Off-center Ratio Research Articles

Effect of inhomogeneity in a patient's body on the accuracy of the pencil beam algorithm in comparison to Monte Carlo

The pencil beam algorithm (PBA) is reasonably accurate and fast. It is, therefore, the primary method used in routine clinical treatment planning for proton radiotherapy; still, it needs to be validated for use in highly inhomogeneous regions. In our investigation of the effect of patient inhomogeneity, PBA was compared with Monte Carlo (MC). A software framework was developed for the MC simulation of radiotherapy based on Geant4. Anatomical sites selected for the comparison were the head/neck, liver, lung and pelvis region. The dose distributions calculated by the two methods in selected examples were compared, as well as a dose volume histogram (DVH) derived from the dose distributions. The comparison of the off-center ratio (OCR) at the iso-center showed good agreement between the PBA and MC, while discrepancies were seen around the distal fall-off regions. While MC showed a fine structure on the OCR in the distal fall-off region, the PBA showed smoother distribution. The fine structures in MC calculation appeared downstream of very low-density regions. Comparison of DVHs showed that most of the target volumes were similarly covered, while some OARs located around the distal region received a higher dose when calculated by MC than the PBA.

A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: a theoretical study on 10-MV X-ray dose calculations in thorax-like phantoms

A convolution/superposition method is proposed for use with primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance. It should be noted that the number of energy bins is usually about ten, and that the reconstructed X-ray spectra can reasonably be applied to media with a wide range of effective Z numbers, ranging from water to lead. The study was carried out for 10-MV X-ray doses in water and thorax-like phantoms with the use of open-jaw-collimated fields. The dose calculations were made separately for primary, scatter, and electron contamination dose components, for which we used two extended radiation sources: one was on the X-ray target and the other on the flattening filter. To calculate the in-air beam intensities at points on the isocenter plane for a given jaw-collimated field, we introduced an in-air output factor (OPF(in-air)) expressed as the product of the off-center jaw-collimator scatter factor (off-center S (c)), the source off-center ratio factor (OCR(source)), and the jaw-collimator radiation reflection factor (RRF(c)). For more accurate dose calculations, we introduce an electron spread fluctuation factor (F (fwd)) to take into account the angular and spatial spread fluctuation for electrons traveling through different media.

Quality Assurance Technique for Absorbed Dose Distribution in External Radiation Therapy with Non-physical Wedges in Consideration of the Character of the Imaging Plate

The film dose distribution method is used for profile measurements of non-physical wedges, because three-dimensional automatic control water phantoms cannot be used. Recently, many hospitals have adopted computed radiography(CR) systems in preference to automatic developing processors and films. This may allow use of automatic processors to be discontinued. In this study, a beam was irradiated to an Imaging Plate(IP), and then IP was exposed to a fixed amount of light with fading, and we then measured the off-center ratio(OCR) absorbed dose distribution in external radiation therapy with non-physical wedge. This was compared with the OCR measured with an ionization chamber dosimeter. It was consequently possible for IP to approximate the value measured by the ionization chamber dosimeter by using a metal filter. This method offers a simple quality assurance technique for absorbed dose distribution in external radiation therapy with non-physical wedges in consideration of the character of the IP.

SU-GG-T-478: A Comparison of Proton Beam Monitor Units Predicted by the Treatment Planning System and Obtained from Physical Calibration

Purpose: To compare the proton monitor units predicted by the treatment planning system and obtained from physical calibration.Method and Materials: Monitor units for proton beam calibration are product of the machine calibration factor (CF), mod factor (MF), Spread out Bragg Peak depth dose (SOBPPdd), field size factor (FSF), inverse square factor (ISF), off center ratio (OCR), and bolus gap factor (BGF). The CF is defined as 1cGy/MU at the center of modulation for the mod wheel 60. The SOBPPdd, MF, FSF, BGF and OCR were measured during the commissioning of proton machines for each beam energy. The proton beam monitor units in treatment planning system are calculated based on the OCR, MF, SOBPdd, ISF from virtual SAD and proton scatter parameters. The differences in the monitor unit calculation between the treatment planning system and from the physical calibration arise from the BGF, FSF and patient scatter. The BGF and FSF are handled correctly in the physical calibration. The patient scattering and heterogeneity are handled by the proton pencil beam dose kernel in the treatment planning system. The proton monitor units from the treatment planning system and physical calibration were compared for prostate, lung and brain patients. Results: The differences in monitor units between the treatment planning system and physical calibration were within 1% for breast cases, and 3%for most of the other cases in the study. The differences increased at higher energy and for thicker compensators. There were also larger differences for some of the cases where the field sizes were smaller than 3 cm. The treatment planning algorithm does not always accommodate special treatment situations adequately. Conclusion:Treatment planning algorithms cannot always accommodate special treatment situations. For such cases, in addition to routine MU calculation, it is important to carry out a physical calibration of treatment fields.

A procedure for calculation of monitor units for passively scattered proton radiotherapy beams

The purpose of this study is to validate a monitor unit (MU) calculation procedure for passively scattered proton therapy beams. The output dose per MU (d/MU) of a therapeutic radiation beam is traditionally calibrated under specific reference conditions. These conditions include beam energy, field size, suitable depth in water or water equivalent phantom in a low dose gradient region with known relative depth dose, and source to point of calibration distance. Treatment field settings usually differ from these reference conditions leading to a different d/MU that needs to be determined for delivering the prescribed dose. For passively scattered proton beams, the proton specific parameters, which need to be defined, are related to the energy, lateral scatterers, range modulating wheel, spread out Bragg peak (SOBP) width, thickness of any range shifter, the depth dose value relative to the normalization point in the SOBP, and scatter both from the range compensator and inhomogeneity in the patient. Following the custom for photons or electrons, a set of proton dosimetry factors, representing the changes in the d/MU relative to a reference condition, can be defined as the relative output factor (ROF), SOBP factor (SOBPF), range shifter factor (RSF), SOBP off-center factor (SOBPOCF), off-center ratio (OCR), inverse square factor (ISF), field size factor (FSF), and compensator and patient scatter factor (CPSF). The ROF, SOBPF, and RSF are the major contributors to the d/MU and were measured using an ion chamber in water tank during the clinical commissioning of each beam to create a dosimetry beam data table to be used for calculating the monitor units. The following simple formula is found to provide an independent method to determine the d/MU at the point of interest (POI) in the patient, namely, (d/MU) = ROF SOBPF. RSF SOBPOCF.OCR.FSF.ISF.CPSF. The monitor units for delivering the intended dose (D) to the POI can be obtained from MU = D / (d/MU). The accuracy and robustness of the above formula were validated by calculating the d/MU in water for many different combinations of beam parameters and comparing it with the corresponding measured d/MU by an ion chamber in a water or water/plastic phantom. This procedure has been in use for MU calculation for patient treatment fields at our facility since May 2006. The differences in the calculated and measured values of the d/MU for 623 distinct fields used for patient treatment during the period of May 2006 to February 2007 are within 2% for 99% of these fields. The authors conclude that an intuitive formula similar to the one used for monitor unit calculation of therapeutic photon beams can be used to compute the monitor units of passively scattered proton therapy beams.

Dose verifications by use of liquid ionization chamber of an electronic portal imaging device (EPID)

In this study, we examined the ability of an L-EPID to verify rectangular and irregular fields and to measure the transmitted exit doses. With respect to the beam profile of rectangular and irregular fields and the doses transmitted through an inhomogeneous phantom, the L-EPID dose obtained from the L-EPID measurement was compared with the conventional dose measured by use of a 0.12-cc ionization chamber and a 3D water phantom. In the comparison of the rectangular and irregular fields, the difference in the off-center ratio (OCR) between the L-EPID dose and the conventional dose was approximately 3% in the steep-dose-gradient region (penumbra regions, >30%/cm) and approximately +/-0.5% in the gentle-dose-gradient region (5%/cm). On the other hand, the dose differences between the L-EPID and the measured doses were less than approximately 2% in the gentle-dose-gradient region. In addition, in the steep-dose-gradient region, the maximum difference was 30%. However, the differences in the distance-to-agreement (DTA) were less than approximately +/-1 mm and were unrelated to the dose gradient. These results suggest that dose verification by L-EPID is very useful in clinical applications.

Examination of Beam Profile Measurement Using an Imaging Plate by the Light Exposure Fading Method for Quality Assurance of External Radiation Therapy

High-resolution film dosimetry has been used for several decades to check and to measure two-dimensional dose distributions. However, in recent years, the automatic processor has been replaced by the spread of computed radiography, or has been little used hospitals. In this study, we measured the off-center ratio (OCR) of the open field, after an irradiating radiation beam was delivered to the imaging plate (IP) under conditions in which the IP was exposed to a fixed amount of light with fading, and compared these data with the OCR measured by an ionization-chamber dosimeter, which is the standard method used for measuring radiation dose. Profile measurement using IP could be achieved by performing light fading, even at a range of more than 100 MU. Further, by using a metallic filter, we succeeded in demonstrating that the profile measurement of IP in an open irradiation field could approximate the values of those obtained by an ionization chamber dosimeter. This method can serve as a simple, easy-to-use method for evaluating the QA of dose distribution in radiation therapy.

Generalized monitor unit calculation for the Varian enhanced dynamic wedge field

The generalized monitor unit (MU) calculation equation for the Varian enhanced dynamic wedge (EDW) is derived. The assumption of this MU calculation method is that the wedge factor of the EDW at the center of the field is a function of field size, the position of the center of the field in the wedge direction, and the final position of the moving jaw. The wedge factors at the center of the field in both symmetric and asymmetric fields are examined. The difference between calculated and measured wedge factors is within 1.0%. The method developed here is easy to implement. The only datum required in addition to the standard set of conventional physical wedge implementation data is the off-axis output factor for the open field in the reference condition. The off-center point calculation is also examined. For the off-center point calculation, the dose profile in the wedge direction for the largest EDW field is used to obtain the relative off-center ratio in any smaller wedge field. The accuracy of the off-center point calculation decreases when the point of calculation is too close to the field edge.

Patient dose simulation in X-ray CT using a radiation treatment-planning system

Medical irradiation dosage has been increasing with the development of new radiological equipment and new techniques like interventional radiology. It is fair to say that patient dose has been increased as a result of the development of multi-slice CT. A number of studies on the irradiation dose of CT have been reported, and the computed tomography dose index (CTDI) is now used as a general means of determining CT dose. However, patient dose distribution in the body varies with the patient's constitution, bowel gas in the body, and conditions of exposure. In this study, patient dose was analyzed from the viewpoint of dose distribution, using a radiation treatment-planning computer. Percent depth dose (PDD) and the off-center ratio (OCR) of the CT beam are needed to calculate dose distribution by the planning computer. Therefore, X-ray CT data were measured with various apparatuses, and beam data were sent to the planning computer. Measurement and simulation doses in the elliptical phantom (Mix-Dp: water equivalent material) were collated, and the CT irradiation dose was determined for patient dose simulation. The rotational radiation treatment technique was used to obtain the patient dose distribution of CT, and patient dose was evaluated through simulation of the dose distribution. CT images of the thorax were sent to the planning computer and simulated. The result was that the patient dose distribution of the thorax was obtained for CT examination.

Using the field edge correction (FEC) method to generate accurate POCRs and OCRs for asymmetric fields.

Dose planning programs originally intended for use with symmetric fields have been adapted for use with asymmetric fields. An accurate representation of the change in primary beam quality with off-axis distance and depth is essential for accurate dose calculation and is usually represented in the computer as a primary radiation profile or primary off-center ratio (POCR). The original field edge correction (FEC) method described by Cadman [Med. Phys. 22, 457 (1995)] to determine POCRs has been extended to allow accurate POCR values to be obtained to an off-axis distance defined by the corners of the largest field, typically at an off-axis distance of 28.3 cm. This technique requires only routine symmetric field measurements including beam profiles, TMRs, and collimator and phantom scatter factors. The POCRs obtained using the FEC technique were used to generate off-center ratios (OCRs) using the boundary factor technique of Chui et al. [Med. Phys. 15, 92 (1988)]. Excellent agreement with measured values was obtained for cross-beam OCRs using a 10 x 10-cm2 field defined by a single set of asymmetric jaws with a field center offset of 15 cm and for diagonal OCRs using a 20 x 20-cm2 field with each pair of jaws in a half-blocked configuration.

Dose computations for asymmetric fields defined by independent jaws.

Asymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF's) for the four jaws. The POCR's and BF's were derived from measured beam profiles, which include the effect of off-axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6-MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.