Dose-rate dependence and IMRT QA suitability of EBT3 radiochromic films for pulse reduced dose-rate radiotherapy (PRDR) dosimetry

BACKGROUND

Pulsed reduced dose rate (PRDR) is an emerging radiotherapy technique for recurrent diseases. It is pertinent that the linac beam characteristics are evaluated for PRDR dose rates and a suitable dosimeter is employed for IMRT QA.

PURPOSE

This study sought to investigate the pulse characteristics of a 6 MV photon beam during PRDR irradiations on a commercial linac. The feasibility of using EBT3 radiochromic film for use in IMRT QA was also investigated by comparing its response to a commercial diode array phantom.

METHODS

A plastic scintillator detector was employed to measure the photon pulse characteristics across nominal repetition rates (NRRs) in the 5-600 MU/min range. Film was irradiated with dose rates in the 0.033-4 Gy/min range to study the dose rate dependence. Five clinical PRDR treatment plans were selected for IMRT QA with the Delta4 phantom and EBT3 film sheets. The planned and measured dose were compared using gamma analysis with a criterion of 3%/3 mm. EBT3 film QA was performed using a cumulative technique and a weighting factor technique.

RESULTS

Negligible differences were observed in the pulse width and height data between the investigated NRRs. The pulse width was measured to be 3.15 ± 0.01μ s $\mu s$ and the PRF was calculated to be 3-357 Hz for the 5-600 MU/min NRRs. The EBT3 film was found to be dose rate independent within 3%. The gamma pass rates (GPRs) were above 99% and 90% for the Delta4 phantom and the EBT3 film using the cumulative QA method, respectively. GPRs as low as 80% were noted for the weighting factor EBT3 QA method.

CONCLUSIONS

Altering the NRRs changes the mean dose rate while the instantaneous dose rate remains constant. The EBT3 film was found to be suitable for PRDR dosimetry and IMRT QA with minimal dose rate dependence.

Post-mastectomy radiotherapy: Impact of bolus thickness and irradiation technique on skin dose

PURPOSE

To determine 10 MV IMRT and VMAT based protocols with a daily bolus targeting a skin dose of 45 Gy in order to replace the 6 MV tangential fields with a 5 mm thick bolus on alternate days method for post-mastectomy radiotherapy.

METHOD

We measured the mean surface dose along the chest wall PTV as a function of different bolus thicknesses for sliding window IMRT and VMAT plans. We analyzed surface dose profiles and dose homogeneities and compared them to our standard 6 MV strategy. All measurements were performed on a thorax phantom with Gafchromic films while dosimetric plans were computed using the Acuros XB algorithm (Varian).

RESULTS

We obtained the best compromise between measured surface dose (mean dose and homogeneity) and skin toxicity threshold obtained from the literature using a daily 3 mm thick bolus. Mean surface doses were 91.4 ± 2.8% [85.7% - 95.4%] and 92.2 ± 2.3% [85.6% - 95.2%] of the prescribed dose with IMRT and VMAT techniques, respectively. Our standard 6 MV alternate days 5 mm thick bolus leads to 89.0 ± 3.7% [83.6% - 95.5%]. Mean dose differences between measured and TPS results were < 3.2% for depths as low as 2 mm depth.

CONCLUSION

10 MV IMRT-based protocols with a daily 3 mm thick bolus produce a surface dose comparable to the standard 6 MV 5 mm thick bolus on alternate days method but with an improved surface dose homogeneity. This allows for a better control of skin toxicity and target volume coverage.

Technical note: Absolute dose measurements of a vault-free radiosurgery system

BACKGROUND

Methods for accurate absolute dose (AD) calibration are essential for the proper functioning of radiotherapy treatment machines. Many systems do not conform to TG-51 calibration standards, and modifications are required. TG-21 calibration is also a viable methodology for these situations with the appropriate setup, equipment, and factors. It has been shown that both these methods result in minimal errors. A similar approach has been taken in calibrating the dose for a recent vault-free radiosurgery system.

PURPOSE

To evaluate modified TG-21 and TG-51 protocols for AD calibrations of the ZAP-X radiosurgery system using ion chambers, film, and thermoluminescent dosimeters (TLDs).

METHODS

The current treatment planning system for ZAP-X requires AD calibration at d_max (7 mm) and 450 mm source-to-axis distance. Both N D , w 60 C o [ G y / C ] $N_{D,w}^{{60}Co}[ {Gy/C} ]$ and N_x [R/C] calibration coefficients were provided by an accredited dosimetry calibration laboratory for a physikalisch technische werkstatten (PTW) 31010 chamber (0.125 cc). The vendor provides an f-bracket that can be mounted on the collimator. Various phantoms can then be attached to the f-bracket. A custom acrylic phantom was designed based on recommendations from TG-21 and technical report series-398 that places the chamber at 500 mm from the source with a depth of 44-mm acrylic and 456-mm SSD. N_x along with other TG-21 parameters was used to calculate the AD. Measurements using a PTW MP3-XS water tank and the same chamber were used to calculate AD using N D , w 60 C o $N_{D,w}^{{60}Co}$ and TG-51 factors. Dose verification was performed using Gafchromic film and 3rd party TLDs.

RESULTS

Measurements from TG-51, TG-21 (utilizing the custom acrylic phantom), film, and TLDs agreed to within ± 2%.

CONCLUSIONS

A modified TG-51 AD calculation in water is preferred but may not be practical due to the difficulty in tank setup. The TG-21 modified protocol using a custom acrylic phantom is an accurate alternative option for dose calibration. Both of these methods are within acceptable agreement and provide confidence in the system's AD calibration.

Evaluating dosimetric accuracy of the 6 MV calibration on EBT3 film in the use of Ir-192 high dose rate brachytherapy

Purpose

To evaluate the dosimetric accuracy of EBT3 film calibrated with a 6 MV beam for high dose rate brachytherapy and propose a novel method for direct film calibration with an Ir‐192 source.

Methods

The 6 MV calibration was performed in water on a linear accelerator (linac). The Ir‐192 calibration was accomplished by irradiating the film wrapped around a cylinder applicator with an Ir‐192 source. All films were scanned 1‐day post‐irradiation to acquire calibration curves for all three (red, blue, and green) channels. The Ir‐192 calibration films were also used for single‐dose comparison. Moreover, an independent test film under a H.A.M. applicator was irradiated and the 2D dose distribution was obtained separately for each calibration using the red channel data. Gamma analysis and point‐by‐point profile comparison were performed to evaluate the performance of both calibrations. The uncertainty budget for each calibration system was analyzed.

Results

The red channel had the best performance for both calibration systems in the single‐dose comparison. We found a significant 4.89% difference from the reference for doses <250 cGy using the 6 MV calibration, while the difference was only 0.87% for doses >600 cGy. Gamma analysis of the 2D dose distribution showed the Ir‐192 calibration had a higher passing rate of 91.9% for the 1 mm/2% criterion, compared to 83.5% for the 6 MV calibration. Most failing points were in the low‐dose region (<200 cGy). The point‐by‐point profile comparison reported a discrepancy of 2%–3.6% between the Ir‐192 and 6 MV calibrations in this low‐dose region. The linac‐ and Ir‐192‐based dosimetry systems had an uncertainty of 4.1% (k = 2) and 5.66% (k = 2), respectively.

Conclusions

Direct calibration of EBT3 films with an Ir‐192 source is feasible and reliable, while the dosimetric accuracy of 6 MV calibration depends on the dose range. The Ir‐192 calibration should be used when the measurement dose range is below 250 cGy.

Development of a heterogeneous phantom to measure range in clinical proton therapy beams

PURPOSE

In particle therapy, determination of range by measurement or calculation can be a significant source of uncertainty. This work investigates the development of a bespoke Range Length Phantom (RaLPh) to allow independent determination of proton range in tissue. This phantom is intended to be used as an audit device.

METHOD

RaLPh was designed to be compact and allows different configurations of tissue substitute slabs, to facilitate measurement of range using radiochromic film. Fourteen RaLPh configurations were tested, using two types of proton fluence optimised water substitutes, two types of bone substitute, and one lung substitute slabs. These were designed to mimic different complex tissue interfaces. Experiments were performed using a 115 MeV mono-energetic scanning proton beam to investigate the proton range for each configuration. Validation of the measured film ranges was performed via Monte Carlo simulations and ionisation chamber measurements. The phantom was then assessed as an audit device, by comparing film measurements with Treatment Planning System (TPS) predicted ranges.

RESULTS

Varying the phantom slab configurations allowed for measurable range differences, and the best combinations of heterogeneous material gave agreement between film and Monte Carlo on average within 0.2% and on average within 0.3% of ionisation chamber measurements. Results against the TPS suggest a material density override is currently required to enable the phantom to be an audit device.

CONCLUSION

This study found that a heterogeneous phantom with radiochromic film can provide range verification as part of a dedicated audit for clinical proton therapy beams.

Assessment of Delivery Quality Assurance for Stereotactic Radiosurgery With Cyberknife

Purpose

The purpose of this study is to establish and assess a practical delivery quality assurance method for stereotactic radiosurgery with Cyberknife by analyzing the geometric and dosimetric accuracies obtained using a PTW31016 PinPoint ionization chamber and EBT3 films. Moreover, this study also explores the relationship between the parameters of plan complexity, target volume, and deliverability parameters and provides a valuable reference for improving plan optimization and validation.

Methods

One hundred fifty cases of delivery quality assurance plans were performed on Cyberknife to assess point dose and planar dose distribution, respectively, using a PTW31016 PinPoint ionization chamber and Gafchromic EBT3 films. The measured chamber doses were compared with the planned mean doses in the sensitive volume of the chamber, and the measured planar doses were compared with the calculated dose distribution using gamma index analysis. The gamma passing rates were evaluated using the criteria of 3%/1 mm and 2%/2 mm. The statistical significance of the correlations between the complexity metrics, target volume, and the gamma passing rate were analyzed using Spearman’s rank correlation coefficient.

Results

For point dose comparison, the averaged dose differences (± standard deviations) were 1.6 ± 0.73% for all the cases. For planar dose distribution, the mean gamma passing rate for 3%/1 mm, and 2%/2 mm evaluation criteria were 94.26% ± 1.89%, and 93.86% ± 2.16%, respectively. The gamma passing rates were higher than 90% for all the delivery quality assurance plans with the criteria of 3%/1 mm and 2%/2 mm. The difference in point dose was lowly correlated with volume of PTV, number of beams, and treatment time for 150 DQA plans, and highly correlated with volume of PTV for 18 DQA plans of small target. DQA gamma passing rate (2%/2 mm) was a moderate significant correlation for the number of nodes, number of beams and treatment time, and a low correlation with MU.

Conclusion

PTW31016 PinPoint ionization chamber and EBT3 film can be used for routine Cyberknife delivery quality assurance. The point dose difference should be within 3%. The gamma passing rate should be higher than 90% for the criteria of 3%/1 mm and 2%/2 mm. In addition, the plan complexity and PTV volume were found to have some influence on the plan deliverability.

Small-field beam data acquisition, detector dependency, and film-based validation for a novel self-shielded stereotactic radiosurgery system

PURPOSE

This study reports a single-institution experience with beam data acquisition and film-based validation for a novel self-shielded sterotactic radiosurgery unit and investigates detector dependency on field output factors (OFs), off-axis ratios (OARs), and percent depth dose (PDD) measurements within the context of small-field dosimetry.

METHODS

The delivery platform for this unit consists of a 2.7-MV S-band linear accelerator mounted on coupled gimbals that rotate around a common isocenter (source-to-axis distance [SAD] = 450 mm), allowing for more than 260 noncoplanar beam angles. Beam collimation is achieved via a tungsten collimator wheel with eight circular apertures ranging from 4 mm to 25 mm in diameter. Three diodes (PTW 60012 Diode E, PTW 60018 SRS Diode, and Sun Nuclear EDGE) and a synthetic diamond detector (PTW 60019 micro Diamond [µD] detector) were used for OAR, PDD, and OF measurements. OFs were also acquired with a PTW 31022 PinPoint ionization chamber. Beam scanning was performed using a 3D water tank at depths of 7, 50, 100, 200, and 250 mm with a source-to-surface distance of 450 mm. OFs were measured at the depth of maximum dose (d_max  = 7 mm) with the SAD at 450 mm. Gafchromic EBT3 film was used to validate OF and profile measurements and as a reference detector for estimating correction factors for active detector OFs. Deviations in field size, penumbra, and PDDs across the different detectors were quantified.

RESULTS

Relative OFs (ROFs) for the diodes were within 1.4% for all collimators except for 5 and 7.5 mm, for which SRS Diode measurements were higher by 1.6% and 2.6% versus Diode E. The µD ROFs were within 1.4% of the diode measurements. PinPoint ROFs were lower by >10% for the 4-mm and 5-mm collimators versus the Diode E and µD. Corrections to OFs using EBT3 film as a reference were within 1.2% for all diodes and the µD detector for collimators 10 mm and greater and within 2.0%, 2.8%, and 1.1% for the 7.5-, 5-, and 4-mm collimators, respectively. The maximum difference in full width at half maximum (FWHM) between the Diode E and the other active detectors was for the 25-mm collimator and was 0.09 mm (µD), 0.16 mm (SRS Diode), and 0.65 mm (EDGE). Differences seen in PDDs beyond the depth of d_max were <1% across the three diodes and the µD. FWHM and penumbra measurements made using EBT3 film were within 1.34% and 3.26%, respectively, of the processed profile data entered into the treatment planning system.

CONCLUSIONS

Minimal differences were seen in OAR and PDD measurements acquired with the diodes and the µD. ROFs measured with the three diodes were within 2.6% and within 1.4% versus the µD. Gafchromic Film measurements provided independent verification of the OAR and OF measurements. Estimated corrections to OFs using film as a reference were <1.6% for the Diode E, EDGE, and µD detector.

An Experimental Slope Method for a More Accurate Measurement of Relative Radiation Doses using Radiographic and Radiochromic Films and Its Application to Megavoltage Small-Field Dosimetry

PURPOSE

An experimental method using the linear portion of the relative film dose-response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.

MATERIALS AND METHODS

The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.

RESULTS

The maximum percentage deviation of PDD_FS with PDD_IC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDD_FS with PDD_IC for electron beams was within 3% for energy range studied of 8-15 MeV. The gamma passing rates of PDD_FS with PDD_IC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDD_FS with PDD_IC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROF_FS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDD_FS and PDD_MC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROF_FS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.

CONCLUSIONS

The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.

EBT3 Films in Low Solar Ultraviolet and X-Ray Dose Measurement: A Comparative Analysis

The purpose of this study is to investigate the potentiality of Gafchromic external beam therapy 3 (EBT3) film to measure low dosage of solar ultraviolet (SUV; 0-10 600 mJ/cm2) and x-ray (0-750 mGy) radiation. In this experiment, 2 groups of EBT3 films were prepared with size 2 cm × 1 cm. The first group of films was exposed by incremental SUV dose in the middle of the day. The other group was irradiated by x-ray at 100 kVp, 100 mA, and 2 S of tube voltage, tube current, and exposure time, respectively. The measured SUV consists of 90% ultraviolet A (UVA) and 10% ultraviolet B. The film discoloration was represented by visible absorbance spectroscopy technique using Jaz spectrometer from Ocean Optics Inc. Simple linear regression produced high accuracy with coefficients of determination, r 2 of 0.9804 and root mean square error (RMSE) of 434.88 mJ/cm2 for the measurement of SUV dose. On the other hand, r 2 of 0.98 and RMSE of 31 mGy was produced for the measurement of x-ray dose. The application of multiple linear regression enhanced the measurement accuracy with R 2 of 99% and 99.7% and RMSE of 327.06 mJ/cm2 and 15.045 mGy for SUV and x-ray dose, respectively. The spectral analysis shows a promising measurement at selected wavelengths for SUV and x-ray dose.

Dosimetric characterization of a new directional low-dose rate brachytherapy source

PURPOSE

CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.TM The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatability and reproducibility of a primary source strength standard for the CivaDot and characterizes the CivaDot dose distribution by performing in-phantom measurements and Monte Carlo (MC) simulations. Existing dosimetric formalisms were adapted to accommodate a directional source, and other distinguishing characteristics including the presence of gold shield x-ray fluorescence were addressed in this investigation.

METHODS

Primary air-kerma strength (SK ) measurements of the CivaDots were performed using two free-air chambers namely, the Variable-Aperture Free-Air Chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center (UWMRRC) and the National Institute of Standards and Technology (NIST) Wide-Angle Free-Air Chamber (WAFAC). An intercomparison of the two free-air chamber measurements was performed along with a comparison of the different assumed CivaDot energy spectra and associated correction factors. Dose distribution measurements of the source were performed in a custom polymethylmethacrylate (PMMA) phantom using GafchromicTM EBT3 film and thermoluminescent dosimeter (TLD) microcubes. Monte Carlo simulations of the source and the measurement setup were performed using MCNP6 radiation transport code.

RESULTS

The CivaDot SK was determined using the two free-air chambers for eight sources with an agreement of better than 1.1% for all sources. The NIST measured CivaDot energy spectrum intensity peaks were within 1.8% of the MC-predicted spectrum intensity peaks. The difference in the net source-specific correction factor determined for the CivaDot free-air chamber measurements for the NIST WAFAC and UW VAFAC was 0.7%. The dose-rate constant analog was determined to be 0.555 cGy h-1 U-1 . The average difference observed in the estimated CivaDot dose-rate constant analog using measurements and MCNP6-predicted value (0.558 cGy h-1 U-1 ) was 0.6% ± 2.3% for eight CivaDot sources using EBT3 film, and -2.6% ± 1.7% using TLD microcube measurements. The CivaDot two-dimensional dose-to-water distribution measured in phantom was compared to the corresponding MC predictions at six depths. The observed difference using a pixel-by-pixel subtraction map of the measured and the predicted dose-to-water distribution was generally within 2-3%, with maximum differences up to 5% of the dose prescribed at the depth of 1 cm.

CONCLUSION

Primary SK measurements of the CivaDot demonstrated good repeatability and reproducibility of the free-air chamber measurements. Measurements of the CivaDot dose distribution using the EBT3 film stack phantom and its subsequent comparison to Monte Carlo-predicted dose distributions were encouraging, given the overall uncertainties. This work will aid in the eventual realization of a clinically viable dosimetric framework for the CivaSheet based on the CivaDot dose distribution.

Technical Note: Construction of heterogeneous head phantom for quality control in stereotactic radiosurgery

PURPOSE

Stereotactic radiosurgery is a high precision modality for conformally delivering high doses of radiation to the brain lesion with a large dose volume. Several studies for the quality control of this technique were performed to measure the dose delivered to the target with a homogenous head phantom and some dosimeters. Some studies were also performed with one or two instances of heterogeneity in the head phantom to measure the dose delivered to the target. But these studies assumed the head as a sphere and simple shape heterogeneity. The construction of an adult human head phantom with the same size, shape, and real inhomogeneity as an adult human head is needed. Only then is measuring the accurate dose delivered to the area of interest and comparison with the calculated dose possible.

METHODS

According to the ICRU Report 44, polytetrafluoroethylene (PTFE) and methyl methacrylate were selected as a bone and soft tissue, respectively. A set of computed tomography (CT) scans from a standard human head were taken, and simplification of the CT images was used to design the layers of the phantom. The parts of each slice were cut and attached together. Tests of density and CT number were done to compare the material of the phantom with tissues of the head. The dose delivered to the target was measured with an EBT3 film.

RESULTS

The density of the PTFE and Plexiglas that were inserted in the phantom are in good agreement with bone and soft tissue. Also, the CT numbers of these materials have a low difference. The dose distribution from the EBT3 film and the treatment planning system is similar.

CONCLUSIONS

The constructed phantom with a size and inhomogeneity like an adult human head is suitable to measure the dose delivered to the area of interest. It also helps make an accurate comparison with the calculated dose by the treatment planning system. By using this phantom, the actual dose delivered to the target was obtained. This anthropomorphic head phantom can be used in other modalities of radiosurgery as well.

Analysis of Gafchromic EBT3 film calibration irradiated with gamma rays from different systems: Gamma Knife and Cobalt-60 unit

In recent years, Gafchromic films are used as an advanced instrument for dosimetry systems. The EBT3 films are a new generation of Gafchromic films. Our main interest is to compare the response of the EBT3 films exposed to gamma rays provided by the Theratron 780C as a conventional radiotherapy system and the Leksell Gamma Knife as a stereotactic radiotherapy system (SRS). Both systems use Cobalt-60 sources, thus using the same energy. However, other factors such as source-to-axis distance, number of sources, dose rate, direction of irradiation, shape of phantom, the field shape of radiation, and different scatter contribution may influence the calibration curve. Calibration curves for the 2 systems were measured and plotted for doses ranging from 0 to 40 Gy at the red and green channels. The best fitting curve was obtained with the Levenberg-Marquardt algorithm. Also, the component of dose uncertainty was obtained for any calibration curve. With the best fitting curve for the EBT3 films, we can use the calibration curve to measure the absolute dose in radiation therapy. Although there is a small deviation between the 2 curves, the p-value at any channel shows no significant difference between the 2 calibration curves. Therefore, the calibration curve for each system can be the same because of minor differences. The results show that with the best fitting curve from measured data, while considering the measurement uncertainties related to them, the EBT3 calibration curve can be used to measure the unknown dose both in SRS and in conventional radiotherapy.