A method to improve the effectiveness of diode in vivo dosimetry

Evaluation of Given Dose Accuracy in Radiation Therapy of Patient with Breast Cancer Using Diode In Vivo Dosimetry

Background: The main goal of radiation therapy is to deliver the highest dose to the tumor and at the same time the lowest dose to the surrounding normal tissue. In vivo dosimetry is a quality control procedure that, instead of controlling the components separately, directly examines the dose reached to the tumor area. Objectives: In this study, the entrance, exit, and middle dose of the breast and supraclavicular area of patients with breast cancer under radiation therapy were measured and compared with calculations. Methods: In this experimental study, the entrance and exit doses of 33 patients with breast tumors treated with 6 MV and 18MV photons were measured simultaneously. The measurement was done, using p-type diodes after calibration and, then, the midpoint dose was calculated, using the transfer method and arithmetic mean method. Also, the entrance dose, exit dose, and midline dose measured with dosimeter were compared with the calculated values in the treatment planning system. Results: There was no significant difference between calculated and measured doses in the entrance, exit, and midline point in breast regions (P > 0.05), but in the supraclavicular region, a challenge was observed. The difference in entrance and midline point between calculation and measurement is not significant based on the transfer method, but there is a significant error based on the arithmetic mean method (P < 0.05). Conclusions: In vivo dosimetry by measured real given dose to the patient can perform a basic role in the quality control of the radiotherapy department. It seems in the entrance dose, the relative error is smaller but due to the smaller value of exit dose, the relative error in small values is more apparent.

Application of spherical diodes for megavoltage photon beams dosimetry

PURPOSE

External beam radiation therapy (EBRT) usually uses heterogeneous dose distributions in a given volume. Designing detectors for quality control of these treatments is still a developing subject. The size of the detectors should be small to enhance spatial resolution and ensure low perturbation of the beam. A high uniformity in angular response is also a very important feature in a detector, because it has to measure radiation coming from all the directions of the space. It is also convenient that detectors are inexpensive and robust, especially to perform in vivo measurements. The purpose of this work is to introduce a new detector for measuring megavoltage photon beams and to assess its performance to measure relative dose in EBRT.

METHODS

The detector studied in this work was designed as a spherical photodiode (1.8 mm in diameter). The change in response of the spherical diodes is measured regarding the angle of incidence, cumulated irradiation, and instantaneous dose rate (or dose per pulse). Additionally, total scatter factors for large and small fields (between 1 × 1 cm(2) and 20 × 20 cm(2)) are evaluated and compared with the results obtained from some commercially available ionization chambers and planar diodes. Additionally, the over-response to low energy scattered photons in large fields is investigated using a shielding layer.

RESULTS

The spherical diode studied in this work produces a high signal (150 nC/Gy for photons of nominal energy of 15 MV and 160 for 6 MV, after 12 kGy) and its angular dependence is lower than that of planar diodes: less than 5% between maximum and minimum in all directions, and 2% around one of the axis. It also has a moderated variation with accumulated dose (about 1.5%/kGy for 15 MV photons and 0.7%/kGy for 6 MV, after 12 kGy) and a low variation with dose per pulse (± 0.4%), and its behavior is similar to commercial diodes in total scatter factor measurements.

CONCLUSIONS

The measurements of relative dose using the spherical diode described in this work show its feasibility for the dosimetry of megavoltage photon beams. A particularly important feature is its good angular response in the MV range. They would be good candidates for in vivo dosimetry, and quality assurance of VMAT and tomotherapy, and other modalities with beams irradiating from multiple orientations, such as Cyberknife and ViewRay, with minor modifications.

An optimized calibration method for surface measurements with MOSFETs in shaped-beam radiosurgery

Nowadays MOSFET dosimeters are widely used for dose verification in radiotherapy procedures. Although their sensitive area satisfies size requirements for small field dosimetry, their use in radiosurgery has rarely been reported. The aim of this study is to propose and optimize a calibration method to perform surface measurements in 6 MV shaped-beam radiosurgery for field sizes down to 18 × 18 mm(2). The effect of different parameters such as recovery time between 2 readings, batch uniformity and build-up cap attenuation was studied. Batch uniformity was found to be within 2% and isocenter dose attenuation due to the build-up cap over the MOSFET was near 2% irrespective of field size. Two sets of sensitivity coefficients (SC) were determined for TN-502RD MOSFET dosimeters using experimental and calculated calibration; the latter being developed using an inverse square law model. Validation measurements were performed on a realistic head phantom in irregular fields. MOSFET dose values obtained by applying either measured or calculated SC were compared. For calibration, optimal results were obtained for an inter-measurement time lapse of 5 min. We also found that fitting the SC values with the inverse square law reduced the number of measurements required for calibration. The study demonstrated that combining inverse square law and Sterling-Worthley formula resulted in an underestimation of up to 4% of the dose measured by MOSFETs for complex beam geometries. With the inverse square law, it is possible to reduce the number of measurements required for calibration for multiple field-SSD combinations. Our results suggested that MOSFETs are suitable sensors for dosimetry when used at the surface in shaped-beam radiosurgery down to 18 × 18 mm(2).

[In vivo dosimetry study of semi-conductors EPD-20 in total body irradiation technique]

PURPOSE

The objective of this work was the study of in vivo dosimetry performed in a series of 54 patients receiving total body irradiation (TBI) at the Salah-Azaiz Institute of Tunis since 2004. In vivo dosimetry measurements were compared to analytically calculated doses from monitor units delivered.

PATIENTS AND METHOD

The irradiation was conducted by a linear accelerator (Clinac 1800, Varian, Palo Alto, USA) using nominal X-rays energies of 6 MV and 18 MV, depending on the thickness of the patient at the abdomen. The dose was measured by semi-conductors p-type EPD-20. These diodes were calibrated in advance with an ionization chamber "PTW Farmer" type of 0.6cm(3) and were placed on the surface of plexiglas phantom in the same TBI conditions. A study of dosimetric characteristics of semi-conductors EPD-20 was carried out as a function of beam direction and temperature. Afterwards, we conducted a comparative analysis of doses measured using these detectors during irradiation to those calculated retrospectively from monitor units delivered to each patient conditioned by TBI.

RESULTS

Experience showed that semi-conductors are sensitive to the angle of beam radiation (0-90 degrees ) and the temperature (22-40 degrees C). The maximum variation is respectively 5 and 7%, but in our irradiation conditions these correction factors are less than 1%. The analysis of the results of the in vivo dosimetry had shown that the ratio of the average measured doses and analytically calculated doses at the abdomen, mediastina, right lung and head are 1.005, 1.007, 1.0135 and 1.008 with a standard deviation "type A" respectively of 3.04, 2.37, 7.09 et 4.15%.

CONCLUSION

In vivo dosimetry by semi-conductors is in perfect agreement with dosimetry by calculation. However, in vivo dosimetry using semiconductors is the only technique that can reflect the dose actually received instantly by the patient during TBI given the many factors that calculation can not take into account: patient and organs motions and the heterogeneity of the targets.

Performance characteristics of a microMOSFET as anin vivodosimeter in radiation therapy

The commercially available microMOSFET dosimeter was characterized for its dosimetric properties in radiotherapy treatments. The MOSFET exhibited excellent correlation with the dose and was linear in the range of 5-500 cGy. No measurable effect in response was observed in the temperature range of 20-40 degrees C. No significant change in response was observed by changing the dose rate between 100 and 600 monitor units (MU) min(-1) or change in the dose per pulse. A 3% post-irradiation fading was observed within the first 5 h of exposure and thereafter it remained stable up to 60 h. A uniform energy response was observed in the therapy range between 4 MV and 18 MV. However, below 0.6 MeV (Cs-132), the MOSFET response increased with the decrease in energy. The MOSFET also had a uniform dose response in 6-20 MeV electron beams. The directional dependence of MOSFET was within +/-2% for all the energies studied. The inherent build-up of the MOSFET was evaluated dosimetrically and found to have varying water equivalent thickness, depending on the energy and the side of the beam entry. At depth, a single calibration factor obtained by averaging the MOSFET response over different field sizes, energies, orientation and depths reproduced the ion chamber measured dose to within 5%. The stereotactic and the penumbral measurements demonstrated that the MOSFET could be used in a high gradient field such as IMRT. The study showed that the microMOSFET dosimeter could be used as an in vivo dosimeter to verify the dose delivery to the patient to within +/-5%.

Comparison ofp-type commercial electron diodes forin vivodosimetry

This paper compares the characteristics of three types of commercial p-type electron diodes specially designed for in vivo dosimetry (Scanditronix EDD2, Sun Nuclear QED 111200-0 and PTW T60010E diodes coupled with a Therados DPD510 dosimeter) in electron fields with energies from 4.5 to 21 MeV, and in conditions similar to those encountered in radiotherapy. In addition to the diodes, a NACP plane parallel ionization chamber and film dosimeters have been used in the experiments. The influence of beam direction on the diode responses (directional effect) was investigated. It was found to be the greatest for the lowest electron beam energy. At 12 MeV and an incidence of +/- 30 degrees, the variation was found to be less than 1% for the Scanditronix and Sun Nuclear diodes and less than 3% for the PTW one. The three diodes exhibited a variation in sensitivity with dose-per-pulse of less than 1% over the range 0.18-0.43 mGy/pulse. The temperature dependence was also studied. The response was linear for the three diodes between 22.2 and 40 degrees C and the sensitivity variations with temperature were (0.25+/-0.01)%/degree C, (0.28+/-0.01)%/degree C, and (0.02 +/-0.01)%/degree C for Scanditronix, Sun Nuclear, and PTW diodes, respectively. Finally the perturbation to the irradiation field induced by the presence of diodes placed at the surface of a homogeneous phantom was investigated and found to be significant, both at the surface and at the depth of maximum dose (several tens of percent) for all three diode types. There is an increase of dose right underneath the diode (close to the surface) and a dose shadow at the depth of maximum. The study shows that electron diodes can be used for in vivo dosimetry provided their characteristics are carefully established before use and taken into consideration at the time of interpretation of the results.

Characterization of an in vivo diode dosimetry system for clinical use

An in vivo dosimetry system that uses p-type semiconductor diodes with buildup caps was characterized for clinical use on accelerators ranging in energy from 4 to 18 MV. The dose per pulse dependence was investigated. This was done by altering the source-surface distance, field size, and wedge for photons. The off-axis correction and effect of changing repetition rate were also investigated. A model was developed to fit the measured two-dimensional diode correction factors.

Dosimetric considerations for patients with HIP prostheses undergoing pelvic irradiation. Report of the AAPM Radiation Therapy Committee Task Group 63

This document is the report of a task group of the Radiation Therapy Committee of the AAPM and has been prepared primarily to advise hospital physicists involved in external beam treatment of patients with pelvic malignancies who have high atomic number (Z) hip prostheses. The purpose of the report is to make the radiation oncology community aware of the problems arising from the presence of these devices in the radiation beam, to quantify the dose perturbations they cause, and, finally, to provide recommendations for treatment planning and delivery. Some of the data and recommendations are also applicable to patients having implanted high-Z prosthetic devices such as pins, humeral head replacements. The scientific understanding and methodology of clinical dosimetry for these situations is still incomplete. This report is intended to reflect the current state of scientific understanding and technical methodology in clinical dosimetry for radiation oncology patients with high-Z hip prostheses.

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.

Accurate in vivo dosimetry of a randomized trial of prostate cancer irradiation

PURPOSE

To guarantee an accurate dose delivery, within +/- 2.5%, in a Phase III randomized trial of prostate cancer irradiation (68 vs. 78 Gy) by means of a comprehensive in vivo dosimetry program.

METHODS AND MATERIALS

Prostate patients are generally treated in our clinic with a 3-field isocentric technique: an 8-MV anteroposterior beam and 2 18-MV wedged laterals. All fields are shaped conformally to the PTV. Patients were randomized between two dose levels of 68 Gy and 78 Gy. During treatment, the entrance and exit dose were measured for each patient with diodes. Special 2.5-mm thick steel build-up caps were applied to make the diodes appropriate for measurements in 18-MV photon beams as well. Portal images were used to verify the correct position of the diodes and to detect and correct for gas filling in the rectum that may influence the exit dose reading. Entrance and exit dose measurements were converted to midplane dose, which was used in combination with a depth dose correction to obtain the dose at the specification point. An action level of 2.5% was applied.

RESULTS

The added build-up for the diodes in the 18-MV beams resulted in correction factors that were only slightly sensitive to changes in beam setup and comparable to the corrections used in the 8-MV beams for diodes without extra build-up. The calibration factor increased almost linearly with cumulative dose: 0.7%/kGy for the 8-MV and 1.2%/kGy for the 18-MV photon beams. The introduction of average correction factors made the analysis easier, while keeping the accuracy within acceptable limits. In a period of 3 years, 225 patients were analyzed, from which 8 patients needed to be corrected. The average ratio of measured and prescribed dose was 1.009 (standard deviation [SD] 0.012) for the total group treated on two linear accelerators. When the results were analyzed per accelerator, the ratios were 1.002 (SD, 0.001) for Accelerator A and 1.015 (SD, 0.001) for Accelerator B. This difference could be attributed to the cumulative effect of three small imperfections in the performance of Accelerator B that were well within the limits of our quality assurance program.

CONCLUSION

Diodes can be used for accurate in vivo dosimetry during prostate irradiation in high-energy photon beams. The dose delivery in this randomized trial is guaranteed within the 2.5% limits on an individual patient basis. This could not be achieved without the in vivo dosimetry program, despite our high-standard quality assurance program of treatment delivery.

Clinical experience with routine diode dosimetry for electron beam radiotherapy

PURPOSE

Electron beam radiotherapy is frequently administered based on clinical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinical experience with patient dose verification using electron diodes and quantitatively assess the dose perturbations caused by the diodes during electron beam radiotherapy.

METHODS AND MATERIALS

A commercial diode dosimeter was used for the in vivo dose measurements. During patient dosimetry, the patients were set up as usual by the therapists. Before treatment, a diode was placed on the patient's skin surface and secured with hypoallergenic tape. The patient was then treated and the diode response registered and stored in the patient radiotherapy system database via our in-house software. A customized patient in vivo dosimetry report showing patient details, expected and measured dose, and percent difference was then generated and printed for analysis and record keeping. We studied the perturbation of electron beams by diodes using film dosimetry. Beam profiles at the 90% prescription isodose depths were obtained with and without the diode on the beam central axis, for 6-20 MeV electron beams and applicator/insert sizes ranging from a 3-cm diameter circular field to a 25 x 25 cm open field.

RESULTS

In vivo dose measurements on 360 patients resulted in the following ranges of deviations from the expected dose at the various anatomic sites: Breast (222 patients) -20.3 to +23.5% (median deviation 0%); Head and Neck (63 patients) -21.5 to +14.8% (median -0.7%); Other sites (75 patients) -17.6 to +18.8% (median +0.5%). Routine diode dosimetry during the first treatment on 360 patients (460 treatment sites) resulted in 11.5% of the measurements outside our acceptable +/-6% dose deviation window. Only 3.7% of the total measurements were outside +/-10% dose deviation. Detailed investigations revealed that the dose discrepancies, overwhelmingly, were due to inaccurate diode orientation and positioning, especially in the regions with rapidly changing contours and/or sloping surfaces. The presence of a diode in the treatment field was found, in some cases, to cause significant dose reduction, most noticeable with smaller fields and lower energy beams. The reduction in dose ranged from 16% (for a 6 MeV beam and a 3 cm diameter circular field) to 4% (for a 12 MeV beam and a 10 x 10 cm field).

CONCLUSIONS

Diode dosimetry was found to be convenient and valuable for verifying in real time the dose delivery accuracy of electron beam treatments, but with some caveats. When treating a small field by low energy electrons, frequent use of diodes is undesirable, because it might result in appreciable reduction of dose to the target volume.

Entrance dose measurements for in-vivo diode dosimetry: Comparison of correction factors for two types of commercial silicon diode detectors

Silicon diode dosimeters have been used routinely for in-vivo dosimetry. Despite their popularity, an appropriate implementation of an in-vivo dosimetry program using diode detectors remains a challenge for clinical physicists. One common approach is to relate the diode readout to the entrance dose, that is, dose to the reference depth of maximum dose such as d(max) for the 10x10 cm(2) field. Various correction factors are needed in order to properly infer the entrance dose from the diode readout, depending on field sizes, target-to-surface distances (TSD), and accessories (such as wedges and compensate filters). In some clinical practices, however, no correction factor is used. In this case, a diode-dosimeter-based in-vivo dosimetry program may not serve the purpose effectively; that is, to provide an overall check of the dosimetry procedure. In this paper, we provide a formula to relate the diode readout to the entrance dose. Correction factors for TSD, field size, and wedges used in this formula are also clearly defined. Two types of commercial diode detectors, ISORAD (n-type) and the newly available QED (p-type) (Sun Nuclear Corporation), are studied. We compared correction factors for TSDs, field sizes, and wedges. Our results are consistent with the theory of radiation damage of silicon diodes. Radiation damage has been shown to be more serious for n-type than for p-type detectors. In general, both types of diode dosimeters require correction factors depending on beam energy, TSD, field size, and wedge. The magnitudes of corrections for QED (p-type) diodes are smaller than ISORAD detectors.

A system for three-dimensional dosimetric verification of treatment plans in intensity-modulated radiotherapy with heavy ions

The introduction of dynamic intensity modulation into radiotherapy using conventional photon beams or scanning particle beams requires additional and efficient methods of dose verification. Dose measurements in dynamically generated dose distributions with a single ionization chamber require a complete application of the treatment field for each single measurement. Therefore measurements are performed by simultaneous use of multiple ionization chambers. The measurement is performed by a computer controlled system and is comprised of the following steps: (a) automated positioning of the ionization chambers, (b) measurement at these points, (c) a comparison with the calculated dose from the treatment planning system, and (d) documentation of the measurement. The ionization chambers are read out by a multichannel electrometer and are densely packed into a mounting of polymethylmetacrylate, which is attached to the arm of a three-dimensional motor-driven water phantom. The measured and planned dose values are displayed numerically as well as graphically. The mean deviation between measured and planned doses as well as their standard deviation are calculated and displayed. Through printouts complete documentation of the measurement is obtained and a quick decision can be made whether the dose distribution is acceptable for the patient. The system is now routinely used for dose verification at the heavy ion therapy project at the Gesellschaft für Schwerionenforschung in Darmstadt. Up to now 242 measurements have been performed for heavy ion treatment of 30 patients. The system allows efficient verification and documentation of carbon ion fields and is in principle also applicable to intensity-modulated photon beams.

Guidelines on the implementation of diode in vivo dosimetry programs for photon and electron external beam therapy

Semiconductor diodes offer many advantages for clinical dosimetry: high sensitivity, real-time readout, simple instrumentation, robustness and air pressure independence. The feasibility and usefulness of in vivo dosimetry with diodes has been shown by numerous publications, but very few, if any, refer to the utilization of diodes in electron beam dosimetry. The purpose of this paper is to present our methods for implementing an effective IVD program for external beam therapy with photons and electrons and to evaluate a new type of diodes. Methods of deciding on reasonable action levels along with calibration procedures, established according to the type of measurements intended to be performed and the action limits, are discussed. Correction factors to account for nonreference clinical conditions for new types of diodes (designed for photon and electron beams) are presented and compared with those required by older models commercially available. The possibilities and limitations of each type of diode are presented, emphasizing the importance of using the appropriate diode for each task and energy range.