Dose perturbations due to in vivo dosimetry with diodes

Development of a novel and low-cost anthropomorphic pelvis phantom for 3D dosimetry in radiotherapy

Purpose

The aim of this study was to construct a low-cost, anthropomorphic, and 3D-printed pelvis phantom and evaluate the feasibility of its use to perform 3D dosimetry with commercially available bead thermoluminescent dosimeters (TLDs).

Material and methods

A novel anthropomorphic female phantom was developed with all relevant pelvic organs to position the bead TLDs. Organs were 3D-printed using acrylonitrile butadiene styrene. Phantom components were confirmed to have mass density and computed tomography (CT) numbers similar to relevant tissues. To find out clinically required spatial resolution of beads to cause no perturbation effect, TLDs were positioned with 2.5, 5, and 7.5 mm spacing on the surface of syringe. After taking a CT scan and creating a 4-field conformal radiotherapy plan, 3 dose planes were extracted from the treatment planning system (TPS) at different depths. By using a 2D-gamma analysis, the TPS reports were compared with and without the presence of beads. Moreover, the bead TLDs were placed on the organs’ surfaces of the pelvis phantom and exposed to high-dose-rate (HDR) ⁶⁰Co source. TLDs’ readouts were compared with the TPS calculated doses, and dose surface histograms (DSHs) of organs were plotted.

Results

3D-printed phantom organs agreed well with body tissues regarding both their design and radiation properties. Furthermore, the 2D-gamma analysis on the syringe showed more than 99% points passed 3%- and 3-mm criteria at different depths. By calculating the integral dose of DSHs, the percentage differences were –1.5%, 2%, 5%, and 10% for uterus, rectum, bladder, and sigmoid, respectively. Also, combined standard uncertainty was estimated as 3.5% (k = 1).

Conclusions

A customized pelvis phantom was successfully built and assessed to confirm properties similar to body tissues. Additionally, no significant perturbation effect with different bead resolutions was presented by the external TPS, with 0.1 mm dose grid resolution.

Dose Shadowing and Prosthesis Involvement for Megavoltage Photon In vivo Diode Dosimetry

AIM

The aim of the study is to investigate the photon beam perturbations induced by an in vivo diode in combination with prosthesis involvement in a human-like phantom.

MATERIALS AND METHODS

Beam perturbations for 6 MV and 10 MV photons caused by an EDP-203G in vivo diode in combination with prosthesis involvement were studied in a unique water-equivalent pelvic phantom, equipped with bony structures and Ti prosthesis using single fields between 2 × 2 and 15 cm × 15 cm as well as 10 MV lateral opposing fields and a four-field plan. Dose distributions were measured with Gafchromic EBT3 films with and without the diode included in the beams on the prosthesis (prosthetic fields) and non-prosthesis (non-prosthetic fields) sides of the phantom. Differences between prosthetic and non-prosthetic field dose data were determined to assess the effect of the prosthesis on the diode-induced beam perturbations inside the phantom.

RESULTS

Photon beam dose perturbations ranged from 2% to 7% and from 5% to 12% for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 4%. In addition, d50 depths ranging from 8.7 to 11.5 cm and from 11.5 to 15 cm were acquired in the phantom for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 5%.

CONCLUSION

On the basis of accuracy requirements in radiotherapy noting that a small underdose to tumors could yield a decrease in the probability of tumor control, the diode-induced beam perturbations in combination with prosthesis involvement in the photon fields may affect treatment outcome, as there would be a reduction in the prescribed target dose during treatment delivery.

Effect of correction/calibration factors on accuracy of in vivo dose delivery with cylindrical n-type Isorad diode in conventional radiotherapy

Purpose

The main aim was to use pre-calculated correction factors and calibration factors for measurement of accuracy of dose delivery before implementation of such in vivo dosimetry on real patients visiting for first radiation treatment. These factors were verified by generating the most common treatment plans on human phantom except for breast and colon using cobalt-60 unit.

Materials and methods

Six treatment plans were generated, i.e. nasopharynx, bladder, prostate, brain, larynx and lung of human phantom, total 18 fields were planned keeping in view the correction factors which are to be verified. MULTIDATA Decision Support System 2.5, Shimadzu simulator, Isorad diode-n type, electrometer patient dose monitor and ATOM Adult male human phantom were used.

Results and conclusion

For 18 fields, the dose delivery was accurate in the range 0·29–6·74%. The deviation between measured and expected doses to nasopharynx, lung, bladder, prostate, brain and larynx cases of human phantom ranged from 1·44–3·89%, 0·29–0·54%, 0·44–6·18%, 0·54–5·16%, 0·33–4·90%, 5·58–6·74%, respectively. In 30 palliative patient cases, the first radiation treatment was also monitored. The accuracy of dosimety ranged from 1·05% to 5·35%. This study is helpful to identify areas of improvement in treatment of patients like quality control/quality assurance (QA) of treatment planning system, beam data modifications, machine repair maintenance, QA audit in radiotherapy.

A study of dosimetric characteristics for in vivo dosimetry with cylindrical n-type Isorad diode

Background and purpose

The objective was to determine diode characteristics before actual dose verification on human phantom and patients.

Materials and methods

The reliability and stability of equipment, signal stability, precision, dose response linearity, field flatness, perturbation of radiation dose, plastic to water conversion factor (Kpl), ionisation chambers (ICs) and diode calibration were determined. Correction factors for tray (CFtray), wedge (CFwedge), field size (CFFS), SSD (CFSSD), angle (CFangle) and block (CFblock) were found. Patient dose monitor, Isorad diode (n-type) and IC (PTW Frieburg), Co-60 unit (Theratron), ATOM Adult male human phantom (Model 701-D, CIRS) were used.

Results and conclusion

Good signal stability, precise data, and linear dose response, variation of 0·500% and 5·000% in field flatness and perturbation tests, respectively, were noted. Kpl was 1·006 for IC PTW Frieburg TW30013, 0114. The diode calibration factor was 0·989. CFtray, CFFS, CFSSD, CFangle, CFblock were 1·001, 1·001, 0·997, 1·006 and 0·990, respectively. CFwedge were 1·024, 1·030 and 1·038 for 30°, 45° and 60° wedges, respectively. The verification of above correction factors (CFs) on Nasopharynx and lung of human phantom was also done.

A Monte Carlo and experimental investigation of the dosimetric behavior of low- and medium-perturbation diodes used for entrance in vivo dosimetry in megavoltage photon beams

Full buildup diodes can cause significant dose perturbation if they are used on most or all of radiotherapy fractions. Given the importance of frequent in vivo measurements in complex treatments, using thin buildup (low‐perturbation) diodes instead is gathering interest. However, such diodes are strictly unsuitable for high‐energy photons; therefore, their use requires evaluation and careful measurement of correction factors (CFs). There is little published data on such factors for low‐perturbation diodes, and none on diode characterization for 9 MV X‐rays. We report on MCNP4c Monte Carlo models of low‐perturbation (EDD5) and medium‐perturbation (EDP10) diodes, and a comparison of source‐to‐surface distance, field size, temperature, and orientation CFs for cobalt‐60 and 9 MV beams. Most of the simulation results were within 4% of the measurements. The results suggest against the use of the EDD5 in axial angles beyond ±50° and exceeding the range 0° to +50° tilt angle at 9 MV. Outside these ranges, although the EDD5 can be used for accurate in vivo dosimetry at 9 MV, its CF variations were found to be 1.5–7.1 times larger than the EDP10 and, therefore, should be applied carefully. Finally, the MCNP diode models are sufficiently reliable tools for independent verification of potentially inaccurate measurements.

PACS numbers: 87.10.Rt; 87.50.cm; 87.55.km; 87.56.Fc

Energy and angular anisotropy optimisation of a p-type diode for in vivo dosimetry in photon-beam radiotherapy

We present simulation work using the Monte Carlo code MCNPX that shows that there is a possibility of improving the silicon p-type diode as a radiation dosemeter, by altering the construction of the diode. Altering the diode die thickness can reduce the inherent angular anisotropy of the diode, with little effect on its energy response. Conversely, the contact material and geometry have a large impact on the energy response with little effect on the inherent angular anisotropy. By correct choice of contact material, the typical over-response -100 keV relative to the response at 60Co energy can be reduced from approximately 20 to 4. It is expected that further enhancements may be made with different geometries and materials.

An assessment of the characteristics of n-type diodes for use in electron beam radiotherapy

Diodes are now commercially available for use in electron beams. This paper aims to assess their response and limitations under different clinical conditions. Parameters considered were applicator factors, focus skin distance (FSD), angle of incidence, and temperature dependence. The beam perturbation was also measured. The beam energies used varied from 5 to 17 MeV. Applicator factors, measured at the surface of a phantom, differed by up to 7% compared with ion chamber measurements at Dmax. Similarly, large differences were found in FSD dependence. The temperature dependence was found to be approximately double the manufacturer's specification at 0.7% per degree Centigrade and the angular dependence was within the specification of +/- 2% for angles of +/- 30 degrees. The beam perturbation was measured as a maximum of 25% for 5-MeV electrons. The measurements were compared to other published data but this is highly dependent on the methodology employed. It was concluded that the diodes could be used in some circumstances, but only if used with extreme caution. An extensive set of commissioning measurements would be required before introducing the diodes into use clinically.

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.

A linear diode array (JFD-5) for match line in vivo dosimetry in photon and electron beams; evaluation for a chest wall irradiation technique

BACKGROUND AND PURPOSE

In vivo dosimetry using thermoluminiscence detectors (TLD) is routinely performed in our institution to determine dose inhomogeneities in the match line region during chest wall irradiation. However, TLDs have some drawbacks: online in vivo dosimetry cannot be performed; generally, doses delivered by the contributing fields are not measured separately; measurement analysis is time consuming. To overcome these problems, the Joined Field Detector (JFD-5), a detector for match line in vivo dosimetry based on diodes, has been developed. This detector and its characteristics are presented.

MATERIALS AND METHODS

The JFD-5 is a linear array of 5 p-type diodes. The middle three diodes, used to measure the dose in the match line region, are positioned at 5-mm intervals. The outer two diodes, positioned at 3-cm distance from the central diode, are used to measure the dose in the two contributing fields. For three JFD-5 detectors, calibration factors for different energies, and sensitivity correction factors for non-standard field sizes, patient skin temperature, and oblique incidence have been determined. The accuracy of penumbra and match line dose measurements has been determined in phantom studies and in vivo.

RESULTS

Calibration factors differ significantly between diodes and between photon and electron beams. However, conversion factors between energies can be applied. The correction factor for temperature is 0.35%/ degrees C, and for oblique incidence 2% at maximum. The penumbra measured with the JFD-5 agrees well with film and linear diode array measurements. JFD-5 in vivo match line dosimetry reproducibility was 2.0% (1 SD) while the agreement with TLD was 0.999+/-0.023 (1 SD).

CONCLUSION

The JFD-5 can be used for accurate, reproducible, and fast on-line match line in vivo dosimetry.

In vivo dosimetry using a single diode for megavoltage photon beam radiotherapy: implementation and response characterization

The AAPM Task Group 40 reported that in vivo dosimetry can be used to identify major deviations in treatment delivery in radiation therapy. In this paper, we investigate the feasibility of using one single diode to perform in vivo dosimetry in the entire radiotherapeutic energy range regardless of its intrinsic buildup material. The only requirement on diode selection would be to choose a diode with the adequate build up to measure the highest beam energy. We have tested the new diodes from Sun Nuclear Corporation (called QED and ISORAD-p--both p-type) for low-, intermediate-, and high-energy range. We have clinically used both diode types to monitor entrance doses. In general, we found that the dose readings from the ISORAD (p-type) are closer of the dose expected than QED diodes in the clinical setting. In this paper we report on the response of these newly available ISORAD (p-type) diode detectors with respect to certain radiation field parameters such as source-to-surface distance, field size, wedge beam modifiers, as well as other parameters that affect detector characteristics (temperature and detector-beam orientation). We have characterized the response of the high-energy ISORAD (p-type) diode in the low- (1-4 MV), intermediate- (6-12 MV), and high-energy (15-25 MV) range. Our results showed that the total variation of the response of high-energy ISORAD (p-type) diodes to all the above parameters are within +/-5% in most encountered clinical patient treatment setups in the megavoltage photon beam radiotherapy. The usage of the high-energy buildup diode has the additional benefit of amplifying the response of the diode reading in case the wrong energy is used for patient treatment. In the light of these findings, we have since then switched to using only one single diode type, namely the "red" diode; manufacturer designation of the ISORAD (p-type) high-energy (15-25 MV) range diode, for all energies in our institution and satellites.

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.

In vivo dosimetry: intercomparison between p-type based and n-type based diodes for the 16-25 MV energy range

This paper compares two different types of diodes designed to cover the energy range from 16 to 25 MV, one n-type (diode-A) and the other p-type (diode-B). A 18 MV x-ray beam has been used for all tests. Signal stability postirradiation, intrinsic precision and linearity of response with dose, front-back symmetry, and dose decrease under the diode were studied. Also, the water equivalent thickness of the build up caps was determined. Both types of diodes were calibrated to give entrance dose. Entrance correction factors for field size, tray, source skin distance, angle, and wedge were determined. Finally, the effect of dose rate, temperature and accumulated dose on the diode's response were studied. Only diode-A had full build-up for 18 MV x rays and standard irradiation conditions. Field size correction factor was about 2%-4% for field sizes bigger than 20 x 20 cm2 for both diodes. Tray correction factor was negligible for diode-A while diode-B would overestimate the dose by a 2% for a 40 x 40 cm2 field size if the correction factor was not applied. Wedge correction factors are only relevant for the 60 degrees wedge, being the correction factor for diode-A significantly higher than for diode-B. Diode-A showed less temperature dependence than diode-B. Sensitivity dependence on dose per pulse was a 1.5% higher for diode-A than for diode-B and therefore a higher SSD dependence was found for diode-A. The loss of sensitivity with accumulated radiation dose was only about 0.3% for diode-A, after 300 Gy, while it amounted to 8% for diode-B. Weighing the different correction factors for both types of diodes no conclusions about which type is better can be driven. From these results it can be also seen that the dependence of the diode response on dose rate in a pulsed beam does not seem to be associated with the fact of being n-type or p-type but could be related to the doping level of the diodes.

Build-up modification of commercial diodes for entrance dose measurements in 'higher energy' photon beams

BACKGROUND AND PURPOSE

Several commercially available p-type diodes do not provide sufficient build-up for in-vivo dosimetry in 'higher' energy photon beams, and only limited information could be found in the literature describing the correction factor variation and/or the achievable accuracy for in-vivo dosimetry methods in this energy range. The first aim of this study is to assess and analyze the variation of diode correction factors for entrance dose measurements at higher photon energies. In a second step the total build up thickness of the diode has been modified in order to minimize the correction factor variation.

MATERIALS AND METHODS

Diode correction factors accounting for non-reference conditions (field size, source surface distance, tray, wedge, and block) are determined in 18-25 MV photon beams provided by different treatment units for Scanditronix p-type diodes recommended for higher energy photon beams: old type and new type EDP-20, and EDP-30 diodes. Hemispherical build-up caps of different materials (copper, iron, lead) are used to increase the total build-up thickness. Perturbation effects with and without additional build-up caps are assessed for the three diode types.

RESULTS

For unmodified diodes field size correction factors (C(FS)) vary between 1.7% and 6%, dependent on diode type and treatment unit. For example, for an old type EDP-20 the C(FS) variation at 18 MV is much higher on a GE linac (5%) as compared to the Philips machine (1.7%). Depending on diode type, this variation can be reduced to 1-2% when adding additional build-up. The variation of source to surface distance correction factors is almost independent of build-up thickness. By adding additional build-up the influence of trays and blocks can be almost eliminated.

CONCLUSIONS

The correction factor variation of unmodified diodes reflects the variation of the electron contamination with treatment geometry. A total build-up thickness of 30 mm is found to be the 'best compromise' for the three types of diodes investigated when measuring entrance doses in the energy range between 18 and 25 MV.

Selective in vivo dosimetry in radiotherapy using P-type semiconductor diodes: a reliable quality assurance procedure

Since 1994, our center has conducted entrance dose measurements on selected patients receiving 6MV x-ray therapy by utilizing a commercial set of p-type semiconductor diodes. We report on three years results representing 386 patients having 1005 measurements and the usefulness of such a system in a radiotherapy department. The 386 patients represent approximately 20% of our total radical treatments. Minimal disruption to patient treatment was achieved. Measurements showed an average variation from expected dose of 0.5% +/- 2.2%. Specific treatment site groups were investigated. Our results show that in vivo dosimetry on a selected group of patients is an effective method of providing an independent verification of dose delivery accuracy.

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.

In vivo dosimetry during external photon beam radiotherapy

In this critical review of the current practice of patient dose verification, we first demonstrate that a high accuracy (about 1-2%, 1 SD) can be obtained. Accurate in vivo dosimetry is possible if diodes and thermoluminescence dosimeters (TLDs), the main detector types in use for in vivo dosimetry, are carefully calibrated and the factors influencing their sensitivity are taken into account. Various methods and philosophies for applying patient dose verification are then evaluated: the measurement of each field for each fraction of each patient, a limited number of checks for all patients, or measurements of specific patient groups, for example, during total body irradiation (TBI) or conformal radiotherapy. The experience of a number of centers is then presented, providing information on the various types of errors detected by in vivo dosimetry, including their frequency and magnitude. From the results of recent studies it can be concluded that in centers having modern equipment with verification systems as well as comprehensive quality assurance (QA) programs, a systematic error larger than 5% in dose delivery is still present for 0.5-1% of the patient treatments. In other studies, a frequency of 3-10% of errors was observed for specific patient groups or when no verification system was present at the accelerator. These results were balanced against the additional manpower and other resources required for such a QA program. It could be concluded that patient dose verification should be an essential part of a QA program in a radiotherapy department, and plays a complementary role to treatment-sheet double checking. As the radiotherapy community makes the transition from the conventional two-dimensional (2D) to three-dimensional (3D) conformal and intensity modulated dose delivery, it is recommended that new treatment techniques be checked systematically for a few patients, and to perform in vivo dosimetry a few times for each patient for situations where errors in dose delivery should be minimized.

Commissioning a p-type silicon diode for use in clinical electron beams

Commissioning measurements were carried out on a p-type silicon diode detector for use in patient monitoring in high energy electron beams. Characteristics specific to the diode were examined. The variation in diode sensitivity with dose per pulse was found to be less than 1% over a range 0.069-0.237 mGy/pulse. The diode exhibited a sensitivity variation with accumulated dose of 10% per kGy and a sensitivity variation with surface temperature of 0.26%/degree C. The dependence of the diode response on the direction of the incident electron beam was investigated. Results were found to exceed the manufacturer's specifications. Output factors measured with the diode agree to within 1.5% of those measured with an NACP-02 air ionization chamber. The detector showed a variation in response with energy of 0.8% over the energy range 4-15 MeV. Prior to introducing the diode into clinical use, an assessment of beam perturbation directly behind the diode was made. The maximum reduction in local dose directly behind the diode at a depth of 1.0 cm below the surface was approximately 13% at 4 and 15 MeV.

A method to improve the effectiveness of diode in vivo dosimetry

A routine diode in vivo dosimetry program based on a combination of entrance and exit dose measurements was clinically implemented in the radiation oncology department of Grace Hospital, Detroit, in January 1995. The delivered dose has been monitored by taking weekly measurements. The calibration of the diodes and the in vivo dosimetry protocol for this new, more effective type of dose verification is presented. The problems encountered within the program are discussed along with our solutions.