Computer-aided analysis of star shot films for high-accuracy radiation therapy treatment units

As mechanical stability of radiation therapy treatment devices has gone beyond sub-millimeter levels, there is a rising demand for simple yet highly accurate measurement techniques to support the routine quality control of these devices. A combination of using high-resolution radiosensitive film and computer-aided analysis could provide an answer. One generally known technique is the acquisition of star shot films to determine the mechanical stability of rotations of gantries and the therapeutic beam. With computer-aided analysis, mechanical performance can be quantified as a radiation isocenter radius size. In this work, computer-aided analysis of star shot film is further refined by applying an analytical solution for the smallest intersecting circle problem, in contrast to the gradient optimization approaches used until today. An algorithm is presented and subjected to a performance test using two different types of radiosensitive film, the Kodak EDR2 radiographic film and the ISP EBT2 radiochromic film. Artificial star shots with a priori known radiation isocenter size are used to determine the systematic errors introduced by the digitization of the film and the computer analysis. The estimated uncertainty on the isocenter size measurement with the presented technique was 0.04 mm (2σ) and 0.06 mm (2σ) for radiographic and radiochromic films, respectively. As an application of the technique, a study was conducted to compare the mechanical stability of O-ring gantry systems with C-arm-based gantries. In total ten systems of five different institutions were included in this study and star shots were acquired for gantry, collimator, ring, couch rotations and gantry wobble. It was not possible to draw general conclusions about differences in mechanical performance between O-ring and C-arm gantry systems, mainly due to differences in the beam-MLC alignment procedure accuracy. Nevertheless, the best performing O-ring system in this study, a BrainLab/MHI Vero system, and the best performing C-arm system, a Varian Truebeam system, showed comparable mechanical performance: gantry isocenter radius of 0.12 and 0.09?mm, respectively, ring/couch rotation of below 0.10 mm for both systems and a wobble of 0.06 and 0.18 mm, respectively. The methodology described in this work can be used to monitor mechanical performance constancy of high-accuracy treatment devices, with means available in a clinical radiation therapy environment.     

Verification of IMRT fields by film dosimetry

In intensity modulated radiation therapy (IMRT) the aim of an accurate conformal dose distribution is obtained through a complex process. This ranges from the calculation of the optimal distribution of fluence by the treatment planning system (TPS), to the dose delivery through a multilamellar collimator (MLC), with several segments per beam in the step and shoot approach. The above-mentioned consideration makes mandatory an accurate dosimetric verification of the IM beams. A high resolution and integrating dosimeter, like the radiographic film, permits one to simultaneously measure the dose in a matrix of points, providing a good means of obtaining dose distributions. The intrinsic limitation of film dosimetry is the sensitivity dependence on the field size and on the measurement depth. However, the introduction of a scattered radiation filter permits the use of a single calibration curve for all field sizes and measurement depths. In this paper the quality control procedure developed for dosimetric verification of IMRT technique is reported. In particular a system of film dosimetry for the verification of a 6 MV photon beam has been implemented, with the introduction of the scattered radiation filter in the clinical practice that permits one to achieve an absolute dose determination with a global uncertainty within 3.4% (1 s.d.). The film has been calibrated to be used both in perpendicular and parallel configurations. The work also includes the characterization of the Elekta MLC. Ionimetric independent detectors have been used to check single point doses. The film dosimetry procedure has been applied to compare the measured absolute dose distributions with the ones calculated by the TPS, both for test and clinical plans. The agreement, quantified by the gamma index that seldom reaches the 1.5 value, is satisfying considering that the comparison is performed between absolute doses.     

An MLC-based linac QA procedure for the characterization of radiation isocenter and room lasers' position

We have designed and implemented a new stereotactic linac QA test with stereotactic precision. The test is used to characterize gantry sag, couch wobble, cone placement, MLC offsets, and room lasers' positions relative to the radiation isocenter. Two MLC star patterns, a cone pattern, and the laser line patterns are recorded on the same imaging medium. Phosphor plates are used as imaging medium due to their sensitivity to red light. The red light of room lasers erases some of the irradiation information stored on the phosphor plates enabling accurate and direct measurements for the position of room lasers and radiation isocenter. Using film instead of the phosphor plate as imaging medium is possible, however, it is less practical. The QA method consists of irradiating four phosphor plates that record the gantry sag between the 0 degrees and 180 degrees gantry angles, the position and stability of couch rotational axis, the sag between the 90 degrees and 270 degrees gantry angles, the accuracy of cone placement on the collimator, the MLC offsets from the collimator rotational axis, and the position of laser lines relative to the radiation isocenter. The estimated accuracy of the method is +/- 0.2 mm. The observed reproducibility of the method is about +/- 0.1 mm. The total irradiation/ illumination time is about 10 min per image. Data analysis, including the phosphor plate scanning, takes less than 5 min for each image. The method characterizes the radiation isocenter geometry with the high accuracy required for the stereotactic radiosurgery. In this respect, it is similar to the standard ball test for stereotactic machines. However, due to the usage of the MLC instead of the cross-hair/ball, it does not depend on the cross-hair/ball placement errors with respect to the lasers and it provides more information on the mechanical integrity of the linac/couch/laser system. Alternatively, it can be used as a highly accurate QA procedure for the nonstereotactic machines. Noteworthy is its ability to characterize the MLC position accuracy, which is an important factor in IMRT delivery.     

Pretreatment verification of IMRT absolute dose distributions using a commercial a-Si EPID

A commercial amorphous silicon electronic portal imaging device (EPID) has been studied to investigate its potential in the field of pretreatment verifications of step and shoot, intensity modulated radiation therapy (IMRT), 6 MV photon beams. The EPID was calibrated to measure absolute exit dose in a water-equivalent phantom at patient level, following an experimental approach, which does not require sophisticated calculation algorithms. The procedure presented was specifically intended to replace the time-consuming in-phantom film dosimetry. The dosimetric response was characterized on the central axis in terms of stability, linearity, and pulse repetition frequency dependence. The a-Si EPID demonstrated a good linearity with dose (within 2% from 1 monitor unit), which represent a prerequisite for the application in IMRT. A series of measurements, in which phantom thickness, air gap between the phantom and the EPID, field size and position of measurement of dose in the phantom (entrance or exit) varied, was performed to find the optimal calibration conditions, for which the field size dependence is minimized. In these conditions (20 cm phantom thickness, 56 cm air gap, exit dose measured at the isocenter), the introduction of a filter for the low-energy scattered radiation allowed us to define a universal calibration factor, independent of field size. The off-axis extension of the dose calibration was performed by applying a radial correction for the beam profile, distorted due to the standard flood field calibration of the device. For the acquisition of IMRT fields, it was necessary to employ home-made software and a specific procedure. This method was applied for the measurement of the dose distributions for 15 clinical IMRT fields. The agreement between the dose distributions, quantified by the gamma index, was found, on average, in 97.6% and 98.3% of the analyzed points for EPID versus TPS and for EPID versus FILM, respectively, thus suggesting a great potential of this EPID for IMRT dosimetric applications.     

Mechanical and radiation isocenter coincidence: an experience in linear accelerator alignment.

As part of the commissioning procedure of a linear accelerator at our cancer center, the defining laser lines were aligned with the optical and radiation isocenter of the linac. When a mechanical checkout jig was set up at the same point, a discrepancy of 4 mm resulted when the gantry was moved from 0 degrees to 180 degrees. Extensive measurements, some with custom-designed devices, confirmed the observations and provided an explanation. Even though the mechanical isocenter is within the specified tolerance of 1-mm radius, the clinically observable discrepancy of 4-mm results from the noncoincidence of the mechanical and radiation isocenters. The clinical significance of the final setup is discussed and future commissioning procedures are recommended.     

Physical aspects of dynamic stereotactic radiosurgery with very small photon beams (1.5 and 3 mm in diameter)

Stereotactic radiosurgery is often used for treating functional disorders. For some of these disorders, the size of the target can be on the order of a millimeter and the radiation dose required for treatment on the order of 80 Gy. The very small radiation field and high prescribed dose present a difficult challenge in beam calibration, dose distribution calculation, and dose delivery. In this work the dose distribution for dynamic stereotactic radiosurgery, carried out with 1.5 and 3 mm circular fields, was studied. A 10 MV beam from a Clinac-18 linac (Varian, Palo Alto, CA) was used as the radiation source. The BEAM/EGS4 Monte Carlo code was used to model the treatment head of the machine along with the small-field collimators. The models were validated with the EGSnrc code, first through a calculation of percent depth doses (PDD) and dose profiles in a water phantom for the two small stationary circular beams and then through a comparison of the calculated with measured PDD and profile data. The three-dimensional (3-D) dose distributions for the dynamic rotation with the two small radiosurgical fields were calculated in a spherical water phantom using a modified version of the fast XVMC Monte Carlo code and the validated models of the machine. The dose distributions in a horizontal plane at the isocenter of the linac were measured with low-speed radiographic film. The maximum sizes of the Monte Carlo-calculated 50% isodose surfaces in this horizontal plane were 2.3 mm for the 1.5 mm diameter beam and 3.8 mm for the 3 mm diameter beam. The maximum discrepancies between the 50% isodose surface on the film and the 50% Monte Carlo-calculated isodose surfaces were 0.3 mm for both the 1.5 and 3 mm beams. In addition, the displacement of the delivered dose distributions with respect to the laser-defined isocenter of the machine was studied. The results showed that dynamic radiosurgery with very small beams has a potential for clinical use.     

Three-dimensional portal image-based dose reconstruction in a virtual phantom for rapid evaluation of IMRT plans

A new method for rapid evaluation of intensity modulated radiation therapy (IMRT) plans has been developed, using portal images for reconstruction of the dose delivered to a virtual three-dimensional (3D) phantom. This technique can replace an array of less complete but more time-consuming measurements. A reference dose calculation is first created by transferring an IMRT plan to a cylindrical phantom, retaining the treatment gantry angles. The isocenter of the fields is placed on or near the phantom axis. This geometry preserves the relative locations of high and low dose regions and has the required symmetry for the dose reconstruction. An electronic portal image (EPI) is acquired for each field, representing the dose in the midplane of a virtual phantom. The image is convolved with a kernel to correct for the lack of scatter, replicating the effect of the cylindrical phantom surrounding the dose plane. This avoids the need to calculate fluence. Images are calibrated to a reference field that delivers a known dose to the isocenter of this phantom. The 3D dose matrix is reconstructed by attenuation and divergence corrections and summed to create a dose matrix (PI-dose) on the same grid spacing as the reference calculation. Comparison of the two distributions is performed with a gradient-weighted 3D dose difference based on dose and position tolerances. Because of its inherent simplicity, the technique is optimally suited for detecting clinically significant variances from a planned dose distribution, rather than for use in the validation of IMRT algorithms. An analysis of differences between PI-dose and calculation, delta PI, compared to differences between conventional quality assurance (QA) and calculation, delta CQ, was performed retrospectively for 20 clinical IMRT cases. PI-dose differences at the isocenter were in good agreement with ionization chamber differences (mean delta PI = -0.8%, standard deviation sigma = 1.5%, against delta CQ = 0.3%, sigma = 1.0%, respectively). PI-dose plane differences had significantly less variance than film plane differences (sigma = 1.1 and 2.1%, respectively). Twenty-two further cases were evaluated using 3D EPI-dosimetry alone. The mean difference delta over volumes with doses above 80% of the isocenter value was delta = -0.3%, sigma(delta) = 0.7%, and standard deviations of the distributions ranged from 1.0 to 2.0%. Verification time per plan, from initial calculation, delivery, dose reconstruction to evaluation, takes less than 1.5 h and is more than four times faster than conventional QA.     

Algorithm for dosimetry of multiarc linear-accelerator stereotactic radiosurgery.

Treatment planning for multiarc radiosurgery is an inherently complex three-dimensional dosimetry problem. Characteristics of small-field x-ray beams suggest that major simplification of the dose computation algorithm is possible without significant loss of accuracy compared to calculations based on large-field algorithms. The simplification makes it practical to efficiently implement accurate multiplanar dosimetry calculations on a desktop computer. An algorithm is described that is based on data from fixed-beam tissue-maximum-ratio (TMR) and profile measurements at isocenter. The profile for each fixed beam is scaled geometrically according to distance from the x-ray source. Beam broadening due to scatter is taken into account by a simple formula that interpolates the full width at half maximum (FWHM) between profiles at isocenter at different depths in phantom. TMR and profile data for two representative small-field collimators (10- and 25-mm projected diameter) were obtained by TLD and film measurements in a phantom. The accuracy of the calculational method and the associated computer program were verified by TLD and film measurements of noncoplanar multiarc irradiations from these collimators on a 4-MV linear accelerator. Comparison of film measurements in two orthogonal planes showed close agreement with calculations in the shape of the dose distribution. Maximal separation of measured and calculated 90%, 80%, and 50% isodose curves was less than or equal to 0.5 mm for all planes and collimators. All TLD and film measurements of dose to isocenter agreed with calculations to within 2%.     

Dosimetry of therapeutic photon beams using an extended dose range film

For intensity modulated radiation therapy (IMRT) dose distribution verification, multidimensional measurements are required to quantify the steep dose-gradient regions. High resolution, two-dimensional dose distributions can be measured using radiographic film. However, the photon energy response of film is known to be a function of depth, field size, and photon beam energy, potentially reducing the accuracy of dose distribution measurements. The dosimetric properties of the recently developed Kodak EDR2 film were investigated and compared to those of Kodak XV film. The dose responses of both film types to 6 MV and 18 MV photon beams were investigated for depths of 5 cm, 10 cm, and 15 cm and field sizes of 4x4 cm2 and 15x15 cm2. This analysis involved the determination of sensitometric curves for XV and EDR2 films, the determination of dose profiles from exposed XV and EDR2 films, and comparison of the film-generated dose profiles to ionization chamber measurements. For the combinations of photon beam energy, depth, and field size investigated here, our results indicate that the sensitometric curves are nearly independent of field size and depth of calibration. For a field size of 4x4 cm2, a single sensitometric curve for either EDR2 and XV film can be used for the determination of relative dose profiles. For the larger field size, the sensitometric curve for EDR2 film is superior to XV film in regions where the dose falls below 20% of the central axis dose, due to the effects that the increased low energy scattered photon contributions have on film response. The limited field size and depth dependence of sensitometric data measured using EDR2 film, along with the inherently wide linear dose-response range of EDR2 film, makes it better suited to the verification of IMRT dose distributions.     

Design and evaluation of a variable aperture collimator for conformal radiotherapy of small animals using a microCT scanner

Treatment of small animals with radiation has in general been limited to planar fields shaped with lead blocks, complicating spatial localization of dose and treatment of deep-seated targets. In order to advance laboratory radiotherapy toward what is accomplished in the clinic, we have constructed a variable aperture collimator for use in shaping the beam of microCT scanner. This unit can image small animal subjects at high resolution, and is capable of delivering therapeutic doses in reasonable exposure times. The proposed collimator consists of two stages, each containing six trapezoidal brass blocks that move along a frame in a manner similar to a camera iris producing a hexagonal aperture of variable size. The two stages are offset by 30 degrees and adjusted for the divergence of the x-ray beam so as to produce a dodecagonal profile at isocenter. Slotted rotating driving plates are used to apply force to pins in the collimator blocks and effect collimator motion. This device has been investigated through both simulation and measurement. The collimator aperture size varied from 0 to 8.5 cm as the driving plate angle increased from 0 to 41 degrees. The torque required to adjust the collimator varied from 0.5 to 5 N x m, increasing with increasing driving plate angle. The transmission profiles produced by the scanner at isocenter exhibited a penumbra of approximately 10% of the collimator aperture width. Misalignment between the collimator assembly and the x-ray source could be identified on the transmission images and corrected by adjustment of the collimator location. This variable aperture collimator technology is therefore a feasible and flexible solution for adjustable shaping of radiation beams for use in small animal radiotherapy as well as other applications in which beam shaping is desired.     

Three-dimensional dose reconstruction of breast cancer treatment using portal imaging

In this study, we present an algorithm for three-dimensional (3-D) dose reconstruction using portal images obtained with an electronic portal imaging device (EPID). For this purpose an algorithm for 2-D dose reconstruction, which was previously developed in our institution, was adapted. The external contour of the patient was used to correct for absorption of primary photons, but the presence of inhomogeneities was not taken into account. The accuracy of the algorithm was determined by irradiating two anthropomorphic breast phantoms with 6 MV photons. The dose values derived from portal images were compared with results from 3-D dose calculations, which, in turn, were verified with data obtained with an ionization chamber and film dosimetry. It was found that the application of contour information significantly improves the accuracy of 2-D dose reconstruction. If the total dose at the isocenter plane resulting from all treatment beams is reconstructed, the average deviation from the planned dose is 0.1%+/-1.7% (1 SD). If contour information is not available, the differences increase up to +/-20% for the individual beams. In that case, the dose can only be reconstructed with reasonable accuracy when (nearly) opposing beams are used. The average deviation of the 3-D reconstructed dose from the planned dose in the irradiated volume is 1.4%+/-5.4% (1 SD). If the irradiated volume is enclosed by planes less than 5 cm distant from the isocenter plane, then the average deviation is only 0.5%+/-3.4% (1 SD). It can be concluded that the proposed algorithm for a 3-D dose reconstruction allows a determination of the dose at the isocenter plane and the dose-volume histogram with an accuracy acceptable for an independent verification of the treatment.     

Effects of finite angular steps and extent of profile data on the calculation of rotational x-ray dose distributions

Computer algorithms for rotational therapy beams, in most cases, perform dose calculations by summing stored fixed beam data at finite angular steps. Such an algorithm, based on the Bentley beam model, was evaluated by comparing calculations with measured data for an 18-MV x-ray beam. Measurements were made in a specially constructed cylindrical water phantom of 15-cm radius using a 0.1-cm3 ionization chamber for an arc of 180 degrees and for a field size of 7.2 X 7.2 cm2 at 100-cm source-axis distance. This study revealed that the Bentley beam model, with fixed beams summed every 10 degrees, predicts the dose in the treatment volume, centered about the isocenter, with an accuracy of approximately 2%. However, dose at depths between the phantom surface and the treatment volume could be underestimated by as much as 10% (3% of isocenter). This was shown to be partially due to the truncated tails of the off-axis profiles in the Bentley model, which extend only 8 mm outside the edge of the radiation field, and the large angular increment of integration (10 degrees). Using beam profiles extending to 4 cm outside the edge of the radiation field and angular steps of 5 degrees or less for summation of fixed beams reduced errors to less than 5%. Therefore, extended beam profiles and smaller angular steps for summing fixed beams are recommended for photon rotation calculation when increased accuracy is required.     

Effect of organ size and position on out-of-field dose distributions during radiation therapy

Mantle field irradiation has historically been the standard radiation treatment for Hodgkin lymphoma. It involves treating large regions of the chest and neck with high doses of radiation (up to 30 Gy). Previous epidemiological studies on the incidence of second malignancies following radiation therapy for Hodgkin lymphoma have revealed an increased incidence of second tumors in various organs, including lung, breast, thyroid and digestive tract. Multiple other studies, including the Surveillance, Epidemiology and End Results, indicated an increased incidence in digestive tract including stomach cancers following mantle field radiotherapy. Assessment of stomach dose is challenging because the stomach is outside the treatment field but very near the treatment border where there are steep dose gradients. In addition, the stomach can vary greatly in size and position. We sought to evaluate the dosimetric impact of the size and variable position of the stomach relative to the field border for a typical Hodgkin lymphoma mantle field irradiation. The mean stomach dose was measured using thermoluminescent dosimetry for nine variations in stomach size and position. The mean doses to the nine stomach variations ranged from 0.43 to 0.83 Gy when 30 Gy was delivered to the treatment isocenter. Statistical analyses indicated that there were no significant differences in the mean stomach dose when the stomach was symmetrically expanded up to 3 cm or shifted laterally (medial, anterior or posterior shifts) by up to 3 cm. There was, however, a significant (P > 0.01) difference in the mean dose when the stomach was shifted superiorly or inferiorly by ≥2.5 cm.     

Modeling the extrafocal radiation and monitor chamber backscatter for photon beam dose calculation

A simple analytical approach has been developed to model extrafocal radiation and monitor chamber backscatter for clinical photon beams. Model parameters for both the extrafocal source and monitor chamber backscatter are determined simultaneously using conventional measured data, i.e., in-air output factors for square and rectangular fields defined by the photon jaws. The model has been applied to 6 MV and 15 MV photon beams produced by a Varian Clinac 2300C/D accelerator. Contributions to the in-air output factor from the extrafocal radiation and monitor chamber backscatter, as predicted by the model, are in good agreement with the measurements. The model can be used to calculate the in-air output factors analytically, with an accuracy of 0.2% for symmetric or asymmetric rectangular fields defined by jaws when the calculation point is at the isocenter and 0.5% when the calculation point is at an extended SSD. For MLC-defined fields, with the jaws at the recommended positions, calculated in-air output factors agree with the measured data to within 0.3% at the isocenter and 0.7% at off-axis positions. The model has been incorporated into a Monte Carlo dose algorithm to calculate the absolute dose distributions in patients or phantoms. For three MLC-defined irregular fields (triangle shape, C-shape, and L-shape), the calculations agree with the measurements to about 1% even for points at off-axis positions. The model will be particularly useful for IMRT dose calculations because it accurately predicts beam output and penumbra dose.     

MCNP-based computational model for the Leksell gamma knife

We have focused on the usage of MCNP code for calculation of Gamma Knife radiation field parameters with a homogenous polystyrene phantom. We have investigated several parameters of the Leksell Gamma Knife radiation field and compared the results with other studies based on EGS4 and PENELOPE code as well as the Leksell Gamma Knife treatment planning system Leksell GammaPlan (LGP). The current model describes all 201 radiation beams together and simulates all the sources in the same time. Within each beam, it considers the technical construction of the source, the source holder, collimator system, the spherical phantom, and surrounding material. We have calculated output factors for various sizes of scoring volumes, relative dose distributions along basic planes including linear dose profiles, integral doses in various volumes, and differential dose volume histograms. All the parameters have been calculated for each collimator size and for the isocentric configuration of the phantom. We have found the calculated output factors to be in agreement with other authors' works except the case of 4 mm collimator size, where averaging over the scoring volume and statistical uncertainties strongly influences the calculated results. In general, all the results are dependent on the choice of the scoring volume. The calculated linear dose profiles and relative dose distributions also match independent studies and the Leksell GammaPlan, but care must be taken about the fluctuations within the plateau, which can influence the normalization, and accuracy in determining the isocenter position, which is important for comparing different dose profiles. The calculated differential dose volume histograms and integral doses have been compared with data provided by the Leksell GammaPlan. The dose volume histograms are in good agreement as well as integral doses calculated in small calculation matrix volumes. However, deviations in integral doses up to 50% can be observed for large volumes such as for the total skull volume. The differences observed in treatment of scattered radiation between the MC method and the LGP may be important in this case. We have also studied the influence of differential direction sampling of primary photons and have found that, due to the anisotropic sampling, doses around the isocenter deviate from each other by up to 6%. With caution about the details of the calculation settings, it is possible to employ the MCNP Monte Carlo code for independent verification of the Leksell Gamma Knife radiation field properties.     

A Monte Carlo based three-dimensional dose reconstruction method derived from portal dose images

The verification of intensity-modulated radiation therapy (IMRT) is necessary for adequate quality control of the treatment. Pretreatment verification may trace the possible differences between the planned dose and the actual dose delivered to the patient. To estimate the impact of differences between planned and delivered photon beams, a three-dimensional (3-D) dose verification method has been developed that reconstructs the dose inside a phantom. The pretreatment procedure is based on portal dose images measured with an electronic portal imaging device (EPID) of the separate beams, without the phantom in the beam and a 3-D dose calculation engine based on the Monte Carlo calculation. Measured gray scale portal images are converted into portal dose images. From these images the lateral scattered dose in the EPID is subtracted and the image is converted into energy fluence. Subsequently, a phase-space distribution is sampled from the energy fluence and a 3-D dose calculation in a phantom is started based on a Monte Carlo dose engine. The reconstruction model is compared to film and ionization chamber measurements for various field sizes. The reconstruction algorithm is also tested for an IMRT plan using 10 MV photons delivered to a phantom and measured using films at several depths in the phantom. Depth dose curves for both 6 and 10 MV photons are reconstructed with a maximum error generally smaller than 1% at depths larger than the buildup region, and smaller than 2% for the off-axis profiles, excluding the penumbra region. The absolute dose values are reconstructed to within 1.5% for square field sizes ranging from 5 to 20 cm width. For the IMRT plan, the dose was reconstructed and compared to the dose distribution with film using the gamma evaluation, with a 3% and 3 mm criterion. 99% of the pixels inside the irradiated field had a gamma value smaller than one. The absolute dose at the isocenter agreed to within 1% with the dose measured with an ionization chamber. It can be concluded that our new dose reconstruction algorithm is able to reconstruct the 3-D dose distribution in phantoms with a high accuracy. This result is obtained by combining portal dose images measured prior to treatment with an accurate dose calculation engine.     

Film dosimetry for intensity modulated radiation therapy: dosimetric evaluation

X-ray film has been used for the dosimetry of intensity modulated radiation therapy (IMRT). However, the over-response of the film to low-energy photons is a significant problem in photon beam dosimetry, especially in regions outside penumbra. In IMRT, the radiation field consists of multiple small fields and their outside-penumbra regions; thus, the film dosimetry, for it involves the source of over-response in its radiation field. In this study we aim to verify and possibly improve film dosimetry for IMRT. Two types of modulated beams were constructed by combining five to seven different static radiation fields using 6 MV x rays. For verifying film dosimetry, x-ray films and an ion chamber were used to measure dose profiles at various depths in a phantom. The film setups include both parallel and perpendicular arrangements against the beam incident direction. In addition, to reduce an over-response, we placed 0.01 in. (0.25 mm) thick lead filters on both sides of the film. Compared with ion-chamber measurement, measured dose profiles showed the film over-response at outside-penumbra and low-dose regions. The error increased with depths and approached 15% as a maximum for the field size of 15 cm x 15 cm at 10 cm depth. The use of filters reduced the error down to 3%. In this study we demonstrated that film dosimetry for IMRT involves sources of error due to its over-response to low-energy photons, with the error most transparent in the low-dose region. The use of filters could enhance the accuracy in film dosimetry for IMRT. In this regard, the use of an optimal filter condition is recommended.     

Evaluation of the precision of portal-image-guided head-and-neck localization: an intra- and interobserver study

There is increasing evidence that, for some patients, image-guided intensity-modulated radiation therapy (IMRT) for head-and-neck cancer patients may maintain target dose coverage and critical organ (e.g., parotids) dose closer to the planned doses than setup using lasers alone. We investigated inter- and intraobserver uncertainties in patient setup in head-and-neck cancer patients. Twenty-two sets of orthogonal digital portal images (from five patients) were selected from images used for daily localization of head-and-neck patients treated with IMRT. To evaluate interobserver variations, five radiation therapists compared the portal images with the plan digitally reconstructed radiographs and reported shifts for the isocenter (approximately C2) and for a supraclavicular reference point. One therapist repeated the procedure a month later to evaluate intraobserver variations. The procedure was then repeated with teams of two therapists. The frequencies for which agreement between the shift reported by the observer and the daily mean shift (average of all observers for a given image set) were less than 1.5 and 2.5 mm were calculated. Standard errors of measurement for the intra- and interobserver uncertainty (SEMintra and SEMinter) for the individual and teams were calculated. The data showed that there was very little difference between individual therapists and teams. At isocenter, 80%-90% of all reported shifts agreed with the daily average within 1.5 mm, showing consistency in the ways both individuals and teams interpret the images (SEMinter approximately 1 mm). This dropped to 65% for the supraclavicular point (SEMinter approximately 1.5 mm). Uncertainties increased for larger setup errors. In conclusion, image-guided patient positioning allows head-and-neck patients to be controlled within 3-4 mm. This is similar to the setup uncertainties found for most head-and-neck patients, but may provide some improvement for the subset of patients with larger setup uncertainties.     

Verification of the alignment of a therapeutic radiation beam relative to its patient positioner

An easily-used system has been developed for routine measurements of the alignment of beams used for radiation therapy. The position of a beam of circular cross section is measured with respect to a steel sphere fixed to the patient positioning table and which should coincide with the isocenter. Since measurements can be done at all gantry angles (if one is available) and with all possible orientations of the patient table, the system is particularly suited for rapid and accurate measurements of gantry and/or couch isocentricity. Because it directly measures beam-to-positioner offset, the system provides an inclusive alignment verification of the total treatment system. The system has been developed for use with proton beams, but it could equally be used for alignment checks of an x-ray beam from a linear accelerator or other source. The measuring instrument consists of a scintillation screen viewed by a CCD camera, mounted on the gantry downstream of the sphere. The steel sphere is not large enough to stop protons of all energies of interest; however, it will always modify the energy and direction of protons which intersect it, creating a region of lower intensity (a "shadow") in the light spot created by the proton beam hitting the screen. The position of the shadow with respect to the light spot is a measure of the alignment of the system. An image-analysis algorithm has been developed for an automatic determination of the position of the shadow with respect to the light spot. The specifications and theoretical analysis of the system have been derived from Monte Carlo simulations, which are validated by measurements. We have demonstrated that the device detects beam misalignments with an accuracy (1 s.d.) of 0.05 mm, which is in agreement with the expected performance. This accuracy is more than sufficient to detect the maximum allowed misalignment of +/-0.5 mm.