P Jursinic

A 2‐D diode array and analysis software for verification of intensity modulated radiation therapy delivery

An analysis is made of a two-dimensional array of diodes that can be used for measuring dose generated in a plane by a radiation beam. This measuring device is the MapCHECK Model 1175 (Sun Nuclear, Melbourne, FL). This device has 445 N-type diodes in a 22 x 22 cm2 2-D array with variable spacing. The entire array of diodes is easily calibrated to allow for measurements in absolute dose. For IMRT quality assurance, each beam is measured individually with the beam central axis oriented perpendicular to the plane of diodes. Software is available to do the analytical comparison of measurements versus dose distributions calculated by a treatment planning system. Comparison criteria of percent difference and distance-to-agreement are defined by the operator. Data are presented that show the diode array has linear response when beam fluence changes by over 300-fold, which is typical of the level of modulation in intensity modulated radiation therapy, IMRT, beams. A linear dependence is also shown for a 100-fold change in monitors units delivered. Methods for how this device can be used in the clinic for quality assurance of IMRT fields are described. Measurements of typical IMRT beams that are modulated by compensators and MLCs are presented with comparisons to treatment planning system dose calculations. A time analysis is done for typical IMRT quality assurance measurements. The setup, calibration, and analysis time for the 2-D diode array are on the order of 20 min, depending on numbers of fields. This is significantly less time than required to do similar analysis with radiographic film. The 2-D diode array is ideal for per-plan quality assurance after an IMRT system is fully commissioned.

Evaluation of Kodak EDR2 film for dose verification of intensity modulated radiation therapy delivered by a static multileaf collimator.

A new type of radiographic film, Kodak EDR2 film, was evaluated for dose verification of intensity modulated radiation therapy (IMRT) delivered by a static multileaf collimator (SMLC). A sensitometric curve of EDR2 film irradiated by a 6 MV x-ray beam was compared with that of Kodak X-OMAT V (XV) film. The effects of field size, depth and dose rate on the sensitometric curve were also studied. It is found that EDR2 film is much less sensitive than XV film. In high-energy x-ray beams, the double hit process is the dominant mechanism that renders the grains on EDR2 films developable. As a result, in the dose range that is commonly used for film dosimetry for IMRT and conventional external beam therapy, the sensitometric curves of EDR2 films cannot be approximated as a linear function, OD = c * D. Within experimental uncertainty, the film sensitivity does not depend on the dose rate (50 vs 300 MU/min) or dose per pulse (from 1.0 x 10(-4) to 4.21 x 10(-4) Gy/pulse). Field sizes and depths (up to field size of 10 x 10 cm2 and depth = 10 cm) have little effect on the sensitometric curves. Percent depth doses (PDDs) for both 6 and 23 MV x rays were measured with both EDR2 and XV films and compared with ion chamber data. Film data are within 2.5% of the ion chamber results. Dose profiles measured with EDR2 film are consistent with those measured with an ion chamber. Examples of measured IMRT isodose distributions versus calculated isodoses are presented. We have used EDR2 films for verification of all IMRT patients treated by SMLC in our clinic. In most cases, with EDR2 film, actual clinical daily fraction doses can be used for verification of composite isodose distributions of SMLC-based IMRT.

Characteristics of sensitometric curves of radiographic films.

A new type of radiographic film, EDR (extended dose range) film, has been recently become available for film dosimetry. It is particularly attractive for composite isodose verification of intensity modulated radiation therapy because of its low sensitivity relative to the more common Kodak XV film. For XV film, the relationship between optical density and dose, commonly known as the sensitometric curve, depends linearly on the dose at low densities. Unlike XV film, the sensitometric curve of EDR film irradiated by megavoltage x rays is not linearly dependent on the dose at low densities. In this work, to understand the mechanisms governing the shape of the sensitometric curves, EDR film was studied with kilovoltage x rays, 60Co gamma rays, megavoltage x rays, and electron beams. As a comparison, XV film was also studied with the same beams mentioned above. The model originally developed by Silberstein [J. Opt. Soc. Am. 35, 93-107, 1945)] is used to fit experimental data. It is found that the single hit model can be used to predict the sensitometric curve for XV films irradiated by all beams used in this work and for EDR films exposed to kilovoltage x rays. For EDR film irradiated by 60Co gamma rays, megavoltage x rays, and electron beams, the double hit model is used to fit the sensitometric curves. For doses less than 100 cGy, a systematic difference between measured densities and that predicted by the double hit model is observed. Possible causes of the observed differences are discussed. The results of this work provide a theoretical explanation of the sensitometric behavior of EDR film.

Implementation of an in vivo diode dosimetry program and changes in diode characteristics over a 4-year clinical history.

An in vivo dosimetry system that used n-type semiconductor diodes with integral build-up caps was introduced into the clinic. Measurements were made on the entrance surface of the patient and were compared to calculated diode readings expected from monitor units delivered by each beam. A method is given for calibration and correction for changes in diode sensitivity, dose-per-pulse effects, collimated field-size (head-scatter factor), wedges, compensators, and scatter from blocks and block trays. Clinically relevant temperature corrections are determined based on temperature measurements made with the diode used as a thermistor. Changes in diode characteristics over 4 years of clinical use are presented. With proper correction for clinical variables it is shown that difference between calculated and measured diode readings are within +/- 1% for phantom measurements and within +/- 3% for clinical measurements at a 95% confidence level. The correlation of dose measurements on the patient surface to dose inside a target volume is discussed.

Comparison of dosimetric characteristics of Siemens virtual and physical wedges

Dosimetric properties of Virtual Wedge (VW) and physical wedge (PW) in 6 and 23 MV photon beams from a Siemens Primus linear accelerator, including wedge factors, depth doses, dose profiles, peripheral doses and surface doses, are compared. While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have a stronger depth dependence than VWF due to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with PW, except in the toe area of large wedge angles with large field sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields while PW fields have lower surface doses. Surface doses for both VW and PW increase with field size and slightly with wedge angle. For VW fields with wedge angles 45 degrees and less, the initial gap up to 3 cm is dosimetrically acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields.

Clinical implementation of AAPM TG61 protocol for kilovoltage x-ray beam dosimetry

Historically, there have been a variety of dosimetry protocols used for kilovoltage x-ray therapy beams with a set of conversion factors and correction factors taken from different references. Corresponding to the continued installation and use of kilovoltage machines, the American Association of Physicists in Medicine (AAPM) presented a unified protocol developed by Task Group 61 (TG61). TG61 determines the absorbed dose to water with an ionization chamber calibrated in air in terms of air kerma (Nk). TG61 presents both an in-air method and an in-phantom method. In this work we only examine the TG61 in-air method. Our traditional dosimetry procedure, which is based upon NCRP Report 69 and on material found in standard medical physics texts, has been compared to the TG61. A variety of kilovoltage beam energies were examined with a set of various field sizes and source to surface distances. TG61 published updated data for the mass absorption coefficient ratios, backscatter factors, and the average energy per ion pair factor. The following conclusions have been reached: (1) Our traditional procedures and the TG61 protocol for in-air measurements are equivalent. (2) The conversion and correction factors used in TG61 are different by up to 4.5% compared to the old factors that we have used. (3) The application of the TG61 factors can result in up to 5% differences in the determination of the absorbed dose.

SU‐FF‐T‐442: Use Of A 2D Array Of Diodes To Test The Accuracy Of MLC Leaf Position And Gap Width

Purpose: For MLC‐modulated IMRT fields, it is important that the leaf positioning be accurate and that the gap between opposing leaf pairs be precisely set. One test of the MLC is the “picket fence” pattern, which consists of narrow, sequentially delivered, abutting sub‐fields of radiation. Typically in current practice, a radiographic film is irradiated and visually inspected for a discontinuous radiation pattern. Here, a 2D‐array of 455 diodes (MapCheck, Sun Nuclear, Melbourne, FL) was used to detect the radiation pattern. Method and Materials: The array was oriented such that 120 diodes aligned with the center of the MLC leaves at each of their sub‐field edges. Measurements were compared against a dose distribution calculated by the XiO treatment planning system (Computerized Medical Systems, St. Louis, MO) for a picket‐fence segment pattern delivered to a flat phantom. MapCheck software was used to obtain the relative percent difference, normalized to the center diode, between the 455 diode‐measured and calculated points. Spreadsheet software was written that extracted the desired 120 sample points from the 455 measured points and performed routine data analysis in less than 30 s. Results: Four different Siemens accelerators equipped with 29‐leaf pair MLCs were measured over a two‐month period. By careful adjustment of MLC leaves, the percent difference between measurement and calculation, averaged for 120 points, could be kept within ±5% with a standard deviation of 6.0%. This standard deviation is attributable to individual MLC leaves and deviations in leaf position for various set positions across the field. The trial‐to‐trial variation is about 2% for the average difference for the 120 points. A 1‐mm leaf offset in the picket fence delivery corresponded to an average difference of about 17%. Conclusion: A rapid, quantitative method, sensitive to submillimeter changes in MLC leaf positioning and gap width has been devised.

TU‐FF‐A2‐03: Image‐Guided Helical Tomotherapy to Treat Advanced Cancers of the Scalp: Prospects for Dose Conformity and Clinical Outcome

Purpose: to demonstrate the improved target‐dose uniformity of helical tomotherapy over IMRT and electron techniques to treat scalp lesions, to summarize our institutions scalp‐treatment outcomes using tomotherapy, and to estimate the required expansion margins for non‐image‐guided scalp treatments. Method and Materials: helical‐tomotherapy planning and delivery was done using the TomoTherapy Hi‐Art System (TomoTherapy, Inc., Madison, WI). The prescribed dose for the tomotherapy plans (average 50.4 Gy) covered 95% of the PTV. For two patients, we also generated a seven‐field photonIMRT plan and a single‐field electron plan, scaling the monitor units so that the tumor DVH passed through the corresponding tomotherapy plans prescription point. We followed up 15 patients with advanced cutaneous scalp lesions (average length 5.2 cm) treated with tomotherapy, noting the incidence of subsequent local recurrences. Using the systematic and random deviations among the MVCT‐guided pre‐treatment shifts, we calculated disease‐site specific expansion margins for non‐image‐guided scalp treatments. Results: the tumor DVH fell off more sharply for the tomotherapy plans than for either the IMRT or electron techniques. The electron plans yielded a better brain DVH than either tomotherapy or IMRT; however, the brain DVH was clinically acceptable (V20 < 10% and V30 negligible) for all three techniques. Among the 15 patients reviewed, a local recurrence arose only for a palliative angiosarcoma case. Among vertex‐scalp cases, an overall margin of approximately 4 mm was obtained. The margin was approximately 5 mm for forehead and neck treatments, and within 2 mm for auricular cases. Conclusions: our dosimetric comparisons demonstrate that helical tomotherapy provides superior tumor dose uniformity over IMRT or electrons, with adequate region‐at‐risk sparing. Curative‐intent treatments with helical tomotherapy indicate favorable short‐term local control of advanced scalp lesions. Data from the MVCT‐guided patient shifts illustrate the appropriate expansion margins, for non‐image‐guided scalp treatment techniques.

TU‐E‐ValB‐06: Direct Aperture Optimization Based Step‐And‐Shoot IMRT with Respiratory Gating

Purpose: Validation of respiration gated IMRT on Siemens linear accelerator has not been reported. This work investigates the reliability, accuracy and efficiency of the delivery of respiratory gated IMRT on a Siemens accelerator. Method and Materials: A Siemens Primus accelerator was interfaced with a pressure sensor belt (Anzai Gating system) to deliver step‐and‐shoot IMRT. A series of IMRT fields, including actual patient, as well as custom segmented fields including the “Picket Fence” were delivered with and without gating (interruption) using a variety of different gating parameters (e.g., duty cycle). Radiographic films and 2D diode array (MapCheck , Sun Nuclear) were used to measure dose distributions. The dose distributions measured with and without gating were compared to identify any delivery error from gating. IMRT with multiple beam angles was also delivered, with and without gating, on a cubic motion phantom. Special measurements were made to individually evaluate dark current, small MU non‐linearity and flatness degradation and their cumulative effects on multiple times interrupted fields. Delivery times for MLC and compensator IMRT plans with various segmentations for sample lung and breast cases were compared. Results: Beam characteristics for the Siemens accelerator was not altered by gating and gated IMRT with Siemens/Anzai systems was found to be accurate and reliable. Measured dose distributions agreed with the calculated results and/or with those delivered without gating. Picket fence results with and without beam gating indicated same MLC positioning accuracy during the gating. The delivery times for the DAO‐based IMRT plans that had small numbers of segments were shorter than those for any other types of IMRT plans including compensators and were comparable with those for 3DCRT. Conclusion: The delivery of gated‐IMRT with Siemens/Anzai systems is reliable and accurate. The DAO‐based IMRT is preferred for gated delivery in terms of treatment times.

MO‐D‐224A‐03: Dependence of Planar IMRT QA On MLC Positional Inaccuracies

Purpose: This work aims to investigate the sensitivity of IMRT QA done by means of planar dosimetry to MLC positional inaccuracies. Also we propose an accurate method for measurement of MLC positioning errors using the 2D diode array. Methods: A method to measure the MLC position errors by using 2D‐arrary of 455 diodes (MapCheck, Sun Nuclear) is developed. Our method utilizes the fact that each diodes signal will be most sensitive to the MLC position error when its center coincides with the edge of a MLC leaf where a small deviation from the accurate position produces sharp increase or decrease in the diode output. We designed various multi‐segmented test patterns based on this principle that can evaluate the MLC deviations with sub‐mm accuracy at multiple MLC banks simultaneously. By using this information as a deviation histogram, we created deliberately erroneous IMRT fields with varying standard deviations of MLC leaf position error. MapCHECK planar dose analysis is performed and the sensitivity of the IMRT QA procedure to MLC position inaccuracy using MapCHECK device is evaluated. Results: Our results indicate that for SIEMENS Primus and MD machines, the MLC positional errors show a standard deviation of about +/− 0.7mm. Right after the MLCcalibration, this deviation might reduce to 0.55mm. It has been found that a single leaf position can vary by as much as 1mm between two consecutive measurements. Fields with less number of segments that are generated by Direct Aperture Optimization are found to be less sensitive to these errors by measurements with deliberately modified fields with random MLC inaccuracies. Conclusion: A method to accurately quantify MLC position is proposed and used to obtain a distribution of leaf position errors for many leaves at multiple banks. The sensitivity of planar dosimeter to the MLC positioning errors is investigated.