[en] Purpose: The identification of appropriate high dose-rate parameters required to produce a 'uniform' dose distribution on the surface of a vaginal cylinder. The high dose-rate dose distribution is then compared to the traditional low dose-rate dose distributions obtained with Burnett cylinders. Methods and Materials: Dose distributions were calculated for 2, 3, and 3.5 cm diameter Burnett cylinders with and without crossing sources. Three models for the high dose-rate cylinders were developed and compared. High dose-rate dose distributions were calculated for 2, 3, and 3.5 cm diameter cylinders with and without anisotropic corrections for various dose specification points. Results: Low dose-rate distributions are not uniform over the surface of the applicator. The exact distribution depends upon cylinder diameter and upon the exact source loading. High dose rate dose distributions can be configured to provide for a 'uniform' dose on the surface, if an apex dose specification point is used together with dose specification points on the surface of the applicator opposite each dwell position. Conclusions: The conversion of low dose rate techniques to high dose rate techniques for vaginal cylinders involves an appreciation of the details of dose distributions of both approaches. The comparison between traditional low dose-rate distributions and high dose-rate distributions shows that, unlike the low dose-rate distributions, a relatively uniform high dose-rate distribution can be obtained independent of cylinder diameter. The clinical significance of the differences in the low dose-rate and high dose-rate dose distributions remains to be determined by long-term follow up of patients treated with high dose-rate techniques

[en] Purpose: To investigate the effect of lung density corrections on the dose delivered to lung cancer radiotherapy patients in a multi-institutional clinical trial, and to determine whether commonly available density-correction algorithms are sufficient to improve the accuracy and precision of dose calculation in the clinical trials setting. Methods and Materials: A benchmark problem was designed (and a corresponding phantom fabricated) to test density-correction algorithms under standard conditions for photon beams ranging from ⁶⁰Co to 24 MV. Point doses and isodose distributions submitted for a Phase III trial in regionally advanced, unresectable non-small-cell lung cancer (Radiation Therapy Oncology Group 88-08) were calculated with and without density correction. Tumor doses were analyzed for 322 patients and 1236 separate fields. Results: For the benchmark problem studied here, the overall correction factor for a four-field treatment varied significantly with energy, ranging from 1.14 (⁶⁰Co) to 1.05 (24 MV) for measured doses, or 1.17 (⁶⁰Co) to 1.05 (24 MV) for doses calculated by conventional density-correction algorithms. For the patient data, overall correction factors (calculated) ranged from 0.95 to 1.28, with a mean of 1.05 and distributional standard deviation of 0.05. The largest corrections were for lateral fields, with a mean correction factor of 1.11 and standard deviation of 0.08. Conclusions: Lung inhomogeneities can lead to significant variations in delivered dose between patients treated in a clinical trial. Existing density-correction algorithms are accurate enough to significantly reduce these variations

[en] We have studied the response of factory calibrated implantable MOSFET detectors to absorbed doses from 100 to 400 cGy. The average measured dose is quite close to the true delivered dose, with the standard deviation falling between 1.4 and 3.6%. The measured dose tends to be slightly underestimated for smaller doses, while it tends to be slightly overestimated for larger doses. Thus, although the calibration of the detector is most accurate for doses close to the calibration dose of 200 cGy, it may be used over the range of commonly used doses in fractionated radiotherapy

[en] The authors have studied the performance of a new version of the implantable MOSFET detector designed for clinical use. Detectors were irradiated under 6- and 18-MV beams in water at body temperature to the calibration dose of 200 cGy/fraction for 20 daily fractions to determine their response and reproducibility. Additional measurements were performed in a solid phantom under 6-MV irradiation at room temperature to daily doses of 100, 150, 200, 250, 400 and 600 cGy/fraction. Finally, the angular dependence with respect to rotation about the detector's longitudinal axis was studied. At body temperature, the detectors were found to have an average response within ±3% of the calibration dose with a standard deviation of 2% or less. At room temperature, doses lower than the calibration dose led to a slight overresponse while doses higher than the calibration dose led to a slight underresponse. Angular dependence was not significant, on average within 1.2% of the mean. When used as specified by the manufacturer, these detectors should provide data useful to verify the delivered dose for external beam radiation therapy within a certain tolerance

[en] Purpose: The purpose of this work was to clarify the interactions between the parameters used in the γ index with the surface-based distance method, which itself can be viewed as a generalized version of the γ index. The examined parameters included the distance to agreement (DTA)/dose difference (DD) criteria, the percentage used as a passing criterion, and the passing percentage for given DTA/DD criteria. The specific aims of our work were (1) to understand the relationships between the parameters used in the γ index, (2) to determine the detection limit, or the minimum detectable error, of the γ index with a given set of parameters, and (3) to establish a procedure to determine parameters that are consistent with the capacity of an IMRT QA system. Methods: The surface-based distance technique with dose gradient factor was derived, and then the relationship between surface-based distance and γ index was established. The dose gradient factor for plans and measurements of 10 IMRT patients, 10 spine stereotactic radiosurgery (SRS) patients, and 3 Radiological Physics Center (RPC) head and neck phantom were calculated and evaluated. The detection limits of the surface-based distance and γ index methods were examined by introducing known shifts to the 10 IMRT plans. Results: The means of the dose gradient factors were 0.434 mm/% and 0.956 mm/% for the SRS and IMRT plans, respectively. Key quantities (including the mean and 90th and 99th percentiles of the distance distribution) of the surface-based distance distribution between two dose distributions were linearly proportional to the actual shifts. However, the passing percentage of the γ index for a given set of DTA/DD criteria was not associated with the actual shift. For IMRT, using the standard quality assurance criteria of 3 mm/3% DTA/DD and a 90% passing rate, we found that the detection limit of the γ index in terms of global shift was 4.07 mm/4.07 % without noise. Conclusions: Surface-based distance is a direct measure of the difference between two dose distributions and can be used to evaluate or determine parameters for use in calculating the γ index. The dose gradient factor represents the weighting between spatial and dose shift and should be determined before DTA/DD criteria are set. The authors also present a procedure to determine γ index parameters from measurements.

[en] Basic dosimetric properties of 6 MV and 18 MV photon beams from a Varian Clinac 21EX accelerator operating without the flattening filter have been measured. These include dose rate data, depth dose dependencies and lateral profiles in a water phantom, total scatter factors and transmission factors of a multileaf collimator. The data are reviewed and compared with measurements for the flattened beams. The unflattened beams have the following: a higher dose rate by factors of 2.3 (6 MV) and 5.5 (18 MV) on the central axis; lower out-of-field dose due to reduced head scatter and softer spectra; less variation of the total scatter factor with field size; and less variation of the shape of lateral dose profiles with depth. The findings suggest that with a flattening filter free accelerator better radiation treatments can be developed, with shorter delivery times and lower doses to normal tissues and organs

[en] In conventional clinical linear accelerators, the flattening filter scatters and absorbs a large fraction of primary photons. Increasing the beam-on time, which also increases the out-of-field exposure to patients, compensates for the reduction in photon fluence. In recent years, intensity modulated radiation therapy has been introduced, yielding better dose distributions than conventional three-dimensional conformal therapy. The drawback of this method is the further increase in beam-on time. An accelerator with the flattening filter removed, which would increase photon fluence greatly, could deliver considerably higher dose rates. The objective of the present study is to investigate the dosimetric properties of 6 and 18 MV photon beams from an accelerator without a flattening filter. The dosimetric data were generated using the Monte Carlo programs BEAMnrc and DOSXYZnrc. The accelerator model was based on the Varian Clinac 2100 design. We compared depth doses, dose rates, lateral profiles, doses outside collimation, total and collimator scatter factors for an accelerator with and without a flatteneing filter. The study showed that removing the filter increased the dose rate on the central axis by a factor of 2.31 (6 MV) and 5.45 (18 MV) at a given target current. Because the flattening filter is a major source of head scatter photons, its removal from the beam line could reduce the out-of-field dose

[en] Purpose: The purpose of this work was to investigate the relationship between dynamically accumulated dose (dynamic dose) and 4D accumulated dose (4D dose) for irradiation of moving tumors, and to quantify the dose uncertainty induced by tumor motion. Methods: The authors established that regardless of treatment modality and delivery properties, the dynamic dose will converge to the 4D dose, instead of the 3D static dose, after multiple deliveries. The bounds of dynamic dose, or the maximum estimation error using 4D or static dose, were established for the 4D and static doses, respectively. Numerical simulations were performed (1) to prove the principle that for each phase, after multiple deliveries, the average number of deliveries for any given time converges to the total number of fractions (K) over the number of phases (N); (2) to investigate the dose difference between the 4D and dynamic doses as a function of the number of deliveries for deliveries of a “pulsed beam”; and (3) to investigate the dose difference between 4D dose and dynamic doses as a function of delivery time for deliveries of a “continuous beam.” A Poisson model was developed to estimate the mean dose error as a function of number of deliveries or delivered time for both pulsed beam and continuous beam. Results: The numerical simulations confirmed that the number of deliveries for each phase converges to K/N, assuming a random starting phase. Simulations for the pulsed beam and continuous beam also suggested that the dose error is a strong function of the number of deliveries and/or total deliver time and could be a function of the breathing cycle, depending on the mode of delivery. The Poisson model agrees well with the simulation. Conclusions: Dynamically accumulated dose will converge to the 4D accumulated dose after multiple deliveries, regardless of treatment modality. Bounds of the dynamic dose could be determined using quantities derived from 4D doses, and the mean dose difference between the dynamic dose and 4D dose as a function of number of deliveries and/or total deliver time was also established.

[en] We have developed a quantitative computed tomography (CT)-based quality assurance (QA) tool for evaluating the accuracy of manufactured compensators used in passively scattered proton therapy. The thickness of a manufactured compensator was measured from its CT images and compared with the planned thickness defined by the treatment planning system. The difference between the measured and planned thicknesses was calculated with use of the Euclidean distance transformation and the kd-tree search method. Compensator accuracy was evaluated by examining several parameters including mean distance, maximum distance, global thickness error and central axis shifts. Two rectangular phantoms were used to validate the performance of the QA tool. Nine patients and 20 compensators were included in this study. We found that mean distances, global thickness errors and central axis shifts were all within 1 mm for all compensators studied, with maximum distances ranging from 1.1 to 3.8 mm. Although all compensators passed manual verification at selected points, about 5% of the pixels still had maximum distances of >2 mm, most of which correlated with large depth gradients. The correlation between the mean depth gradient of the compensator and the percentage of pixels with mean distance <1 mm is -0.93 with p < 0.001, which suggests that the mean depth gradient is a good indicator of compensator complexity. These results demonstrate that the CT-based compensator QA tool can be used to quantitatively evaluate manufactured compensators.

[en] We investigated the applicability of single-use MOSFET detectors as quality-assurance devices. Using ten accelerators available at our institution, we performed output measurements in both water and solid phantoms under photon irradiation. The MOSFET detectors performed well within the manufacturer's specifications, with average deviations of 2.1% and 0.7% for the 6 and 18 MV beams, respectively. The strength of the detector's design, including its wireless set-up, factory calibration and direct read-out, makes the system an acceptable independent quality-assurance device for use in verifying machine output within an accuracy of ±5%. The MOSFET detectors provide a quick check of machine output, which can be efficacious in detecting gross errors in machine calibrations