[Investigation of Estimation Accuracy of Phantom Scatter Factor by Clarkson Method Considering Depth in MLC Irregular Field].

In monitor unit (MU) independent verification by calculation for irregular field (MLC field) using multileaf collimator in X-ray therapy, it has become common to use collimator scatter factor (Sc) and phantom scatter factor (Sp) instead of total scatter factor (Sc, p). It is usually expressed as Sc, p (A)=Sc (A)×Sp (A), and the field size A is considered but the depth d is not. Sc is data of in-air output, and measure with a mini-phantom at constant depth to remove electron contamination. On the other hand, Sp is obtained from measurement data of Sc, p and Sc, and can be expressed as Sc, p (d, A)=Sc (constant depth, A)×Sp (d, A) at an arbitrary depth d, thus Sp depends on the depth of Sc, p. Therefore, Sp needs to consider depth. In addition, a linear accelerator equipped with the tertiary MLC has two field sizes, that are collimator field by upper and lower collimators and MLC field by tertiary MLC below them. In MU independent verification by calculation, it is often used that the estimated value of Sp obtained by converting MLC field to equivalent square field and referring to data of Sp in square field. To convert the MLC field to equivalent square field, a conversion formula from sector radius r to equivalent square field L by Clarkson's sector integration (Clarkson method) is used. In this study, using 24 types of MLC fields to evaluate estimation accuracy due to the difference of conversion formula in Clarkson method, we estimated value of Sp using r=0.5611L of B-Clarkson method and using r=0.5580L of A-Clarkson method. And the difference with the measured value of Sp obtained by measuring Sc, p and Sc in the same MLC fields was compared. While, to evaluate estimation accuracy due to the different depths using these Clarkson methods, the difference between estimated value and measured value of Sp similarly obtained at depth of 5, 10 and 15 cm was compared. As results, estimated value of Sp using A-Clarkson method than using B-Clarkson method was close to measured value, and it was the same trend at depth of 5, 10 and 15 cm. Therefore, it was suggested that estimation accuracy of Sp by A-Clarkson method is higher than B-Clarkson method when verifying beams with different depths in MU independent verification by calculation for MLC field.

[Investigation of the Influence of the Conversion Method to Equivalent Square Field on the Depth Direction of Phantom Scatter Factor].

In X-ray therapy, equivalent square field (side of equivalent square field) is important because it influences the accuracy of independent verification of monitor unit (MU) by calculation. To calculate the side of equivalent square field for rectangular fields, we often use a table of domestic standard measurement method (Day's method), or A/P method calculated by area-perimeter ratio. The sides of equivalent square fields of these methods are assumed to be unchanged by depth and energy, but there are reports that it is not valid. Therefore, the depth dependency of side of equivalent square fields of Day's method, A/P method, and area ratio correction (ARC) method was compared by measuring phantom scatter factors (Sp). From the analysis of Sp measured at different depths, the estimated value of Sp on the equivalent square side of the Day's method and A/P method had a depth dependency that the difference from the measured value was large when the measurement depth was deep. The estimated value of Sp on the equivalent square side of the ARC method had a small difference from the measured value even when the measurement depth was deep, and the depth dependency was small compared with the Day's method and the A/P method. Side of equivalent square field of ARC method had a smaller difference of depth dependency than in the case of Day's method and A/P method. Therefore, in the independent verification of MU for rectangular field, using the equivalent square side of the ARC method is better.

[Development of Monitoring Method of Respiratory Waveform in Thoracicoabdominal Part Using Web Camera].

Countermeasures against respiratory movement are important for tumors of thorax and abdomen in stereotactic body radiation therapy. In the present paper, a web-camera-based-respiratory monitoring method without contact with patient's body was proposed for respiratory study. Thoracic and abdominal motion images were taken by a web camera, and were analyzed using simple image-processing techniques for obtaining respiratory waveforms. Four motion images with different respiration rate were obtained from resusci anne simulator. Respiration waveforms were estimated from the moving images by the proposed method, and were compared with respiration waveforms obtained by the conventional respiratory monitoring device. That was found to have a strong correlation. In addition, the two waveforms were similar in Bland-Altman method comparison. The proposed method can provide non-contact, non-invasive, simple, and realistic respiratory monitoring system for radiotherapy.

A study on a dental device for the prevention of mucosal dose enhancement caused by backscatter radiation from dental alloy during external beam radiotherapy.

The changes in dose distribution caused by backscatter radiation from a common commercial dental alloy (Au-Ag-Pd dental alloy; DA) were investigated to identify the optimal material and thicknesses of a dental device (DD) for effective prevention of mucositis. To this end, 1 cm3 of DA was irradiated with a 6-MV X-ray beam (100 MU) in a field size of 10 × 10 cm2 using a Novalis TX linear accelerator. Ethylene vinyl acetate copolymer, polyolefin elastomer, and polyethylene terephthalate (PET) were selected as DD materials. The depth dose along the central axis was determined with respect to the presence/absence of DA and DDs at thicknesses of 1-10 mm using a parallel-plate ionization chamber. The dose in the absence of DDs showed the lowest value at a distance of 5 mm from the DA surface and gradually increased with distance between the measurement point and the DA surface for distances of ≥5 mm. Except for PET, no significant difference between the DA dose curves for the presence and absence of DDs was observed. In the dose curve, PET showed a slightly higher dose for DA with DD than for DA without DD for thicknesses of ≥4 mm. The findings herein suggest that the optimal DD material for preventing local dose enhancement of the mucosa caused by DA backscatter radiation should have a relatively low atomic number and physical density and that optimal DD thickness should be chosen considering backscatter radiation and percentage depth dose.

Initial implementation of the conversion from the energy-subtracted CT number to electron density in tissue inhomogeneity corrections: an anthropomorphic phantom study of radiotherapy treatment planning.

PURPOSE: To achieve accurate tissue inhomogeneity corrections in radiotherapy treatment planning, the authors had previously proposed a novel conversion of the energy-subtracted computed tomography (CT) number to an electron density (ΔHU-ρ(e) conversion), which provides a single linear relationship between ΔHU and ρ(e) over a wide range of ρ(e). The purpose of this study is to present an initial implementation of the ΔHU-ρ(e) conversion method for a treatment planning system (TPS). In this paper, two example radiotherapy plans are used to evaluate the reliability of dose calculations in the ΔHU-ρ(e) conversion method. METHODS: CT images were acquired using a clinical dual-source CT (DSCT) scanner operated in the dual-energy mode with two tube potential pairs and an additional tin (Sn) filter for the high-kV tube (80-140 kV/Sn and 100-140 kV/Sn). Single-energy CT using the same DSCT scanner was also performed at 120 kV to compare the ΔHU-ρ(e) conversion method with a conventional conversion from a CT number to ρ(e) (Hounsfield units, HU-ρ(e) conversion). Lookup tables for ρ(e) calibration were obtained from the CT image acquisitions for tissue substitutes in an electron density phantom (EDP). To investigate the beam-hardening effect on dosimetric uncertainties, two EDPs with different sizes (a body EDP and a head EDP) were used for the ρ(e) calibration. Each acquired lookup table was applied to two radiotherapy plans designed using the XiO TPS with the superposition algorithm for an anthropomorphic phantom. The first radiotherapy plan was for an oral cavity tumor and the second was for a lung tumor. RESULTS: In both treatment plans, the performance of the ΔHU-ρ(e) conversion was superior to that of the conventional HU-ρ(e) conversion in terms of the reliability of dose calculations. Especially, for the oral tumor plan, which dealt with dentition and bony structures, treatment planning with the HU-ρ(e) conversion exhibited apparent discrepancies between the dose distributions and dose-volume histograms (DVHs) of the body-EDP and head-EDP calibrations. In contrast, the dose distributions and DVHs of the body-EDP and head-EDP calibrations coincided with each other almost perfectly in the ΔHU-ρ(e) conversion for 100-140 kV/Sn. The difference between the V100's (the mean planning target volume receiving 100% of the prescribed dose; a DVH parameter) of the body-EDP and head-EDP calibrations could be reduced to less than 1% using the ΔHU-ρ(e) conversion, but exceeded 11% for the HU-ρ(e) conversion. CONCLUSIONS: The ΔHU-ρ(e) conversion can be implemented for currently available TPS's without any modifications or extensions. The ΔHU-ρ(e) conversion appears to be a promising method for providing an accurate and reliable inhomogeneity correction in treatment planning for any ill-conditioned scans that include (i) the use of a calibration EDP that is nonequivalent to the patient's body tissues, (ii) a mismatch between the size of the patient and the calibration EDP, or (iii) a large quantity of high-density and high-atomic-number tissue structures.

Conversion of the energy-subtracted CT number to electron density based on a single linear relationship: an experimental verification using a clinical dual-source CT scanner.

In radiotherapy treatment planning, the conversion of the computed tomography (CT) number to electron density is one of the main processes that determine the accuracy of patient dose calculations. However, in general, the CT number and electron density of tissues cannot be interrelated using a simple one-to-one correspondence. This study aims to experimentally verify the clinical feasibility of an existing novel conversion method proposed by the author of this note, which converts the energy-subtracted CT number (ΔHU) to the relative electron density (ρe) via a single linear relationship by using a dual-energy CT (DECT). The ΔHU-ρe conversion was performed using a clinical second-generation dual-source CT scanner operated in the dual-energy mode with tube potentials of 80 kV and 140 kV with and without an additional tin filter. The ΔHU-ρe calibration line was obtained from the DECT image acquisition for tissue substitutes in an electron density phantom. In addition, the effect of object size on ΔHU-ρe conversion was also experimentally investigated. The plot of the measured ΔHU versus nominal ρe values exhibited a single linear relationship over a wide ρe range from 0.00 (air) to 2.35 (aluminum). The ΔHU-ρe conversion performed with the tin filter yielded a lower dose and more reliable ρe values that were less affected by the object-size variation when compared to the corresponding values obtained for the case without the tin filter.

[Estimation of the phantom scatter factor (Sp) of rectangular fields].

This study proposes a method to accurately estimate the phantom scatter factor (Sp) of arbitrary rectangular fields. We measured output doses in water and air; these measured values were based on square fields and a limited number of symmetric rectangular fields using 4 MV and 10 MV X-rays of a Varian Clinac-iX. We calculated Sp from these measured values. Then, using these Sp values, we estimated equations of Sp on square fields consisting of the primary dose, Day's scatter, and forward scatter. This equation may be used to estimate the Sp value on a square field, but it cannot estimate the Sp value on a rectangular field. We investigated the calculation method for an equivalent square of a rectangular field. As a result, this study's calculation method for an equivalent square, the area ratio correction method, was more accurate than the conventional Bjärngard's method. Therefore, when using the approximate equation of Sp on a square field and the equivalent square calculated by the area ratio correction method, a Sp value of an arbitrary rectangular field may be accurately estimated.

[Estimation of collimator scatter factor, S(c), of rectangular fields by a saturation model].

We estimated collimator scatter factor, S(c), of symmetric rectangular fields of any size by applying a two-component scatter model to measured in-air output data in width and length directions of measured rectangles. The in-air output was measured for symmetric rectangles with combined width and length sizes of 7 x 7 and 6 x 6 using 10 MV and 4 MV X-rays of Varian's Clinac 2100 C/D, respectively. The model consists of scatter components from the primary collimator and flattening filter and from the collimator jaws: the former shows a saturation curve and the latter increases linearly with enlarging field size. This model was fitted to the measured dataset firstly in the width and secondly in the length directions of rectangles; then, by compiling interpolated matrix data, the S(c) table of symmetric rectangles was constructed. In addition, using this S(c) table, values of S(c) were calculated for a few asymmetric rectangles by Day's method, and were in good agreement with measured values. Therefore, we think that our method is practical and precise for constructing the S(c) table of symmetric rectangles from measured data, and that using this table, the S(c) of any asymmetric rectangles may be calculated.

[Estimation of collimator scatter factor, S(c), of small field sizes using long-SCD method and two saturation models].

To estimate the collimator scatter factor, S(c) of small field sizes in which a mini-phantom cannot be fully included at the nominal treatment distance (NTD=100 cm), we measured the in-air output of 4 MV and 10 MV X-rays of a Varian's Clinac 2100 C/D using a mini-phantom at NTD and at a long source-to-chamber distance (SCD=200 cm) with field-size defined at the isocenter down to 4.6 x 4.6 cm(2) and 2.3 x 2.3 cm(2), respectively. We then compared the fitted curve to the NTD dataset by a cumulative exponential distribution model with that by a cumulative Gaussian distribution (error function) model containing a zero-field extrapolated term derived from the long SCD dataset. The results showed that the zero-field extensions of two fitted curves coincided for a 4 MV X-ray, but a large discrepancy was seen between them for a 10 MV X-ray. Therefore, the S(c) of small field sizes not measurable using a mini-phantom at the NTD can be well estimated by applying the cumulative exponential model to the NTD dataset in the case of a 4 MV X-ray beam filtrated with a cone-shaped flattener. However, to estimate the S(c) of such small field sizes in the case of a 10 MV X-ray beam filtrated with a bell-shaped flattener, we consider it preferable to also measure in-air output at a long SCD and to apply the cumulative Gaussian model as described here.

[Approximation of collimator scatter factor Sc by a saturation model].

To more easily estimate accurate values of collimator scatter facor, S(c), we suggest a two-component saturation model that accounts for scatter from the primary collimator and flattening filter and from the collimator jaws. This model, which assumes an exponential distribution of scatter intensity, was tested by in-air measurements using a mini-phantom for 4 MV and 10 MV X-rays of a Clinac 2100 C/D linear accelerator. The results showed a good fit of this model to our measured data (R(2)>0.9993). When the measured value was divided into the primary collimator/flattening filter component and the collimator jaw component, as expected, the former component showed a rapid and full saturation curve with increased field size, while the latter showed an almost linearly increasing curve. Therefore, we think that this saturation model is useful for the estimation of S(c) and is applicable to monitor unit calculation for an asymmetric field.