Evaluation of GafChromic EBT prototype B for external beam dose verification

Characteristics of breathing-adapted gating using surface guidance for use in particle therapy: A phantom-based end-to-end test from CT simulation to dose delivery

To account for intra-fractional tumor motion during dose delivery in radiotherapy, various treatment strategies are clinically implemented such as breathing-adapted gating and irradiating the tumor during specific breathing phases. In this work, we present a comprehensive phantom-based end-to-end test of breathing-adapted gating utilizing surface guidance for use in particle therapy. A commercial dynamic thorax phantom was used to reproduce regular and irregular breathing patterns recorded by the GateRT respiratory monitoring system. The amplitudes and periods of recorded breathing patterns were analysed and compared to planned patterns (ground-truth). In addition, the mean absolute deviations (MAD) and Pearson correlation coefficients (PCC) between the measurements and ground-truth were assessed. Measurements of gated and non-gated irradiations were also analysed with respect to dosimetry and geometry, and compared to treatment planning system (TPS). Further, the latency time of beam on/off was evaluated. Compared to the ground-truth, measurements performed with GateRT showed amplitude differences between 0.03 ± 0.02 mm and 0.26 ± 0.03 mm for regular and irregular breathing patterns, whilst periods of both breathing patterns ranged with a standard deviation between 10 and 190 ms. Furthermore, the GateRT software precisely acquired breathing patterns with a maximum MAD of 0.30 ± 0.23 mm. The PCC constantly ranged between 0.998 and 1.000. Comparisons between TPS and measured dose profiles indicated absolute mean dose deviations within institutional tolerances of ±5%. Geometrical beam characteristics also varied within our institutional tolerances of 1.5 mm. The overall time delays were <60 ms and thus within both recommended tolerances published by ESTRO and AAPM of 200 and 100 ms, respectively. In this study, a non-invasive optical surface-guided workflow including image acquisition, treatment planning, patient positioning and gated irradiation at an ion-beam gantry was investigated, and shown to be clinically viable. Based on phantom measurements, our results show a clinically-appropriate spatial, temporal, and dosimetric accuracy when using surface guidance in the clinical setting, and the results comply with international and institutional guidelines and tolerances.

Dosimetric verification and quality assurance for intensity-modulated radiation therapy using Gafchromic® EBT3 film

Purpose

This study aimed to examine the dosimetric properties of Gafchromic® EBT3 film and intensity-modulated radiation therapy quality assurance (IMRT QA).

Materials and methods

Beams characteristics dosimetric properties and 20 IMRT plans were created and irradiated on Varian dual-energy DHX-S Linac for 6 and 15 MV energies. EBT3 films were analysed using ‘film Pro QA 2014’ software.

Results

The dosimetric comparison of EBT3 film (for red channel dosimetry) and ionisation ion chamber measurement showed that average deviations of symmetry, flatness, central axis, penumbra (left) and penumbra (right) of dose profile were 0·18, 1·34, 0·49%, 3·68 and 3·61 mm for 6 MV and 0·10, 1·3, 0·45, 2·65 and 2·71 mm for 15 MV, respectively. The blue and green channels dosimetry showed greater dose deviation as compared with red channel. IMRT QA verification plan complied about 95% at all different criteria. Reproducibility, stability and face orientation of film were within 1·4% for red channel.

Conclusions

The results advocate that the film can be used not only for dosimetric assessment but also as a reliable IMRT QA tool.

In vivo dosimetry using radiochromic films (EBT-2) during intraoperative radiotherapy

Background

Intraoperative radiotherapy is a method of choice to deliver a critical radiation dose to the tumour bed immediately after surgical excision.

Aim

The purpose of this work is to check the dose delivered to the patients during intraoperative electron beam radiation therapy (IOERT) in the conservative treatment of breast cancer, by means of reference dose measurement using radiochromic (EBT-2) films.

Material and methods

Ninety patients with early-stage breast cancer underwent exclusive IOERT to the tumour bed using a LIAC linear accelerator. Absolute dose measurements were done with film pieces. After irradiation, the pixel values of the films were obtained via MATLAB and ImageJ softwares. Calibration curve was also used for calculating net optical density. Expected dose was compared to the patient delivered dose.

Results

The mean deviation of the delivered dose from the expected one was 2·56% that is well in the accepted criteria. Only in one case, there was a larger deviation due to barometer miscalibration.

Findings

EBT-2 film response is independent from dose-per-pulse and as it was shown in this study it can be robustly used during breast IOERT for dosimetric and also positioning verifications.

Comparison of CCC and ETAR dose calculation algorithms in pituitary adenoma radiation treatment planning; Monte Carlo evaluation

Aims

To verify the accuracy of two common absorbed dose calculation algorithms in comparison to Monte Carlo (MC) simulation for the planning of the pituitary adenoma radiation treatment.

Materials and methods

After validation of Linac's head modelling by MC in water phantom, it was verified in Rando phantom as a heterogeneous medium for pituitary gland irradiation. Then, equivalent tissue-air ratio (ETAR) and collapsed cone convolution (CCC) algorithms were compared for a conventional three small non-coplanar field technique. This technique uses 30 degree physical wedge and 18 MV photon beams.

Results

Dose distribution findings showed significant difference between ETAR and CCC of delivered dose in pituitary irradiation. The differences between MC and dose calculation algorithms were 6.40 ± 3.44% for CCC and 10.36 ± 4.37% for ETAR. None of the algorithms could predict actual dose in air cavity areas in comparison to the MC method.

Conclusions

Difference between calculation and true dose value affects radiation treatment outcome and normal tissue complication probability. It is of prime concern to select appropriate treatment planning system according to our clinical situation. It is further emphasised that MC can be the method of choice for clinical dose calculation algorithms verification.

Is It Possible to Publish a Calibration Function for Radiochromic Film?

PURPOSE

To assess the possibility of using a public calibration function for radiochromic film dosimetry in dose QA of highly conformal treatment plans.

METHODS

EBT3 film calibration strips (3.5 × 20 cm2 from lots A101212 and A011713) were exposed on a Varian Trilogy at a facility to a 10 × 10 cm2 open field at doses of 80, 160, 320 cGy using 6MV photons. Together with a strip of unexposed film from the same lot the exposed films were digitized in a single scan using different Epson 10,000 XL scanners at two different facilities. The dose-response data for each color-channel from each facility were generated using the same calibration function X(D) = a + b/(D - c), where X(D) is the response at dose D and a, b and c are the coefficients. Different batches of EBT3 film were exposed to a VMAT beam. These films, plus two reference strips exposed to doses of zero and 160 cGy, were digitized on the scanners at the two facilities. Using the multi-channel dosimetry method and One-scan protocol (Med Phys, 39:6339-49, 2012) the recorded doses on the VMAT films were calculated and the results were compared with the VMAT plan using a Gamma index of 3%/3 mm.

RESULTS

The passing rates obtained for dose maps calculated for all combinations of VMAT images and calibration functions were nearly unchanged, using the One-scan protocol. Also, in all cases a passing rate of >99% was obtained for Gamma index of 3%/3 mm. On the other hand, if the One-scan protocol was not employed, the dose maps for VMAT images and calibration functions from different scanners showed poor correlation with the treatment plan. This is probably due to the scan-to-scan variability.

CONCLUSIONS

The authors have found that it is feasible to use a public calibration function for a given radiochromic film lot using the same methodology, One-scan protocol, for patient-specific QA.

An efficient protocol for radiochromic film dosimetry combining calibration and measurement in a single scan

PURPOSE

Radiochromic film provides dose measurement at high spatial resolution, but often is not preferred for routine evaluation of patient-specific intensity modulated radiation therapy (IMRT) plans owing to ease-of-use factors. The authors have established an efficient protocol that combines calibration and measurement in a single scan and enables measurement results to be obtained in less than 30 min. This avoids complications due to postexposure changes in radiochromic film that delay the completion of a measurement, often for up to 24 h, in commonly used methods. In addition, the protocol addresses the accuracy and integrity of the measurement by eliminating environmental and interscan variability issues.

METHODS

The authors collected dose-response data from six production lots of Gafchromic EBT3 film and three production lots of EBT2 film at doses up to 480 cGy. In this work, the authors used seven different scanners of two different models-Epson 10000XL and V700; postexposure times before scanning from 30 min to 9 days; ambient temperatures for scanning spanning 11 °C; and two film orientations. Scanning was in 48-bit RGB format at 72 dpi resolution. Dose evaluation was conducted using a triple-channel dosimetry method. To evaluate the measurement protocol, patient specific IMRT and volumetric modulated arc therapy (VMAT) plans were exposed onto EBT3 films on a Varian Trilogy Linac. Film scanning was done following the protocol under a number of different conditions and the dose maps were analyzed to demonstrate the equivalence of results.

RESULTS

The results indicated that the dose-response data could be fit by a set of related rational functions leading to the description of a generic calibration curve. A simplified dosimetry protocol was established where dose-response data for a specific film lot, scanner, and scanning conditions could be derived from two films exposed to known doses. In most cases only one calibrated exposure was required since the dose for one of the films could be zero. Using the Gamma test criterion of 2%∕2 mm to evaluate the measurements, similar passing rates ranging between about 95% and 99% for the fields studied were obtained from application films digitized under a variety of conditions all of them different than the conditions under which the calibration films were scanned.

CONCLUSIONS

The authors have developed a simplified and efficient protocol to measure doses delivered by an IMRT or VMAT plan using only the patient film, one calibration film, one unexposed film, and applying a single scan to acquire a digital image for calculation and analysis. The simplification and timesaving offer a potential practical solution for using radiochromic film for routine treatment plan quality assurance without sacrificing spatial resolution for convenience.

Confirmation of Skin Doses Resulting from Bolus Effect of Intervening Alpha-cradle and Carbon Fiber Couch in Radiotherapy

In this study, we verified the treatment planning calculations of skin doses with the incorporation of the bolus effect due to the intervening alpha-cradle (AC) and carbon fiber couch (CFC) using radiochromic EBT2 films. A polystyrene phantom (25 × 25 × 15 cm3) with six EBT2 films separated by polystyrene slabs, at depths of 0, 0.1, 0.2, 0.5, 1, 1.4 cm, was positioned above an AC, which was ~1 cm thick. The phantom and AC assembly were CT scanned and the CT-images were transferred to the treatment planning system (TPS) for calculations in three scenarios: (A) ignoring AC and CFC, (B) accounting for AC only, (C) accounting for both AC and CFC. A single posterior 10 × 10 cm2 field, a pair of posterior-oblique 10 × 10 cm2 fields, and a posterior IMRT field (6 MV photons from a Varian Trilogy linac) were planned. For each radiation field configuration, the same MU were used in all three scenarios in the TPS. Each plan for scenario C was delivered to expose a stack of EBT2 films in the phantom through AC and CFC. In addition, in vivo EBT2 film measurement on a lung cancer patient immobilized with AC undergoing IMRT was also included in this study. Point doses and planar distributions generated from the TPS for the three scenarios were compared with the data from the EBT2 film measurements. For all the field arrangements, the EBT2 film data including the in vivo measurement agreed with the doses calculated for scenario (C), within the uncertainty of the EBT2 measurements (~4%). For the single posterior field (a pair of posterior-oblique fields), the TPS generated doses were lower than the EBT2 doses by 34%, 33%, 31%, 13% (34%, 31%, 31%, 11%) for scenario A and by 27%, 25%, 22%, 8% (25%, 21%, 21%, 6%) for scenario B at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. For the IMRT field, the 2D dose distributions at each depth calculated in scenario C agree with those measured data. When comparing the central axis doses for the IMRT field, we found the TPS generated doses for scenario A (B) were lower than the EBT2 data by 35%, 34%, 31%, 16% (29%, 26%, 23%, 10%) at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. There were no significant differences for the depths of 1.0 and 1.4 cm for all the radiation fields studied. TPS calculation of doses in the skin layers accounting for AC and CFC was verified by EBT2 film data. Ignoring the presence of AC and/or CFC in TPS calculation would significantly underestimate the doses in the skin layers. For the clinicians, as more hypofractionated regimens and stereotactic regimens are being used, this information will be useful to avoid potential serious skin toxicities, and also assist in clinical decisions and report these doses accurately to relevant clinical trials/cooperative groups, such as RTOG.

Super-resolution non-parametric deconvolution in modelling the radial response function of a parallel plate ionization chamber

The signal of the dosimetric detector is generally dependent on the shape and size of the sensitive volume of the detector. In order to optimize the performance of the detector and reliability of the output signal the effect of the detector size should be corrected or, at least, taken into account. The response of the detector can be modelled using the convolution theorem that connects the system input (actual dose), output (measured result) and the effect of the detector (response function) by a linear convolution operator. We have developed the super-resolution and non-parametric deconvolution method for determination of the cylinder symmetric ionization chamber radial response function. We have demonstrated that the presented deconvolution method is able to determine the radial response for the Roos parallel plate ionization chamber with a better than 0.5 mm correspondence with the physical measures of the chamber. In addition, the performance of the method was proved by the excellent agreement between the output factors of the stereotactic conical collimators (4-20 mm diameter) measured by the Roos chamber, where the detector size is larger than the measured field, and the reference detector (diode). The presented deconvolution method has a potential in providing reference data for more accurate physical models of the ionization chamber as well as for improving and enhancing the performance of the detectors in specific dosimetric problems.

Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams

PURPOSE

Dose verification in highly conformal radiation therapy such as IMRT or proton therapy can benefit from the high spatial resolution offered by radio-chromic films such as Gafchromic EBT or EBT2. Recently, a new generation of these films, EBT3, has become available. The composition and thickness of the sensitive layer are the same as for the previous EBT2 films. The most important change is the symmetric layer configuration to eliminate side orientation dependence, which is reported for EBT2 films.

METHODS

The general film characteristics such as sensitivity to read-out orientation and postexposure darkening evolution of the new EBT3 film are evaluated. Film response has been investigated in clinical photon and proton beams and compared to former EBT2 films. Quenching effects in the proton Bragg peak region have been studied for both, EBT2 and EBT3 films.

RESULTS

The general performance of EBT3 is comparable to EBT2, and the orientation dependence with respect to film side is completely eliminated in EBT3 films. Response differences of EBT2 and EBT3 films are of the same order of magnitude as batch-to-batch variations observed for EBT2 films. No significant difference has been found for both generations of EBT films between photon and proton exposure. Depth dose measurements of EBT2 and EBT3 show an excellent agreement, though underestimating dose by up to 20% in the Bragg peak region.

CONCLUSIONS

The symmetric configuration of EBT3 presents a major improvement for film handling. EBT3 has similar dosimetric performance as its precursor EBT2 and can, thus, be applied to dose verification in IMRT in the same way. For dose verification in proton therapy the underresponse in the Bragg peak region has to be taken into account.

Homogeneity of GAFCHROMIC EBT2 film among different lot numbers

EBT2 film is widely used for quality assurance in radiation therapy. The purpose of this study was to investigate the homogeneity of EBT2 film among various lots, and the dose dependence of heterogeneity. EBT2 film was positioned in the center of a flatbed scanner and scanned in transmission mode at 75 dpi. Homogeneity was investigated by evaluating gray value and net optical density (netOD) with the red color channel. The dose dependence of heterogeneity in a single sheet from five lots was investigated at 0.5, 2, and 3 Gy. Maximum coefficient of variation as evaluated by netOD in a single film was 3.0% in one lot, but no higher than 0.5% in other lots. Dose dependence of heterogeneity was observed on evaluation by gray value but not on evaluation by netOD. These results suggest that EBT2 should be examined in each lot number before clinical use, and that the dose calibration curve should be constructed using netOD.

PACS number: 87

Monte Carlo simulations to replace film dosimetry in IMRT verification

Patient-specific verification of intensity-modulated radiation therapy (IMRT) plans can be done by dosimetric measurements or by independent dose or monitor unit calculations. The aim of this study was the clinical evaluation of IMRT verification based on a fast Monte Carlo (MC) program with regard to possible benefits compared to commonly used film dosimetry. 25 head-and-neck IMRT plans were recalculated by a pencil beam based treatment planning system (TPS) using an appropriate quality assurance (QA) phantom. All plans were verified both by film and diode dosimetry and compared to MC simulations. The irradiated films, the results of diode measurements and the computed dose distributions were evaluated, and the data were compared on the basis of gamma maps and dose-difference histograms. Average deviations in the high-dose region between diode measurements and point dose calculations performed with the TPS and MC program were 0.7 ± 2.7% and 1.2 ± 3.1%, respectively. For film measurements, the mean gamma values with 3% dose difference and 3mm distance-to-agreement were 0.74 ± 0.28 (TPS as reference) with dose deviations up to 10%. Corresponding values were significantly reduced to 0.34 ± 0.09 for MC dose calculation. The total time needed for both verification procedures is comparable, however, by far less labor intensive in the case of MC simulations. The presented study showed that independent dose calculation verification of IMRT plans with a fast MC program has the potential to eclipse film dosimetry more and more in the near future. Thus, the linac-specific QA part will necessarily become more important. In combination with MC simulations and due to the simple set-up, point-dose measurements for dosimetric plausibility checks are recommended at least in the IMRT introduction phase.

Characterizing the marker-dye correction for Gafchromic(®) EBT2 film: a comparison of three analysis methods

PURPOSE

Gafchromic(®) EBT2 film has a yellow marker dye incorporated into the active layer of the film that can be used to correct the film response for small variations in thickness. This work characterizes the effect of the marker-dye correction on the uniformity and uncertainty of dose measurements with EBT2 film. The effect of variations in time postexposure on the uniformity of EBT2 is also investigated.

METHODS

EBT2 films were used to measure the flatness of a (60)Co field to provide a high-spatial resolution evaluation of the film uniformity. As a reference, the flatness of the (60)Co field was also measured with Kodak EDR2 films. The EBT2 films were digitized with a flatbed document scanner 24, 48, and 72 h postexposure, and the images were analyzed using three methods: (1) the manufacturer-recommended marker-dye correction, (2) an in-house marker-dye correction, and (3) a net optical density (OD) measurement in the red color channel. The field flatness was calculated from orthogonal profiles through the center of the field using each analysis method, and the results were compared with the EDR2 measurements. Uncertainty was propagated through a dose calculation for each analysis method. The change in the measured field flatness for increasing times postexposure was also determined.

RESULTS

Both marker-dye correction methods improved the field flatness measured with EBT2 film relative to the net OD method, with a maximum improvement of 1% using the manufacturer-recommended correction. However, the manufacturer-recommended correction also resulted in a dose uncertainty an order of magnitude greater than the other two methods. The in-house marker-dye correction lowered the dose uncertainty relative to the net OD method. The measured field flatness did not exhibit any unidirectional change with increasing time postexposure and showed a maximum change of 0.3%.

CONCLUSIONS

The marker dye in EBT2 can be used to improve the response uniformity of the film. Depending on the film analysis method used, however, application of a marker-dye correction can improve or degrade the dose uncertainty relative to the net OD method. The uniformity of EBT2 was found to be independent of the time postexposure.

Dose verification of helical tomotherapy intensity modulated radiation therapy planning using 2D-array ion chambers

PURPOSE

To investigate the clinical usage of dose verification of Helical Tomotherapy plans by using 2D-array ion chambers, and to develop an efficient way to validate the dose delivered for the patients during treatments.

MATERIALS AND METHODS

A pixel-segmented ionisation chamber device, IMRT MatriXX™ and Multicube™ phantom from IBA were used on ten selected Tomotherapy IMRT/IGRT head and neck plans in this study. The combined phantom was set up to measure the dose distribution from coronal and sagittal planes. The setup of phantom was guided for verifying the correction position by pre-treatment Tomotherapy MVCT images. After the irradiation, the measured dose distributions of coronal and sagittal planes were compared with those from calculation by the planning system for cross verification. The results were evaluated by the absolute and relative doses as well as Gamma (γ) function. The feasibility of the different measuring methods was studied for this rotational treatment technique.

RESULTS

The dose distributions measured by the MatriXX 2D array were in good agreements with plans calculated by Tomotherapy planning system. The discrepancy between the measured dose and predicted dose in the selected points was within ±3%. In the comparison of the pixel-segmented ionisation chamber versus treatment planning system using the 3 mm/3% γ-function criteria, the mean passing rates of 2 mm dose grid with γ-parameter ≤1 were 97.37% and 96.91%, in two orthogonal planes (coronal and sagittal directions), respectively.

CONCLUSION

MatriXX with Multicube is a new system created for rotational delivery quality assurance (QA) and found to be reliable to measure both absolute dose and relative dose distributions, simultaneously. It achieves the goal of an efficient and accurate dosimetry validation method of the helical delivery pattern for the Helical Tomotherapy IMRT planning.

Evaluation of two-dimensional bolus effect of immobilization/support devices on skin doses: a radiochromic EBT film dosimetry study in phantom

PURPOSE

In this study, the authors have quantified the two-dimensional (2D) perspective of skin dose increase using EBT film dosimetry in phantom in the presence of patient immobilization devices during conventional and IMRT treatments.

METHODS

For 6 MV conventional photon field, the authors evaluated and quantified the 2D bolus effect on skin doses for six different common patient immobilization/support devices, including carbon fiber grid with Mylar sheet, Orfit carbon fiber base plate, balsa wood board, Styrofoam, perforated AquaPlast sheet, and alpha-cradle. For 6 and 15 MV IMRT fields, a stack of two film layers positioned above a solid phantom was exposed at the air interface or in the presence of a patient alpha-cradle. All the films were scanned and the pixel values were converted to doses based on an established calibration curve. The authors determined the 2D skin dose distributions, isodose curves, and cross-sectional profiles at the surface layers with or without the immobilization/support device. The authors also generated and compared the dose area histograms (DAHs) and dose area products from the 2D skin dose distributions.

RESULTS

In contrast with 20% relative dose [(RD) dose relative to dmax on central axis] at 0.0153 cm in the film layer for 6 MV 10 x 10 cm2 open field, the average RDs at the same depth in the film layer were 71%, 69%, 55%, and 57% for Orfit, balsa wood, Styrofoam, and alpha-cradle, respectively. At the same depth, the RDs were 54% under a strut and 26% between neighboring struts of a carbon fiber grid with Mylar sheet, and between 34% and 56% for stretched perforated AquaPlast sheet. In the presence of the alpha-cradle for the 6 MV (15 MV) IMRT fields, the hot spot doses at the effective measurement depths of 0.0153 and 0.0459 cm were 140% and 150%, (83% and 89%), respectively, of the isocenter dose. The enhancement factor was defined as the ratio of a given DAH parameter (minimum dose received in a given area) with and without the support device. For 6 MV conventional 10 x 10 cm2 field, the enhancement factor was the highest (3.4) for the Orfit carbon fiber plate. As for the IMRT field, the enhancement factors varied with the size of the area of interest and were as high as 3.8 (4.3) at the hot spot of 5 cm2 area in the top film layer (0.0153 cm) for 6 MV (15 MV) beams.

CONCLUSIONS

Significant 2D bolus effect on skin dose in the presence of patient support and immobilization devices was confirmed and quantified with EBT film dosimetry. Furthermore, the EBT film has potential application for in vivo monitoring of the 2D skin dose distributions during patient treatments.

Potential errors in optical density measurements due to scanning side in EBT and EBT2 Gafchromic film dosimetry

PURPOSE

The purpose of this study is to determine the effect on the measured optical density of scanning on either side of a Gafchromic EBT and EBT2 film using an Epson (Epson Canada Ltd., Toronto, Ontario) 10000XL flat bed scanner.

METHODS

Calibration curves were constructed using EBT2 film scanned in landscape orientation in both reflection and transmission mode on an Epson 10000XL scanner. Calibration curves were also constructed using EBT film. Potential errors due to an optical density difference from scanning the film on either side ("face up" or "face down") were simulated.

RESULTS

Scanning the film face up or face down on the scanner bed while keeping the film angular orientation constant affects the measured optical density when scanning in reflection mode. In contrast, no statistically significant effect was seen when scanning in transmission mode. This effect can significantly affect relative and absolute dose measurements. As an application example, the authors demonstrate potential errors of 17.8% by inverting the film scanning side on the gamma index for 3%-3 mm criteria on a head and neck intensity modulated radiotherapy plan, and errors in absolute dose measurements ranging from 10% to 35% between 2 and 5 Gy.

CONCLUSIONS

Process consistency is the key to obtaining accurate and precise results in Gafchromic film dosimetry. When scanning in reflection mode, care must be taken to place the film consistently on the same side on the scanner bed.

Commissioning of a double-focused micro multileaf collimator (muMLC)

Double-focused muMLCs are able to create fields with steeper dose gradients at the field edges and are, therefore, an advancement in delivering stereotactic treatments. A double-focused muMLC has been installed at a Siemens Primus linear accelerator (linac) as a first research installation in Europe. The basic dosimetric parameters, such as leakage, output factors, depth-dose curves and penumbra, have been measured in 6 and 15 MV-mode by use of radiochromic films (GafChromic EBT), ionization chambers and our solid water QA-phantom (Easy Cube). The leakage between the leaves is minimal and lower than that of most commercially available MLCs. Therefore, the field size of the linac can be kept constant while the leaves of the muMLC are creating different aperture shapes. Percentage depth doses (PDDs) generated by the double-focused muMLC are equal to depth-dose curves of the original linac. That means the muMLC affects only the off-axis ratio (OAR). Based on the fact that the muMLC is double-focused and the source-to-collimator distance is larger, the penumbra is sharper than that for fields defined by the original linac MLC. The mechanical and dosimetric investigations show the benefit of the double-focused muMLC attached to a Siemens Primus linear accelerator.

Evaluation and optimization of the new EBT2 radiochromic film dosimetry system for patient dose verification in radiotherapy

A new radiochromic film, the yellow Gafchromic EBT2, has been marketed as a drop-in replacement for the discontinued blue EBT film. In order to verify the manufacturer's claims prior to clinical use, EBT2 was characterized in transmission, and the less commonly used, reflection modes with an Epson Expression 10000XL A3 flatbed scanner. The red channel was confirmed to provide the greatest sensitivity and was used for all measurements. The post-irradiation darkening of the film was investigated, and the relative response was found to be dose dependent with higher doses stabilizing earlier than lower doses. After 13 h all dose levels had stabilized to within 1% of their value at 24 h. Uniformity of irradiated EBT2 films was within 0.8% and 1.2% (2SD of signal), in reflection and transmission modes, respectively. The light scattering effect, arising from the structure and thickness of EBT2, was found to give rise to an apparent scanner non-uniformity of up to 5.5% in signal. In reflection mode, differences of up to 1.2% were found between the signal obtained from a small film fragment (5 x 5 cm(2)) and the signal obtained from the same fragment bordered by extra film. Further work is needed to determine the origin of this effect, as there will be implications for reflection dosimetry of intensity modulated fields; reflection mode cannot yet be regarded as a viable alternative to transmission mode. Our results suggest that EBT2 film is a valid alternative, rather than a direct replacement for EBT film.

An easy method to account for light scattering dose dependence in radiochromic films

PURPOSE

To date no detector can offer the unbeatable characteristics of film dosimetry in terms of spatial resolution and this is why it has been chosen by many institutions for treatment verification and, in that respect, radiochromic films are becoming increasingly popular due to their advantageous properties. It is the aim of this work to suggest an easy method to overcome one of the drawbacks in radiochromic film dosimetry associated with the scanning device, namely, the nonuniform dose dependent response, mainly due to the light scattering effect.

METHODS

The suggested procedure consists of building four correction matrices by sequentially scanning one, two, three, and four unexposed blank films. The color level of these four matrices is compatible with four points in the calibration curve dose range. Therefore, the dose dependent correction to the scanned irradiated film will be obtained by interpolating between the four correction matrices.

RESULTS

The validity of the suggested method is checked against an ion chamber 2D array. The use of the proposed flattening correction improves considerably the dose agreement when compared with the cases in which no correction is applied.

CONCLUSIONS

The method showed to be fast and easy and practically overcomes the dependence on the dose of light scattering of flatbed scanners.

Use of new radiochromic devices for peripheral dose measurement: potential in-vivo dosimetry application

The authors have studied the feasibility of using three new high-sensitivity radiochromic devices in measuring the doses to peripheral points outside the primary megavoltage photon beams. The three devices were GAFCHROMIC® EBT film, prototype Low Dose (LD) Film, and prototype LD Card. The authors performed point dosimetry using these three devices in water-equivalent solid phantoms at x = 3,5,8,10, and 15 cm from the edge of 6 MV and 15 MV photon beams of 10x10 cm(2), and at depths of 0, 0.5 cm, and depth of maximum dose. A full sheet of EBT film was exposed with 5000 MU. The prototype LD film pieces were 1.5x2 cm(2) in size. Some LD films were provided in the form of a card in 1.8x5 cm(2) holding an active film in 1.8x2 cm(2). These are referred to as "LD dosimeter cards". The small LD films and cards were exposed with 500 MU. For each scanned film, a 6 mm circular area centered at the measurement point was sampled and the mean pixel value was obtained. The calibration curves were established from the calibration data for each combination of film/cards and densitometer/scanner. The doses at the peripheral points determined from the films were compared with those obtained using ion chamber at respective locations in a water phantom and general agreements were found. It is feasible to accurately measure peripheral doses of megavoltage photon beams using the new high-sensitivity radiochromic devices. This near real-time and inexpensive method can be applied in a clinical setting for dose measurements to critical organs and sensitive patient implant devices.

Photon beam dosimetry in the superficial buildup region using radiochromic EBT film stack

It has been a challenge to perform accurate 2D or 3D dosimetry in the surface region with steep dose gradient for megavoltage photon beams. We developed a dosimetry method in the superficial buildup region for the 6 and 15 MV photon beams using a radiochromic EBT film stack. Eight radiochromic EBT film strips (3 x 20 x 0.024 cm3) stacked together formed a 3D dosimeter. The film stack was positioned above a polystyrene phantom and surrounded by Solid Water slabs (0.2 cm) with the top film layer at 100 cm SSD. A 10 x 10 cm2 open field was used to irradiate the film stack with 1000 MU. All films were scanned using Epson 4870 flatbed scanner with transmission mode, 48 bit color, and 150 dpi (0.017 cm pixel resolution). The pixel values were converted to doses using an established calibration curve. This method allowed dose measurement for depths from 0.012 to 0.18 cm with fine spatial resolution (0.017 cm horizontally and 0.024 cm vertically). For each energy modality, we obtained both the central axis percent depth doses and the beam profiles along the central line covering the primary field and peripheral region at each depth. The primary field doses varied steeply with depth, while those in the peripheral region were weakly dependent on depth. For the 6 MV and 15 MV photon beams, (1) the central axis percent depth doses in the eight film layers ranged from 22% to 66% and from 15% to 44%, respectively; (2) the extrapolated percent depth doses at d = 0 were 15% and 14%, respectively. Agreement with the previously reported central axis percent depth doses in this region using parallel plate thin window ion chamber and ultrathin TLD was observed. The percent depth doses and beam profiles data can be incorporated in the treatment planning system for more accurate assessment of the doses to skin and shallow tumors to accomplish more accurate calculation results in the clinical usage.

Optimization of the gafchromic EBT protocol for IMRT QA

Quality assurance of external beam (radio)therapy (EBT) requires tools with specific characteristics. A radiochromic film dubbed "Gafchromic EBT" (G-EBT) that is particularly suited for external beam therapy because of its features was introduced in 2004. Its characteristics, especially the high spatial resolution, make it suitable for measurement of dose distributions in radiotherapy, especially intensity-modulated radiation therapy (IMRT). While several aspects of the film characteristics have been previously reported separately, we present a comprehensive evaluation centered on practical IMRT verification, leading to an optimized protocol. Therefore the constancy within one batch, the relationship between optical density (OD) and dose (dose range between 1.4 Gy and 8.4 Gy) and the dose rate dependence for four dose rates (55, 108, 217, 441 MU/min) were investigated. In addition to these characteristics, energy dependence between two energies (50kV and 6 MV), tissue equivalency, post irradiation coloration over one month, pressure and temperature sensitivity were evaluated. We then optimized the protocol using the G-EBT films, in combination with an EPSON-Expression 1680 pro flatbed scanner, for IMRT QA, while either striving to keep the compound error as small as possible or trying to reduce evaluation time. As a basis for this protocol optimization, the characteristics of the scanner (such as inhomogeneity of the scanning field) and its software (such as consequences of extracting only the red color channel) had to be determined first. The interaction of film and scanner (variation of the OD depending on the scanning direction or the scanning resolution) was assessed as well. Using the optimized protocol for IMRT QA, the compound error could be reduced to approximately 2% for a quality-driven approach and maximum 5.5% for an approach attempting to reduce procedure time. While the quality-driven approach provides appropriate accuracy for individual patient QA, the procedure-time driven approach can only be used for preliminary measurements.

Evaluation of the eclipse electron Monte Carlo dose calculation for small fields

Varian Medical Systems (Palo Alto, CA) has implemented the Monte Carlo electron dose calculation algorithm (eMC) in the Eclipse treatment planning system. Previous algorithms for electron treatment planning were limited in their calculation ability for small field depth doses and monitor units. An old rule of thumb to approximate the limiting cutout size for an electron field was determined by the lateral scatter equilibrium and approximated by E (MeV)/2.5 in centimeters of water. In this study, we compared eMC calculations and measurements of depth doses, isodose distributions, and monitor units for several different energy and small field cutout size combinations at different SSDs. Measurements were made using EBT film (International Specialty Products, Wayne, NJ) and a PinPoint ion chamber (PTW‐New York Corp., Hicksville, NY). Our results indicate that the eMC algorithm can accurately predict depth doses, isodose distributions, and monitor units (within 2.5%) for field sizes as small as 3.0 cm diameter for energies in the 6 to 20 MeV range at 100 cm SSD. Therefore, the previous energy dependent rule of thumb does not apply to the Eclipse electron Monte Carlo code. However, at extended SSDs (105–110 cm), the results show good agreement (within 4%) only for higher energies (12, 16, and 20 MeV) for a field size of 3 cm.

PACS number: 87.53.Hv

Commissioning of a novel microCT/RT system for small animal conformal radiotherapy

The purpose of this work was to commission a 120 kVp photon beam produced by a micro-computed tomography (microCT) scanner for use in irradiating mice to therapeutic doses. A variable-aperture collimator has been integrated with a microCT scanner to allow the delivery of beams with pseudocircular profiles of arbitrary width between 0.1 and 6.0 cm. The dose rate at the isocenter of the system was measured using ion chamber and gafchromic EBT film as 1.56-2.13 Gy min(-1) at the water surface for field diameters between 0.2 and 6.0 cm. The dose rate decreases approximately 10% per every 5 mm depth in water for field diameters between 0.5 and 1.0 cm. The flatness, symmetry and penumbra of the beam are 3.6%, 1.0% and 0.5 mm, respectively. These parameters are sufficient to accurately conform the radiation dose delivered to target organs on mice. The irradiated field size is affected principally by the divergence of the beam. In general, the beam has appropriate dosimetric characteristics to accurately deliver the dose to organs inside the mice's bodies. Using multiple beams delivered from a variety of angular directions, targets as small as 2 mm may be irradiated while sparing surrounding tissue. This microCT/RT system is a feasible tool to irradiate mice using treatment planning and delivery methods analogous to those applied to humans.

Optimizing the dynamic range extension of a radiochromic film dosimetry system

The authors present a radiochromic film dosimetry protocol for a multicolor channel radiochromic film dosimetry system consisting of the external beam therapy (EBT) model GAFCHROMIC film and the Epson Expression 1680 flat-bed document scanner. Instead of extracting only the red color channel, the authors are using all three color channels in the absorption spectrum of the EBT film to extend the dynamic dose range of the radiochromic film dosimetry system. By optimizing the dose range for each color channel, they obtained a system that has both precision and accuracy below 1.5%, and the optimized ranges are 0-4 Gy for the red channel, 4-50 Gy for the green channel, and above 50 Gy for the blue channel.

Temperature and hydration effects on absorbance spectra and radiation sensitivity of a radiochromic medium

The effects of temperature on real time changes in optical density (DeltaOD) of GAFCHROMIC EBT film were investigated. The spectral peak of maximum change in absorbance (lambdamax) was shown to downshift linearly when the temperature of the film was increased from 22 to 38 degrees C. The DeltaOD values were also shown to decrease linearly with temperature, and this decrease could not be attributed to the shift in lambdamax. A compensation scheme using lambdamax and a temperature-dependent correction factor was investigated, but provided limited improvement. Part of the reason may be the fluctuations in hydration of the active component, which were found to affect both position of absorbance peaks and the sensitivity of the film. To test the effect of hydration, laminated and unlaminated films were desiccated. This shifted both the major and minor absorbance peaks in the opposite direction to the change observed with temperature. The desiccated film also exhibited reduced sensitivity to ionizing radiation. Rehydration of the desiccated films did not reverse the effects, but rather gave rise to another form of the polymer with absorbance maxima upshifted further 20 nm. Hence, the spectral characteristics and sensitivity of the film can be dependent on its history, potentially complicating both real-time and conventional radiation dosimetry.