Percentage Depth Dose Measurements Research Articles

SU-FF-T-96: Characteristics of Narrow Field Photon Beam Measurements for a Micro-MLC Based Radiosurgery System

Purpose: To determine the response of various detectors in the commissioning of narrow field megavoltage photon beams for a micro-MLC based radiosurgery system. Method and Materials: We commissioned both a standard and a high dose rate 6 MV photon beams from a Trilogy unit (Varian Medical systems, Palo Alto CA) for the pencil beam algorithm using a micro-MLC based Radiosurgery system (BrainLAB AG Germany). The loss of lateral equilibrium, high dose gradients, and spectral changes within high energy narrow photon beams imposes restrictions on detector composition and size used for relative measurements. We investigated the response of two p-type Si-diodes (Scanditronix photon and stereotactic diodes), a diamond (PTW) detector, a cylindrical (Scanditronix RK chamber) and a micro-ion-chamber (Exradin #a16) for depth dose and relative output measurements. Results: Percent depth dose measurement with photon diode (guarded) yielded consistently higher measurements by 1–2% compared to ionization chamber results for larger depths with increasing field sizes. Stereotactic diode measurements of depth doses for field sizes ⩾18×18 mm2 yielded 1–2% lower values compared to ionization chamber results. For field sizes ⩽12×12mm2 photon diode depth dose measurements were higher by 1–5% at depths > 5cm compared to those measured by stereotactic diode. The relative output factors measured by photon diode, diamond detector and both ion chambers were within 1% agreement with each other for all field sizes ⩾ 30×30mm2. The stereotactic diode measurements were lower by ∼2.5% for all but the smallest field size of 6×6 mm2 compared to other detectors. Relative output factors for the high dose rate beam were higher by 2–4% for fields < 18×18mm2 compared to the standard 6 MV photon beam. Conclusion: Measurement of relative output factors and depth doses for megavoltage narrow photon fields require multiple detectors to correctly estimate the true response over the range of measurements.

SU‐FF‐T‐259: In‐Phantom Measurements of the Bremsstrahlung Dose for Megavoltage Electron Beams Incident On High Z Materials for Use in An Electron Multileaf Collimator

Purpose: There is a growing interest in the use of a multileaf collimator (MLC) for electron beam therapy to shape the fields as well as provide intensity modulated beams. One important consideration is the material and thickness of the collimating leaves. They must be sufficiently thick to stop the highest electron energy as well as minimize the bremsstrahlung radiation produced by the high Z material. In this work the total bremsstrtahlung dose is measured for megavoltage electron beams incident on three high Z materials used to simulate collimating leaves. Materials and Methods: A Varian Cl 2100 C is used to provide 6, 9, 12, 16 and 20 MeV electron beams. At the end of the standard cone are positioned high Z materials with thickness appropriate for multileaves. The high Z materials studied are cerrobend, brass and tungsten. Thermoluminescent dosimeters, radiographic film and a parallel‐plate ionization chamber are used to measure the x‐ray dose along the central beam axis in a solid‐water phantom at a source to surface of 100 cm. Film is used to obtain beam profiles at selected depths as well as provide an estimate of the beam quality. Results: The results show that for 20 MeV electrons the leakage as a percentage of the central‐axis (electron and x‐rays) dose is 4.4, 2.0 and 1.0 % for cerrobend, brass and tungsten, respectively, and 4.5% for the open field. The estimated beam quality of the x‐ray spectra inferred from the percentage depth dose measurements are 0.32, 0.63 and 0.86 MeV for brass, cerrobend and tungsten, respectively. Conclusion: The data show that 2 cm thick tungsten stops the 20 MeV electrons while reducing the bremsstrahlung dose to levels lower than that produced by the source in the unblocked field, and they also provide measurements for comparison with Monte Carlo calculations.

SU‐FF‐T‐85: Assessing the Reliability of Film and Semiconductor Detector in Measurement of Central Axis Depth Dose of Electron Beams

Purpose: To investigate the possibility of using the Kodak X‐Omat V film for the central axis depth‐dose measurements of electron beams.Method and materials: The X‐Omat V films were exposed in a MED‐TEC film phantom cassette with parallel orientation to produce central axis depth dose curves. These film data were compared with the measured percentage depth dose curves by using a Scanditronix scanning water phantom with both RK ion chamber and electron diode. Measurements were made at a source to surface distance (SSD) of 100 cm for the electron energies between 6 and 20 MeV on a Varian Clinac 2100C linear accelerator for field sizes of 6 × 6 cm2 to 20 × 20 cm2. Results: The comparison of the central axis depth dose parameters, R100, R90, R50, and RP measured by the film and diode with respect to the IC's values showed that the difference did not indicate the dependence either on the energy or cone size. In the buildup region, the film presented higher percent depth dose values for most energies and cone sizes but beyond the depth of maximum dose (R100), the results were opposite. The difference of every parameter for both film and diode was within the acceptable value of ±2 mm. Conclusions: The ionization chamber and diode measurements appear to be more reliable than the film measurements, but film is particularly useful for new electron cutout data where access to accelerator beam time is limited.

TU‐D‐M100F‐07: Simplified Clinical Quality Assurance for Helical Tomotherapy

Purpose: To evaluate a Web‐based Quality Assurance tool for maintaining the clinical integrity of the Tomotherapy machine. Method and Materials: This tool takes advantage of the output of the two distinct ion chamber systems on the Tomotherapy machine. One is the standard sealed ion chamber that resides in the head of a machine near the primary collimators and is used to calculate Monitor Units. The other is a set of pressurized Xenon detectors that are used for imaging. Both systems monitor ionizations independently and are rigidly attached so that their geometry is constant regardless of the orientation of the beam on the gantry which makes possible testing for both static and rotational modes. The collected data can be reviewed via a Web page and a downloadable Report in PDF format is available within seconds after collection of the data. Results: As an example, one performs a 200 second rotational delivery with the couch removed from the beam. The data are analyzed in both an integrated and pulse‐by pulse methodology to determine consistency between the two chamber systems, changes in output and changes in energy. The collected information is evaluated relative to a standard delivery created during machine commissioning. Comparison to standard ion chamber measurements in phantom under static conditions indicate that the analyses are consistent for calibration and more sensitive to energy changes than percentage depth dose measurements made in phantom. Additional information about the machine's performance is identified easily. Conclusion: New technology offers an opportunity to perform standard Quality Assurance tests in a new manner. The simple test described here is more sensitive and performed more quickly than traditional methodology. Other tests are possible. Conflict of Interest: Three authors are employees of Tomotherapy, Inc.

Calculating percent depth dose with the electron pencil‐beam redefinition algorithm

In the present work, we investigated the accuracy of the electron pencil‐beam redefinition algorithm (PBRA) in calculating central‐axis percent depth dose in water for rectangular fields. The PBRA energy correction factor C(E) was determined so that PBRA‐calculated percent depth dose best matched the percent depth dose measured in water. The hypothesis tested was that a method can be implemented into the PBRA that will enable the algorithm to calculate central‐axis percent depth dose in water at a 100‐cm source‐to‐surface distance (SSD) with an accuracy of 2% or 1‐mm distance to agreement for rectangular field sizes ≥2×2 cm. Preliminary investigations showed that C(E), determined using a single percent depth dose for a large field (that is, having side‐scatter equilibrium), was insufficient for the PBRA to accurately calculate percent depth dose for all square fields ≥2×2 cm. Therefore, two alternative methods for determining C(E) were investigated. In Method 1, C(E), modeled as a polynomial in energy, was determined by fitting the PBRA calculations to individual rectangular‐field percent depth doses. In Method 2, C(E) for square fields, described by a polynomial in both energy and side of square W [that is, C=C(E,W)], was determined by fitting the PBRA calculations to measured percent depth dose for a small number of square fields. Using the function C(E, W), C(E) for other square fields was determined, and C(E) for rectangular field sizes was determined using the geometric mean of C(E) for the two measured square fields of the dimension of the rectangle (square root method). Using both methods, PBRA calculations were evaluated by comparison with measured square‐field and derived rectangular‐field percent depth doses at 100‐cm SSD for the Siemens Primus radiotherapy accelerator equipped with a 25×25‐cm applicator at 10 MeV and 15 MeV. To improve the fit of C(E) and C(E, W) to the electron component of percent depth dose, it was necessary to modify the PBRA's photon depth dose model to include dose buildup. Results showed that, using both methods, the PBRA was able to predict percent depth dose within criteria for all square and rectangular fields. Results showed that second‐ or third‐order polynomials in energy (Methods 1 and 2) and in field size (Method 2) were typically required. Although the time for dose calculation using Method 1 is approximately twice that using Method 2, we recommend that Method 1 be used for clinical implementation of the PBRA because it is more accurate (most measured depth doses predicted within approximately 1%) and simpler to implement.PACS number: 87.53.Fs

Output factor measurements for a kilovoltage x-ray therapy unit

Output factors at the surface for treatment cones and lead cut-outs have been measured for a Pantak Therapax SXT 150 superficial therapy unit with x-ray beam qualities from 1 to 13 mm A1 HVL. A variety of phantom materials and two ionisation chambers were tested for their suitability in output factor and percentage depth dose measurement. Solid water proved a useful water-equivalent phantom material with discrepancies between measurements in water and solid water less than 2.3% for percentage depth dose and less than 0.6% for output factors. Larger measurement discrepancies were found for Plastic Water and Perspex. A PTW Markus chamber was found to compare well with a NE 2532/3 low energy chamber in percentage depth dose measurement, but discrepancies arose between the chambers in output factor measurements, up to 5% for small field sizes. Measurements indicated that the Markus chamber had an energy dependent response in the kilovoltage range, which could account for the discrepancy in output factor measurement.

TH-C-J-6B-04: Performance Characterization of Megavoltage CT Imaging On a Helical Tomotherapy Unit

Purpose: This study characterizes the image performance of the megavoltage computed tomography (MVCT) imaging from the Hi‐ART II tomotherapy treatment unit using measures of noise, uniformity, contrast, linearity, and spatial resolution as a function of absorbed dose.Method and Materials: MVCT image performance was characterized using a AAPM CT Performance Phantom. The following tests were performed as a function of collimator pitch and image pixel size: imagenoise,spatial uniformity, objective (modulation transfer function) and subjective (hole patterns) spatial resolution,contrast linearity, and contrast‐detail resolution. The multiple scan average dose (MSAD) was determined in a 20 cm diameter cylindrical phantom as a function of collimator pitch, and the beam energy was estimated from percent depth dose measurements. Results: The uniformity and spatial resolution of MVCT images generated by the HI‐ART II are comparable to that of diagnostic CTimages. The MVCT scan contrast is linear with respect to electron density of target material. MVCT images do not have the same performance characteristics as state‐of‐the‐art diagnostic images based on objective measures of noise and low‐contrast resolution, but large object low contrast resolution is sufficient to for identification of many important soft tissue structures. The MSAD ranges from 0.25 cGy to 1.1 cGy, and the effective beam energy is 3.5 MV. Conclusion: MVCT images from the Hi‐ART II are adequate for verifying a patient's position at the time of radiation therapy. These images also provide sufficient contrast to delineate many soft‐tissue structures. Hence, these images are adequate for reconstruction of the dose actually delivered to anatomical structures during radiotherapy. The dose delivered by the MVCT is low enough that images can be acquired on a daily basis.

TU‐A‐T‐6C‐01: TG‐70: Clinical Electron Beam Dosimetry: Supplement to TG‐25

Purpose: The purpose of this talk is to describe the intent and content of Task Group 70: Clinical electron beam dosimetry: supplement to TG‐25. Task Group 70 of the AAPM was formed originally to address the issues of clinical electron beam dosimetry in light of the changes brought about by the new calibration protocol TG 51. The latter task group gives a clear description of the procedure required to establish the dose rate at one point in a radiation beam. However, clinical dosimetry demands the ability to find the dose rate at any point in a radiation treatments field. TG 70 addresses these issues for electron beam treatments. Originally, Task Group 25 addressed these issues at the introduction of calibration protocol, TG 21 and provided direction for all aspects of clinical electron beam application in radiation therapy. Its initial intent was to give recommendations to practicing medical physicists, techniques for them to follow, and reasons for the particular recommendations. Task Group 70 has the same intent as the original TG 25 but to do so in light of not only TG 51 but also to take into account advances in the field since the time of TG 25. Improvements in the accuracy of correction factors such as the incorporation of realistic stopping powers for electron beams, Pwall, Pgr and Pfl, are describe along with improvements in electron beam treatment planning or improvements in techniques that have taken place since the original TG25 report. Each section states clearly the reason for that particular section, the problems that are addressed, or the new information that has been developed since TG 25. The solutions to those problems, the actions needed to be taken by practicing clinical medical physicist, and other direction is provided in each section. Practical clinical aspects of electron beam dosimetry will be discussed including techniques for determining the output factors, percentage depth dose, and output factors and central axis percentage depth dose measurements for small, irregularly shaped electron beams. The final section of the task group report describes clinical applications of electron beams.

Characterization of electron contamination in megavoltage photon beams

The purpose of the present study is to characterize electron contamination in photon beams in different clinical situations. Variations with field size, beam modifier (tray, shaping block) and source-surface distance (SSD) were studied. Percentage depth dose measurements with and without a purging magnet and replacing the air by helium were performed to identify the two electron sources that are clearly differentiated: air and treatment head. Previous analytical methods were used to fit the measured data, exploring the validity of these models. Electrons generated in the treatment head are more energetic and more important for larger field sizes, shorter SSD, and greater depths. This difference is much more noticeable for the 18 MV beam than for the 6 MV beam. If a tray is used as beam modifier, electron contamination increases, but the energy of these electrons is similar to that of electrons coming from the treatment head. Electron contamination could be fitted to a modified exponential curve. For machine modeling in a treatment planning system, setting SSD at 90 cm for input data could reduce errors for most isocentric treatments, because they will be delivered for SSD ranging from 80 to 100 cm. For very small field sizes, air-generated electrons must be considered independently, because of their different energetic spectrum and dosimetric influence.

Evaluation of the performance of VIPAR polymer gels using a variety of x-ray and electron beams

The aim of this investigation was the evaluation of the usefulness of N-vinyl pyrrolidone argon (VIPAR) polymer gel dosimetry for relative dose measurements using the majority of types and energies of radiation beams used in clinical practice. For this reason, VIPAR polymer gels were irradiated with the following beams: 6 and 23 MV photons (maximum dose: 15 Gy) and 6, 9, 12, 15, 18 and 21 MeV electrons (90% dose: 15 Gy). Using 6 MV x-rays, a linear gel dose response was verified for doses up to 20 Gy. Assuming linearity of response for the rest of the photon and electron beams used in this study, percentage depth dose measurements were derived. For all beams used and the range of relative doses studied, a satisfying agreement was observed between percentage depth dose measurements performed using the VIPAR gel–MRI method and an ion chamber, validating the assumption that a linear gel dose response holds for all photon and electron beams studied. VIPAR gels, therefore, can be used for relative dose distribution measurements using photons or electrons of any typical energy used in external radiotherapy applications. It is also demonstrated that two-dimensional dose distribution measurements through an irradiated (9 MeV electrons, 3 cm × 3 cm cone) VIPAR gel volume can be easily obtained.

Conversion of measured percentage depth dose to tissue maximum ratio values in stereotactic radiotherapy

For many treatment planning systems tissue maximum ratios (TMR) are required as input. These tissue maximum ratios can be measured with a 3D computer-controlled water phantom; however, a TMR measurement option is not always available on such a system. Alternatively TMR values can be measured ‘manually’ by lowering the detector and raising the water phantom with the same distance, but this makes TMR measurements time consuming. Therefore we have derived TMR values from percentage depth dose (PDD) curves.Existing conversion methods express TMR values in terms of PDD, phantom scatter factor (Sp), and inverse square law. For stereotactic treatments circular fields ranging from 5–50 mm (19 cones) are used with the treatment planning system XKnife (Radionics). The calculation of TMR curves for this range is not possible with existing methods. This is because PDD curves of field sizes smaller than 5 mm (smallest cone size) are needed, but these cones are not provided. Besides, for field sizes smaller than 40 mm, the phantom scatter factor is difficult to determine and will introduce significant errors.To overcome these uncertainties, an alternative method has been developed to obtain TMR values from PDD data, where absolute doses are expressed in terms of PDD, total scatter factor and inverse square law. For each depth, the dose as a function of field size is fitted to a double exponential function. Then the TMR is calculated by taking the ratio of this function at the depth of interest and the reference depth, for the correct field size.For all 19 cones the total scatter factor and PDDs have been measured with a shielded diode in water for a 6 MV photon beam. Calculated TMR curves are compared with TMR values measured with a diode. The agreement is within 2%. Therefore this relatively simple conversion method meets the required accuracy for daily dose calculation in stereotactic radiotherapy. In principle this method could also be applied for other small field sizes such as those formed with a mini multileaf collimator.

A study of Rhizophora spp wood phantom for dosimetric purposes using high-energy photon and electron beams

Previous scattering and depth-dose investigations involving use of the Malaysian hardwood Rhizophora spp have shown this medium to produce good agreement with measurements made in water. Present study extends the comparison, now including measurements of percentage depth-dose made for photons at 6 MV and 5 and 12 MeV electron beams. For the 6 MV photon and 5 MeV electron beams, discrepancies between percentage depth-dose for Rhizophora spp and water, at all depths, are found to be within 2.6 and 2.4% respectively. At 12 MeV electron energies, measured percentage depth-doses in Rhizophora spp beyond 3.5 cm depth are found to be in significant discord with those for water. The absorbed dose in water measured in Rhizophora spp at d max for all three beams produces discrepancies of no more than 1.1% when compared with measurements made in water.

Experimental determination of the overall perturbation factor for theNACP chamber in electron beams for dmax < d ⩽ d80

In electron beam dosimetry with an ionization chamber, a factor that corrects forthe cavity perturbation of the medium, Prepl, and one to account forthe disturbance due to the chamber wall material differing from the medium,Pwall, are required. The overall perturbation correction factor, pq = PreplPwall, has been introduced because of the difficulty inseparately measuring these two components. An advantage of parallel-plateionization chambers is that pq has been shown to be close to unity atdmax. However, many dosimetry applications require knowledge of theoverall perturbation factor at depths greater than dmax. Wedetermined pq for the NACP chamber at depths beyond dmax byintercomparing percentage depth dose measurements made with it with thoseobtained with a PTW/diamond detector for which pq was taken as unity at allthe measurement depths. Data were obtained at depths corresponding toapproximately the 90 and 80 per cent of the dose maxima for 20, 16, 12 and6 MeV incident electrons. The beam energy at depth, Ed, and the percentagedepth-dose gradient varied from 1.4 to 14.3 MeV and 0 to 5.8% mm-1respectively. Our results show that within the estimated uncertainty of 1.3%,pq,NACP is unity over the range of energies and dose gradientsstudied.

Electron contamination from the lead cutout used in kilovoltage radiotherapy

In kilovoltage radiotherapy, low-energy electrons are generated on the lead cutout plate frequently used to shape the x-ray field. They can cause surface dose enhancement and affect the percentage depth dose (PDD) and cutout factor when these parameters are measured with a parallel-plate chamber. Although many previous studies concerning a similar surface dose enhancement have suggested using a thin electron filter to eliminate the effect of these low-energy electrons, no details have been given regarding the adequacy of the filter. This paper describes the procedures required to test an electron filter for our Darpec 2000 superficial x-ray machine. A piece of plain paper was found to be an adequate electron filter, which could block all the low-energy electrons without affecting the PDD and cutout factor measurement. The surface dose enhancement caused by the presence of the cutout plate was studied with such a filter. The enhancement was found to increase with x-ray energy and decrease with cutout size as well as applicator size. At the highest x-ray energy in this study (120 kV, HVL 8.6 mmAl), the enhancement ranges from less than 1% for a 8 cm diameter cutout to a maximum of 14% for a 2 cm diameter cutout with a 10 cm applicator. For a 5 cm diameter applicator it ranges from about 1% to about 11%. The difference between the measured and predicted cutout factor displays similar characteristics. Since the low-energy electrons do not generate any useful treatment dose but their presence can lead to potential dosimetry problems, we suggest that a filter tested with the procedure described in this paper should be used when percentage depth dose and cutout factor for kilovoltage x-ray are measured with a parallel-plate chamber.

Dosimetry of hip irradiation for the prevention of heterotopic bone formation after arthroplasty

Purpose: The dosimetry of hip irradiation for the prevention of heterotopic bone formation following arthroplasty is complicated by the use of custom shielding in the treatment portal, and the fact that irradiation is usually required during a 48 hour period following surgery. Both the machine output and depth dose factors of the resulting fields are modified by the presence of the shielding blocks. A simplified dosimetric approach, based on correction factors for both the output and depth dose as a function of field geometry is presented for various megavoltage energy beams.Materials and Methods: Measurements of relative dose factors (RDF) and percentage depth dose (PDD) were carried out for different combinations of field size, block size and separation between adjacent blocks. Both RDF and PDD measurements were made in a water phantom. Ratios of RDF and PDD were obtained by dividing individual measurements or curves by the corresponding values for the open field (i.e., without blocks). The average values of these ratios constitute the correction factors to be applied for a given MU or treatment time calculation.Results: Extensive RDF and PDD measurements reveal that for the field and block dimensions of interest the correction factors for RDF can be parameterized as a function of separation between two adjacent blocks and beam energy alone and the depth correction factors are additionally only a function of depth. The correction factors for depth dose are equally valid for fixed source-skin distance techniques (that use PDD) and fixed source-axis distance techniques (that use TMR).Conclusion: A simple model for the calculation of output in hip irradiation is presented for the situation where the use of computer-based algorithms may not be practical. The model accurately predicts the RDF of the treatment portal to within 2% and the PDD to within 2% for the range of field sizes, block sizes, block gaps and beam energies of interest ignoring other variables.

A commercial IMRT treatment-planning dose-calculation algorithm

Purpose: The dose-calculation algorithm for a commercial arc-based intensity modulated radiation therapy (IMRT) treatment-planning and delivery system (Peacock, NOMOS Corporation) is described. Methods and Materials: The IMRT delivery system uses a dynamically controlled multileaf collimator with 40 leaves that project on our accelerator to either 1.0 × 0.84 cm 2 or 1.0 × 1.68 cm 2 at isocenter arranged in two banks of 20 leaves each. The dose-calculation algorithm uses tissue-phantom ratios derived from percent depth dose measurements, measured relative output data, and single leaf profiles. Some compromises are made in the algorithm terms to enable more straightforward dosimetry measurements and to reduce dose computation times. The dose calculation algorithm is presented, and consequences of the approximations are investigated using previously published 4 MV photon beam data. Results: Most of the approximations lead to dose errors of a few percent. However, the use of depth-invariant single-leaf profiles results in errors as large as 9% for 4 MV fixed beams. Conclusions: Large dosimetric errors are possible for small fixed fields using this algorithm. However, the algorithm is designed for tomotherapy dose delivery, where doses are delivered from multiple directions and depths. Investigations of the algorithm in more clinically relevant conditions have been conducted and show that the algorithm accuracy is 1.3% and therefore is clinically acceptable for tomotherapy.

Use of a new type of radiochromic film, a new parallel-plate micro-chamber, MOSFETs, and TLD 800 microcubes in the dosimetry of small beams

The dosimetry of the fields usually employed in radiosurgery requires the use of small detectors to measure Total Scatter Factor (Sc,p), Tissue Maximum Ratio (TMR), Percentage Depth Dose (PDD), and Off Axis Ratio (OAR). In this paper new dosimeters are investigated: a new type of radiochromic film, a micro parallel-plate chamber (filled with both air and tetramethylsilane, TMS), MOSFETs, and TLD-800 microcubes. Their behavior has been compared with the response of radiographic film and with the values obtained with BEAM Monte Carlo simulation. The experimental data confirm that dosimetry with radiochromic films and TLDs gives consistent results for all beam diameters. The parallel-plate micro chamber underestimates the Sc,p for the smallest field diameters (4.4 mm and 6.7 mm); MOSFETs show an over-estimation for the Sc,p of the 4.4 mm, 6.7 mm, and 10.5 mm field diameters. BEAM Monte Carlo simulation employing a parallel beam and a standard 6 MV x-ray spectrum has been used to obtain a correction factor as a function of the field size for both the parallel-plate micro chamber and MOSFETs. High accuracy measurements of PDD and TMR have been made in a water phantom both with radiochromic film and with the micro parallel-plate chamber and have been compared with the data computed by BEAM Monte Carlo simulation. The latter dosimeter is preferred because of the quicker and simpler use and because it gives immediate readout. Measurements of OAR made with radiochromic films and with radiographic films give differences in the 80%-20% penumbra width within 0.6 mm for field diameters ranging from 4.4 mm to 19 mm.

Application of the BPW-34 Photodiode for Depth Dose Measurements in the Presence of Inhomogeneities

In recent years, semiconductor detectors have found application in radiation dosimetry due to their high sensitivity and small dimensions. In previous papers the response of the common photodiode BPW-34 has been studied to evaluate its stability for use as a radiation dosemeter. In this study, the BPW-34 was used for measuring percentage depth dose in inhomogeneous tissues. Measurements were performed along the central axis of both 60 Co γ ray and 6 MV X ray fields in a Perspex phantom containing low density inhomogeneities. The percentage depth doses measured with the photodiode were compared with those obtained with TLDs irradiated in the same conditions. The results show that the discrepancy between the responses of the photodiode and that of TLDs is less than 1%, indicating that the BPW-34 can be used for measuring dose distributions in heterogeneous tissues.

Optimization of pencil beam widths for electron‐beam dose calculations

The pencil beam method of calculating dose distributions for electron-beam radiotherapy has been very useful, however, several limitations in the approach have been recognized. One such limitation is the lack of a mechanism to model range straggling of electrons. For stationary electron-beam calculations, range straggling is incorporated incompletely in the planar-fluence-to-dose conversion factor, which uses measured percentage depth dose curves to force the calculated percentage depth dose to reproduce the measurement. When calculating the dose distribution for an arced beam using a pencil beam algorithm, insufficient modeling of the pencil beams leads to larger errors than when using a stationary beam algorithm. The calculated depth of maximum dose is systematically over-estimated by the pencil beam calculations. We will show that the lack of a way to account for range straggling in the arc-electron pencil beam calculation is primarily responsible for this discrepancy. Methods of incorporating range straggling into the electron pencil beam dose calculation have been presented before, but no data have been shown to support their use for heterogeneous phantoms (patients). This paper presents a similar range-straggling modification, as well as data to show that this model can predict pencil beam width to within 20% for heterogeneous slab phantoms. For stationary electron-beam calculations, the calculated isodose lines follows the measured isodose lines to within 1 mm down to the 10% dose level.(ABSTRACT TRUNCATED AT 250 WORDS)

Patient dosimetry quality assurance program with a commercial diode system

Purpose : To evaluate a commercial silicone diode dosimeter for a patient dosimetry quality assurance program. Methods and Materials : The diode dosimeter was calibrated against an ion chamber and percentage depth dose, linearity, anisotropy, virtual source position, and field size factor studies were performed. Correction factors for lack of full scatter medium in the diode entrance and exit dose measurements were acquired. Dosimetry equations were proposed for calculation of dose delivered at isocenter. Diode dose accuracy and reproducibility were tested on phantom and on four patients. A patient dosimetry quality assurance program based on diode measured dose was instituted and patient dose data were collected. Results : Diode measured percentage depth dose and field factors agreed to within 3% with those measured with an ion chamber. The diode exhibited less than 1.7% angular dose anisotropy and less than 0.5% nonlinearity up to 4 Gy. Diode dose measurements in phantom showed that the calculated doses differed from the prescribed dose by less than 1.5%; the diode exhibited a daily dose reproducibility of better than 0.2%. On four selected patients, the measured dose reproducibility was 1.5%; the average calculated doses were all within ± 7% of the prescribed doses. For 33 of 40 patients treated with a 6 MV beam, measured doses were within ± 7% of the prescribed doses. For 58 of 63 patients treated with an 18 MV beam, measured doses were within ± 7% of the prescribed doses. For 11 out of 12 patients, a second repeat measurements yielded doses within ± 7% of the prescribed doses. Conclusions : The proposed diode-based patient dosimetry quality assurance program with dose tolerance at ± 7% is simple and feasible. It is capable of detecting certain serious treatment errors such as incorrect daily dose greater than 7%, incorrect wedge use, incorrect photon energy and patient setup errors involving some incorrect source-to-surface-distance vs. source-to-axis-distance treatments.