Volume 38, Issue 8, August 2011
 EDITORIAL
 POINT/COUNTERPOINT


Medical physics graduate programs should adjust enrollment to achieve equilibrium between graduates and residents
View Description Hide Description  Top

 MEDICAL PHYSICS LETTERS


On the Monte Carlo simulation of electron transport in the sub1 keV energy range
View Description Hide DescriptionPurpose:The validity of “classic” Monte Carlo(MC) simulations of electron and positrontransport at sub1 keV energies is investigated in the context of quantum theory.
Methods:Quantum theory dictates that uncertainties on the position and energymomentum fourvectors of radiation quanta obey Heisenberg’s uncertainty relation; however, these uncertainties are neglected in “classical” MC simulations of radiation transport in which position and momentum are known precisely. Using the quantum uncertainty relation and electron mean free path, the magnitudes of uncertainties on electron position and momentum are calculated for different kinetic energies; a validity bound on the classical simulation of electron transport is derived.
Results:In order to satisfy the Heisenberg uncertainty principle,uncertainties of 5% must be assigned to position and momentum for 1 keV electrons in water; at 100 eV, these uncertainties are 17 to 20% and are even larger at lower energies. In gaseous media such as air, these uncertainties are much smaller (less than 1% for electrons with energy 20 eV or greater).
Conclusions:The classical Monte Carlotransport treatment is questionable for sub1 keV electrons in condensed water as uncertainties on position and momentum must be large (relative to electron momentum and mean free path) to satisfy the quantum uncertainty principle. Simulations which do not account for these uncertainties are not faithful representations of the physical processes, calling into question the results of MC track structure codes simulating sub1 keV electron transport. Further, the large difference in the scale at which quantum effects are important in gaseous and condensed media suggests that track structure measurements in gases are not necessarily representative of track structure in condensed materials on a micrometer or a nanometer scale.

Iterative reconstruction for differential phase contrast imaging using spherically symmetric basis functions
View Description Hide DescriptionPurpose:The purpose of this work is to combine two areas of active research in tomographic xray imaging. The first one is the use of iterative reconstruction (IR) techniques. The second one is differential phase contrastimaging (DPCI).
Methods:The authors derive a maximum likelihood (ML) reconstruction algorithm with regularization for DPCI. Forward and backprojection are implemented using spherically symmetric basis functions (blobs) and differential footprints, thus completely avoiding the need for numerical differentiation throughout the reconstruction process. The method is applied to the problem of reconstruction of an object from sparsely sampled projections.
Results:The results show that the proposed method can handle the sparsely sampled data efficiently. In particular no streak artifacts are visible which are present in images obtained by filtered backprojection (FBP).
Conclusions:IR algorithms have a wide spectrum of proven advantages in the area of conventional computed tomography. The present work describes for the first time, how a matched forward and backprojection can be implemented for DPCI, which is furthermore free of any heuristics. The newly developed ML reconstruction algorithm for DPCI shows that for the case of sparsely sampled projection data, an improvement in image quality is obtained that is qualitatively comparable to a corresponding situation in conventional xray imaging. Based on the proposed operators for forward and backprojection, a large variety of IR algorithms is thus made available for DPCI.

Extremely efficient and deterministic approach to generating optimal ordering of diffusion MRI measurements
View Description Hide DescriptionPurpose:DiffusionMRImeasurements are typically acquired sequentially with unit gradient directions that are distributed uniformly on the unit sphere. The ordering of the gradient directions has significant effect on the quality of dMRIderived quantities. Even though several methods have been proposed to generate optimal orderings of gradient directions, these methods are not widely used in clinical studies because of the two major problems. The first problem is that the existing methods for generating highly uniform and antipodally symmetric gradient directions are inefficient. The second problem is that the existing methods for generating optimal orderings of gradient directions are also highly inefficient. In this work, the authors propose two extremely efficient and deterministic methods to solve these two problems.
Methods:The method for generating nearly uniform point set on the unit sphere (with antipodal symmetry) is based upon the notion that the spacing between two consecutive points on the same latitude should be equal to the spacing between two consecutive latitudes. The method for generating optimal ordering of diffusion gradient directions is based on the idea that each subset of incremental sample size, which is derived from the prescribed and full set of gradient directions, must be as uniform as possible in terms of the modified electrostatic energy designed for antipodally symmetric point set.
Results:The proposed method outperformed the stateoftheart method in terms of computational efficiency by about six orders of magnitude.
Conclusions:Two extremely efficient and deterministic methods have been developed for solving the problem of optimal ordering of diffusion gradient directions. The proposed strategy is also applicable to optimal viewordering in threedimensional radial MRI.
 Top

 OBITUARY


Rosalyn Sussman Yalow, 1921–2011
View Description Hide Description  Top

 RADIATION THERAPY PHYSICS


Analyzing the impact of intrafraction motion: Correlation of different dose metrics with changes in target D_{95%}
View Description Hide DescriptionPurpose:A number of techniques are available to determine the dosimetric impact of intrafraction motion during intensity modulated radiation therapy(IMRT). Motioninduced doseperturbations can be determined both computationally and experimentally using a number of different dosimetric metrics. However, these measures may lead to different conclusions regarding the clinical impact of motion. This study compares the analysis of identical doseperturbations using different dosimetric metrics. Calculated changes in target D_{95%} are used as a reference.
Methods:A total of 3768 motionencoded dose distributions were calculated for nine lungtumor patients. The motionencoded dose distributions were compared to static dose distributions using three dosimetric metrics: 2D γ, 3D γ, and histogram analysis. Each of these metrics was used to analyze doseperturbations both globally and within the target structure. Furthermore, the failing voxels were analyzed separately according to failure mode, i.e., under vs. overdosed voxels. Metrics were evaluated based on their agreement with changes in target D_{95%}. Evaluations included the metrics’ maximum average sensitivity and specificity (MASS) in detecting unacceptable deliveries, a coefficient correlated to ranking (τ), and the linear correlation coefficient, r.
Results:Of the evaluated metrics, the histogram metric restricted to the underdosed voxels within the target agreed best with changes in target D_{95%}. This metric achieved a MASS of 0.93, a τ of 0.69, and an rvalue of 0.85. In comparison, the unrestricted 2D γ metric achieved MASS = 0.77, τ = 0.40, and r = 0.67. Restricting the 2D γ test both geographically and in failure mode increased the MASS to 0.85, τ to 0.70, and the rvalue to 0.80.
Conclusions:This study suggests that any clinical decisions based solely on an unrestricted 2D γ metric are suboptimal. A geographic and failure mode restriction can improve results. The remaining uncertainties with nonDVH (dose volume histogram) based metrics should be kept in mind when they are used to evaluate the dosimetric impact of target motion.

Numbers of beam angles required for nearoptimal IMRT: Theoretical limits and numerical studies
View Description Hide DescriptionPurpose:To derive limits on the numbers of beams needed to deliver nearoptimal IMRT, and to assess the accuracy of the limits.
Methods:The authors four different limits have been derived. One, K_{A}, has been obtained by coupling Fourier techniques with a proof used to obtain Bortfeld’s limit, K, that if all the crossprofiles of a manyfield plan can be represented as polynomials of order (K−1) over the range [−R, + R], then within the radius R circle an identical dosedistribution can be created using just K fields. Two further limits, K_{H} and K_{N,} have been obtained using sampling theory, the K_{N} limit describing fields spaced at the Nyquist frequency. K_{N} can be generalized to K_{N,Fbeamlet}, a limit that accounts for the finite size of the beamlets from which modulated fields are constructed. Using Bortfeld’s theoretical framework, the accuracy of the limits has been explored by testing how well the crossprofiles of an 8 MV doubleGaussian pencil beam and of 1 and 4 cm wide fields can be approximated by polynomials of orders equal to the different limits minus one. The dependence of optimized cost function values of IMRT plans, generated for a simple geometry and for a headandneck (oropharynx) case, on the numbers of beams used to construct the plans has also been studied.
Results:The limits are all multiples of R/W (W being the 20%–80% penumbrawidth of a broad field) and work out at K = 27, K_{A} = 43, K_{H} = 34, and K_{N} = 68 fields for R = 10 cm and W = 5.3 mm. All and none of the crossprofiles are approximated well by polynomials of order K_{N}−1 and K−1, respectively, suggesting some inaccuracy in the assumptions used to derive the limit K. Order K_{A}−1 polynomials cannot accurately describe the pencil beam profile, but do approximate the 1 and 4cm profiles reasonably well because higher spatial frequencies are attenuated in these wider fields. All the profiles are represented well by polynomials of order K_{N,Fbeamlet}−1, which decreases from K_{N} as beamlet width increases. Cost functions generated in the IMRT planning study fall as greater numbers of fields are used, before plateauing out around K_{N,Fbeamlet} fields.
Conclusions:Numerical calculations suggest that the minimum number of fields required for nearoptimal IMRT lies around the generalized Nyquist limit K_{N,Fbeamlet}. For a clinically realistic 20%–80% penumbrawidth of 5.3 mm and a radius of interest of 10 cm, K_{N,Fbeamlet} falls from 68 to 47 fields as the beamlet width rises from 0 to 1 cm.

Measuring the wobble of radiation field centers during gantry rotation and collimator movement on a linear accelerator
View Description Hide DescriptionPurpose:The isocenter accuracy of a linear accelerator is often assessed with starshot films. This approach is limited in its ability to quantify three dimensional wobble of radiation field centers (RFCs). The authors report a Winston–Lutz based method to measure the 3D wobble of RFCs during gantry rotation, collimator rotation, and collimatorfield size change.
Methods:A stationary ballbearing phantom was imaged using multileaf collimatorshaped radiation fields at various gantry angles, collimator angles, and field sizes. The center of the ballbearing served as a reference point, to which all RFCs were localized using a computer algorithm with subpixel accuracy. Then, the gantry rotation isocenter and the collimator rotation axis were derived from the coordinates of these RFCs. Finally, the deviation or wobble of the individual RFC from the derived isocenter or rotation axis was quantified.
Results:The results showed that the RFCs were stable as the field size of the multileaf collimator was varied. The wobble of RFCs depended on the gantry angle and the collimator angle and was reproducible, indicating that the mechanical imperfections of the linac were mostly systematic and quantifiable. It was found that the 3D wobble of RFCs during gantry rotation was reduced after compensating for a constant misalignment of the multileaf collimator.
Conclusions:The 3D wobble of RFCs can be measured with submillimeter precision using the proposed method. This method provides a useful tool for checking and adjusting the radiation isocenter tightness of a linac.

Optimized knot placement for Bsplines in deformable image registration
View Description Hide DescriptionPurpose: To develop an automatic knot placement algorithm to enable the use of NonUniform Rational BSplines (NURBS) in deformable image registration.Methods: The authors developed a twostep approach to fit a known displacement vector field (DVF). An initial fit was made with uniform knot spacing. The error generated by this fit was then assigned as an attractive force pulling on the knots, acting against a resistive spring force in an iterative equilibration scheme. To demonstrate the accuracy gain of knot optimization over uniform knot placement, we compared the sum of the squared errors and the frequency of large errors.Results: Fits were made to a onedimensional DVF using 1–20 free knots. Given the same number of free knots, the optimized, nonuniform Bspline fit produced a smaller error than the uniform Bspline fit. The accuracy was improved by a mean factor of 4.02. The optimized Bspline was found to greatly reduce the number of errors more than 1 standard deviation from the mean error of the uniform fit. The uniform Bspline had 15 such errors, while the optimized Bspline had only two. The algorithm was extended to fit a twodimensional DVF using control point grid sizes ranging from 8 × 8 to 15 × 15. Compared with uniform fits, the optimized Bspline fits were again found to reduce the sum of squared errors (mean ratio = 2.61) and number of large errors (mean ratio = 4.50).Conclusions: Nonuniform Bsplines offer an attractive alternative to uniform Bsplines in modeling the DVF. They carry forward the mathematical compactness of Bsplines while simultaneously introducing new degrees of freedom. The increased adaptability of knot placement gained from the generalization to NURBS offers increased local control as well as the ability to explicitly represent topological discontinuities.

A twodimensional deformable phantom for quantitatively verifying deformation algorithms
View Description Hide DescriptionPurpose: The incorporation of deformable image registration into the treatment planning process is rapidly advancing. For this reason, the methods used to verify the underlying deformation algorithms must evolve equally fast. This manuscript proposes a twodimensional deformable phantom, which can objectively verify the accuracy of deformation algorithms, as the next step for improving these techniques.Methods: The phantom represents a single plane of the anatomy for a head and neck patient. Inflation of a balloon catheter inside the phantom simulates tumor growth. CT and cameraimages of the phantom are acquired before and after its deformation. Nonradiopaque markers reside on the surface of the deformable anatomy and are visible through an acrylic plate, which enables an optical camera to measure their positions; thus, establishing the groundtruth deformation. This measured deformation is directly compared to the predictions of deformation algorithms, using several similarity metrics. The ratio of the number of points with more than a 3 mm deformation error over the number that are deformed by more than 3 mm is used for an error metric to evaluate algorithm accuracy.Results: An optical method of characterizing deformation has been successfully demonstrated. For the tests of this method, the balloon catheter deforms 32 out of the 54 surface markers by more than 3 mm. Different deformation errors result from the different similarity metrics. The most accurate deformation predictions had an error of 75%.Conclusions: The results presented here demonstrate the utility of the phantom for objectively verifying deformation algorithms and determining which is the most accurate. They also indicate that the phantom would benefit from more electron density heterogeneity. The reduction of the deformable anatomy to a twodimensional system allows for the use of nonradiopaque markers, which do not influence deformation algorithms. This is the fundamental advantage of this verification technique.

Measured and Monte Carlo calculated k _{ Q } factors: Accuracy and comparison
View Description Hide DescriptionPurpose: The journal Medical Physics recently published two papers that determine beam quality conversion factors, k _{ Q }, for large sets of ion chambers. In the first paper [McEwen Med. Phys. 37, 2179–2193 (2010)], k _{ Q } was determined experimentally, while the second paper [Muir and Rogers Med. Phys. 37, 5939–5950 (2010)] provides k _{ Q } factors calculated using Monte Carlo simulations. This work investigates a variety of additional consistency checks to verify the accuracy of the k _{ Q } factors determined in each publication and a comparison of the two data sets. Uncertainty introduced in calculated k _{ Q } factors by possible variation of W/e with beam energy is investigated further.Methods: The validity of the experimental set of k _{ Q } factors relies on the accuracy of the NE2571 reference chamber measurements to which k _{ Q } factors for all other ion chambers are correlated. The stability of NE2571 absorbed dose to water calibration coefficients is determined and comparison to other experimental k _{ Q } factors is analyzed. Reliability of Monte Carlo calculated k _{ Q } factors is assessed through comparison to other publications that provide Monte Carlo calculations of k _{ Q } as well as an analysis of the sleeve effect, the effect of cavity length and selfconsistencies between graphitewalled Farmerchambers. Comparison between the two data sets is given in terms of the percent difference between the k _{ Q } factors presented in both publications.Results: Monitoring of the absorbed dose calibration coefficients for the NE2571 chambers over a period of more than 15 yrs exhibit consistency at a level better than 0.1%. Agreement of the NE2571 k _{ Q } factors with a quadratic fit to all other experimental data from standards labs for the same chamber is observed within 0.3%. Monte Carlo calculated k _{ Q } factors are in good agreement with most other Monte Carlo calculated k _{ Q } factors. Expected results are observed for the sleeve effect and the effect of cavity length on k _{ Q }. The mean percent differences between experimental and Monte Carlo calculated k _{ Q } factors are −0.08, −0.07, and −0.23% for the Elekta 6, 10, and 25 MV nominal beam energies, respectively. An upper limit on the variation of W/e in photon beams from cobalt60 to 25 MV is determined as 0.4% with 95% confidence. The combined uncertainty on Monte Carlo calculated k _{ Q } factors is reassessed and amounts to between 0.40 and 0.49% depending on the wall material of the chamber.Conclusions: Excellent agreement (mean percent difference of only 0.13% for the entire data set) between experimental and calculated k _{ Q } factors is observed. For some chambers, k _{ Q } is measured for only one chamber of each type—the level of agreement observed in this study would suggest that for those chambers the measured k _{ Q } values are generally representative of the chamber type.

Evaluation of the dosimetric impact of interfractional anatomical variations on prostate proton therapy using daily inroom CT images
View Description Hide DescriptionPurpose:To quantify interfractional anatomical variations and their dosimetric impact during the course of fractionated proton therapy (PT) of prostate cancer and to assess the robustness of the current treatment planning techniques.
Methods:Simulation and daily inroom CT scans from ten prostate carcinoma patients were analyzed. PT treatment plans (78 Gy in 39 fractions of 2 Gy) were created on the simulation CT, delivering 25 fractions to PTV1 (expanded from prostate and seminal vesicles), followed by 14 boost fractions to PTV2 (expanded from prostate). Plans were subsequently applied to daily CT, with beams aligned to the prostate center in the sagittal plane. For five patients having a sufficiently large daily imaging volume, structure contours were manually drawn, and plans were evaluated for all CT sets. For the other five patients, the plans were evaluated for six selected fractions. The daily CT was matched to the simulation CT through deformable registration. The registration accuracy was validated for each fraction, and the three patients with a large number of accurately registered fractions were used for dose accumulation.
Results:In individual fractions, the coverage of the prostate, seminal vesicles, and PTV1 was generally maintained at the corresponding prescription dose. For PTV2, the volume covered by the fractional prescription dose of 2 Gy (i.e.,V2) was, on average, reduced by less than 3% compared to the simulation plan. Among the 225 (39 × 5 + 6 × 5) fractions examined, 15 showed a V2 reduction larger than 5%, of which ten were caused by a large variation in rectal gas, and five were due to a prostate shift in the craniocaudal direction. The fractional dose to the anterior rectal wall was found to increase for one patient who had large rectal gas volume in 25 of the 39 fractions, and another who experienced significant prostate volume reduction during the treatment. The fractional bladder dose generally increased with decreasing fullness. In the total accumulated dose for the three patients after excluding a few fractions with inaccurate registration due to a large amount of rectal gas (a condition inconsistent with RTOG protocol), 98.5%, 96.6%, and 98.2% of the PTV2 received the prescription dose of 78 Gy. The V75 and V70 of the anterior rectal wall and bladder both remained within tolerance.
Conclusions:The results confirm that the PT planning techniques and dose constraints used at our institution ensure that target coverage to the prescription dose is maintained in the presence of interfractional anatomical variations. Dose coverage in individual fractions can be compromised, and normal tissuedose increased, due to deviations in the bladder and rectal volume compared to the simulation plans or progressive changes in the prostate volume during the treatment. Deviations from the plan can be reduced with efforts aimed at maintaining consistent daily patient anatomy.

Dose monitoring and output correction for the effects of scanning field changes with uniform scanning proton beam
View Description Hide DescriptionPurpose: The output of a proton beam is affected by proton energy, SpreadOut Bragg Peak (SOBP) width, aperture size, dose rate, and the point of measurement. In a uniform scanning proton beam (USPB), the scanning field size is adjusted (including the vertical length and the horizontal width) according to the treatment field size with appropriate margins to reduce secondary neutron production. Different scanning field settings result in beam output variations that are investigated in this study.Methods: The measurements are performed with a parallel plate Markus chamber at the center of SOBP under the reference condition with 16 cm range, 10 cm SOBP, and 5 cm air gap. The effect of dose rate on field output is studied by varying proton beam current from 0.5 to 7 nA. The effects of scanning field settings are studied by varying independently the field width and length from 12 × 12 to 30 × 30 cm^{2}.Results: The results demonstrate that scanning field variations can produce output variation up to 3.80%. In addition, larger output variation is observed with scanning field changes along the stem direction of the patient dose monitor (PDM). By investigating the underlying physics of incomplete charge collection and the stem effects of the PDM, an analytical model is proposed to calculate USPB output with consideration of the scanning field area and the PDM stem length that is irradiated. The average absolute difference between the measured output and calculated output using our new correction model are within 0.13 and 0.08% for the 20 and 30 cm snouts, respectively.Conclusions: This study proposes a correction model for accurate USPB output calculation, which takes account of scanning field settings and the PDM stem effects. This model may be used to extend the existing output calculation model from one snout size to other snout sizes with customized scanning field settings. The study is especially useful for calculating field output for treatment without individualized patient specific measurements.

Monte Carlo simulation of contrastenhanced whole brain radiotherapy on a CT scanner
View Description Hide DescriptionPurpose:To perform a feasibility study of contrastenhanced whole brainradiotherapy for treating patients with multiple brainmetastasis using a conventional computed tomography(CT) scanner.
Methods:The treatmentdose was optimized to be applied in a single run using a maximum tube power of kWs at kV. CT scans of a large and a small head were used as reference. Irradiation geometry, shielding, axial beam collimation, radial beam collimation, gantry tilt, and tube current for beam modulation were optimized using a Monte Carlo simulation and a contrast agent concentration of mg/ml iodine in the tumor. The statistical uncertainty of the Monte Carlo simulation was corrected using back convolution.
Results:Using a CT tube with a beam collimation of mm, a mean tumordose of Gy was achieved, while the head bone dose was Gy with a normal braindose of Gy, eye dose of Gy, and lens dose of Gy, respectively. Using a CT tube with dose modulation and a beam collimation of mm, the mean tumordose was Gy with a head bone dose of Gy, normal braindose of Gy, eye dose of Gy, and lens dose of Gy, respectively. Thus a standard CT scanner enables an effective tumordose of Gy to be administered in 13 fractions, while exposing healthy brain to an effective dose of Gy and head bone to Gy. Additional radial collimation implemented in the hardware improves the therapeutic tumordose by in relation to the bone dose.
Conclusions:Contrastenhanced total brainradiotherapy is feasible using a conventional CT tube with optimized dose application.

Radiation hardness of the storage phosphor europium doped potassium chloride for radiation therapy dosimetry
View Description Hide DescriptionPurpose: An important property of a reusable dosimeter is its radiationhardness, that is, its ability to retain its dosimetric merits after irradiation. The radiationhardness of europium doped potassium chloride (KCl:Eu^{2+}), a storage phosphormaterial recently proposed for radiation therapydosimetry, is examined in this study.
Methods: Pelletstyle KCl:Eu^{2+}dosimeters, 6 mm in diameter, and 1 mm thick, were fabricated inhouse for this study. The pellets were exposed by a 6 MV photon beam or in a high dose rate ^{137}Cs irradiator. Macroscopic properties, such as radiation sensitivity, dose response linearity, and signal stability, were studied with a laboratory photostimulated luminescence (PSL) readout system. Since phosphor performance is related to the state of the storage centers and the activator, Eu^{2+}, in the host lattice, spectroscopic and temporal measurements were carried out in order to explore radiationinduced changes at the microscopic level.
Results: KCl:Eu^{2+}dosimeters retained approximately 90% of their initial signal strength after a 5000 Gy dose history. Dose response was initially supralinear over the dose range of 100–700 cGy but became linear after 60 Gy. Linearity did not change significantly in the 0–5000 Gy dose history spanned in this study. Annealing high dose history chips resulted in a return of supralinearity and a recovery of sensitivity. There were no significant changes in the PSL stimulation spectra, PSL emission spectra,photoluminescencespectra, or luminescence lifetime, indicating that the PSL signal process remains intact after irradiation but at a reduced efficiency due to reparable radiationinduced perturbations in the crystal lattice.
Conclusions: Systematic studies of KCl:Eu^{2+}material are important for understanding how the material can be optimized for radiation therapydosimetry purposes. The data presented here indicate that KCl:Eu^{2+} exhibits strong radiationhardness and lends support for further investigations of this novel material.

Microdosimetric study on influence of low energy photons on relative biological effectiveness under therapeutic conditions using 6 MV linac
View Description Hide DescriptionPurpose:Microdosimetry has been developed for the evaluation of radiation quality, and singleevent dosemean lineal energy is wellused to represent the radiation quality. In this study, the changes of the relative biological effectiveness (RBE) values under the therapeutic conditions using a 6 MV linac were investigated with a microdosimetric method.
Methods:The values under the various irradiation conditions for xrays from a 6 MV linac were measured with a tissueequivalent proportional counter (TEPC) at an extremely low dose rate of a few tens of μGy/min by decreasing the gun grid voltage of the linac. According to the microdosimetric kinetic model (MK model), the RBE_{MK} values for cell killing of the human salivary gland (HSG) tumor cells can be derived if the values are obtained from TEPC measurements. The Monte Carlo code GEANT4 was also used to calculate the photon energy distributions and to investigate the changes of the values under the various conditions.
Results:The changes of the values were less than approximately 10% when the field size and the depth in a phantom varied. However, in the measurements perpendicular to a central beam axis, large changes were observed between the values inside the field and those outside the field. The maximum increase of approximately 50% in the value outside the field was obtained compared with those inside the field. The GEANT4 calculations showed that there existed a large relative number of low energy photons outside of the field as compared with inside of the field. The percentages of the photon fluences below 200 keV outside the field were approximately 40% against approximately 8% inside the field. By using the MK model, the field size and the depth dependence of the RBE_{MK} values were less than approximately 2% inside the field. However, the RBE_{MK} values outside the field were 6.6% higher than those inside the field.
Conclusions:The increase of the RBE_{MK} values by 6.6% outside the field was observed. This increase is caused by the change of the photon energy distributions, especially the increase of the relative number of low energy photons outside the field.

Fast IMRT with narrow high energy scanned photon beams
View Description Hide DescriptionPurpose:Since the first publications on intensity modulated radiation therapy(IMRT) in the early 1980s almost all efforts have been focused on fairly time consuming dynamic or segmental multileaf collimation. With narrow fast scanned photonbeams, the flexibility and accuracy in beam shaping increases, not least in combination with fast penumbra trimming multileaf collimators. Previously, experiments have been performed with full range targets, generating a broad bremsstrahlung beam, in combination with multileaf collimators or material compensators. In the present publication, the first measurements with fast narrow high energy (50 MV) scanned photonbeams are presented indicating an interesting performance increase even though some of the hardware used were suboptimal.
Methods:Inverse therapy planning was used to calculate optimal scanning patterns to generate dose distributions with interesting properties for fast IMRT. To fully utilize the dose distributional advantages with scanned beams, it is necessary to use narrow high energy beams from a thin bremsstrahlung target and a powerful purging magnet capable of deflecting the transmitted electron beam away from the generated photons onto a dedicated electron collector. During the present measurements the scanning system, purging magnet, and electron collimator in the treatment head of the MM50 racetrack accelerator was used with 3–6 mm thick bremsstrahlung targets of beryllium. The dose distributions were measured with diodes in water and with EDR2 film in PMMA. Monte Carlo simulations withgeant4 were used to study the influence of the electrons transmitted through the target on the photon pencil beam kernel.
Results:The full width at halfmaximum (FWHM) of the scanned photonbeam was 34 mm measured at isocenter, below 9.5 cm of water, 1 m from the 3 mm Be bremsstrahlung target. To generate a homogeneous dose distribution in a 10 × 10 cm^{2} field, the authors used a spot matrix of 100 equal intensity beam spots resulting in a uniformity of collimated 80%–20% penumbra of 9 mm at a primary electron energy of 50 MeV. For the more complex cardioid shaped dose distribution, they used 270 spots, which at a pulse repetition frequency of 200 Hz is completed every 1.36 s.
Conclusions:The present measurements indicate that the use of narrow scanned photonbeams is a flexible and fast method to deliver advanced intensity modulated beams. Fast scanned photonIMRT should, therefore, be a very interesting modality in the delivery of biologically optimized radiation therapy with the possibility forin vivo treatment verification with PETCT imaging.

In vivo realtime dosimetric verification in high dose rate prostate brachytherapy
View Description Hide DescriptionPurpose:To evaluate the performance of a diode array in the routine verification of planned dose to points inside the rectum from prostate high dose rate (HDR) brachytherapy using a realtime planning system.
Methods:A dosimetric study involving 28 patients was undertaken where measured doses received during treatment were compared to those calculated by the treatment planning system (TPS). After the ultrasound imaging required for treatment planning had been recorded, the ultrasound probe was replaced with a geometric replica that contained an 8 mm diameter cylindrical cavity in which a PTW diode array type 9112 was placed. The replica probe was then positioned inside the rectum with the individual diode positions determined using fluoroscopy. Dose was then recorded during the patients’ treatment and compared to associated coordinates in the planning system.
Results:Factors influencing diode response and experimental uncertainty were initially investigated to estimate the overall uncertainty involved in dose measurements, which was determined to be ±10%. Data was acquired for 28 patients’ first fractions, 11 patients’ second fractions, and 13 patients’ third fractions with collection dependent upon circumstances. Deviations between the diode measurements and predicted values ranged from −42% to +35% with 71% of measurements experiencing less than a 10% deviation from the predicted values. If the ±10% measurement uncertainty was combined with a tolerated dose discrepancy of ±10% then over 95% of the diode results exhibited agreement with the calculated data to within ±20%. It must also be noted that when large dose discrepancies were apparent they did not necessarily occur for all five diodes in the one measurement.
Conclusions:This technique provided a method that could be utilized to detect gross errors in dose delivery of a realtime prostate HDR plan. Limitations in the detection system used must be well understood if meaningful results are to be achieved.

Validation of a new control system for Elekta accelerators facilitating continuously variable dose rate
View Description Hide DescriptionPurpose:Elekta accelerators controlled by the current clinically used acceleratorcontrol system, Desktop 7.01 (D7), uses binned variable dose rate (BVDR) for volumetric modulated arc therapy (VMAT). The next version of the treatment control system (Integrity™) supports continuously variable dose rate (CVDR) as well as BVDR. Using CVDR opposed to BVDR for VMAT has the potential of reducing the treatment time but may lead to lower dosimetric accuracy due to faster moving accelerator parts. Using D7 and a test version of Integrity, differences in ability to control the accelerator, treatment efficiency, and dosimetric accuracy between the two systems were investigated.
Methods:Single parameter tests were designed to expose differences in the way the two systems control the movements of the accelerator. In these tests, either the jaws, multi leaf collimators(MLCs), or gantry moved at constant speed while the dose rate was changed in discrete steps. The positional errors of the moving component and dose rate were recorded using the control systems with a sampling frequency of 4 Hz. The clinical applicability of Integrity was tested using 15 clinically used VMAT plans (5 prostate, 5 H&N, and 5 lung) generated by the SmartArc algorithm inPINNACLE. The treatment time was measured from beamon to beamoff and the accuracy of the dosedelivery was assessed by comparing DELTA4 measurements and PINNACLE calculated doses using gamma evaluation.
Results:The single parameter tests showed that Integrity had an improved feedback between gantry motion and dose rate at the slight expense of MLC control compared to D7. The single parameter test did not reveal any significant differences in the control of either jaws or backup jaws between the two systems. These differences in gantry and MLC control together with the use of CVDR gives a smoother Integrity VMAT delivery compared to D7 with less abrupt changes in acceleratormotion. Gamma evaluation (2% of 2 Gy and 2 mm) of the calculated doses andDELTA4 measured doses corrected for systematic errors showed an average pass rate of more than 97.8% for both D7, Integrity BVDR, and Integrity CVDR deliveries. Direct comparisons between the measured doses using strict gamma criteria of 0.5% and 0.5 mm showed excellent agreement between D7 and Integrity delivereddoses with average pass rates above 95.7%. Finally, the Integrity control system resulted in a significant 35% (55 ± 13 s) reduction in treatment time, on average.
Conclusions:Single parameter tests showed that the two control systems differed in their feedback loops between MLC, gantry, and dose rate. These differences made the VMAT deliveries more smooth using the new Integrity treatment control system, compared to the current Desktop 7.01. Together with the use of CVDR, which results in less abrupt changes in dose rate, this further increases the smoothness of the delivery. The use of CVDR for VMAT with the Integrity desktop results in a significant reduction in treatment time compared to BVDR with an average reduction of 35%. This decrease in delivery time was achieved without compromising the dosimetric accuracy.
