremely high. The dose should be within a few percent and also delivered with high accuracy in space. Vendors and users have successfully managed to implement radiation therapy systems, which are able to achieve these demands with high accuracy and reproducibility. These systems include computerized tomography scanners, treatment planning systems, simulators, treatment machines, and record and verify systems. More and more common are also computer networks to assure data integrity when transferring information between the systems. Even if these systems are commissioned and kept under quality assurance programs to maintain their accuracy, errors may be introduced. Especially, the human factor is an uncontrolled parameter that may introduce errors. Thus, unintentional changes or incorrect handling of data may occur during clinical use of the equipment. Having an independent dose calculation system implemented in the daily quality assurance process may assure a high quality of treatments and avoidance of severe errors.

MATERIALS AND METHODS

To accomplish this, a system of equations for calculating the absorbed dose to the prescription point from the set-up information, has been compiled into a dose-calculation engine. The model is based on data completely independent of the treatment planning system (TPS). The fundamental parameter in the dose engine is the linear attenuation coefficient for the primary photons. This parameter can readily be determined experimentally. The dose calculation engine has been programmed into a hand-held PC allowing direct calculation of the dose to the prescription point when the first treatment is delivered to the patient.

RESULTS AND CONCLUSION

The model is validated with measurements and is shown to be within +/-1.0% (1 SD). Comparison against a state-of-the-art TPS shows an average difference of 0.3% with a standard deviation of +/-2.1%. An action level covering 95% of the cases has been chosen, i.e. +/-4.0%. Deviations larger than this are with a high probability due to erroneous handling of the patient set-up data. This system has been implemented into the daily clinical quality control program.

On beam quality and stopping power ratios for high-energy x-rays

The aim of this work is to quantitatively compare two commonly used beam quality indices, IPR(20/10) and %dd(10)x, with respect to their ability to predict stopping power ratios (water to air), s(w,air), for high-energy x-rays. In particular, effects due to a varied amount of filtration of the photon beam will be studied. A new method for characterizing beam quality is also presented, where the information we strive to obtain is the moments of the spectral distribution. We will show how the moments enter into a general description of the transmission curve and that it is possible to correlate the moments to s(w,air) with a unique and simple relationship. Comparisons with TPR(20/10) and %dd(10), show that the moments are well suited for beam quality specification in terms of choosing the correct s(w,air).