Kwang-Ho Cheong

Variations in dose distribution and optical properties of GafchromicTM EBT2 film according to scanning mode

PURPOSE The authors aim was to investigate the effects of using transmission and reflection scanning modes, the film orientation during scanning, and ambient room light on a dosimetry system based on the Gafchromic(TM) EBT2 film model. METHODS For calibration, the films were cut to 3 × 3 cm(2) and irradiated from 20 to 700 cGy at the depth of maximum dose using 6 and 10 MV photon beams in a 10 × 10 cm(2) field size. Absolute dose calibration of the linear accelerator was done according to the TRS398 protocol. An FG65-G ionization chamber was used to monitor the dose while irradiating the films in solid water. The film pieces were scanned with an EPSON Expression 1680 Pro flatbed scanner in transmission and reflection modes. Authors investigated the effect of orientation on films and examined the optical properties of EBT2 film using an ellipsometer and an ultraviolet (UV)/visible spectrometer to explain the dosimetric dependence of the film on orientation during the scanning process. To investigate the effect of ambient room light, films were preirradiated in 6 and 10 MV photon beams with intensity-modulated radiotherapy (IMRT) quality assurance (QA) plans, and then exposed to room light, either directly for 2 days in a workroom or for 2 months in a film box. Gamma index pass criteria of (3%, 3 mm) were used. RESULTS The dose response curves based on net optical density (NOD) indicated that the reflection scanning mode can provide a better dose sensitivity than the transmission scanning mode, whereas the standard deviation of the dose is greater in reflection mode than in transmission mode. When the film was rotated 90° from the portrait orientation, the average dose of the EBT2 film decreased by 11.5-19.6% in transmission mode and by 1.5-2.3% in reflection mode. Using an ellipsometer, variation of the refractive index of EBT2 film-the birefringence property-was found to be the largest between 45° (1.72 and 1.71) and 135° (1.8 and 1.77) for 300 and 800 cGy. Absorption spectra of EBT2 films measured with spectrometer were the function of film orientation. The readings in reflection scanning mode were more stable against room light than those in transmission scanning mode, although dose readings increased in both modes after the films were exposed to room light. CONCLUSIONS The transmission scanning mode exhibited a strong dependence on film orientation during scanning and a change in optical density resulting from room light exposure, so a constant scanning orientation and minimal exposure to light can reduce uncertainty in the measured dose (23 ± 3%). The angular dependence was analyzed using Jones matrices and optical properties of EBT2 film were obtained using an ellipsometer and an UV/visible spectrometer. The reflection scanning mode has relatively good stability with respect to room light and film orientation on a scanner, although the large standard deviation of dose is a disadvantage in measurements of absolute dose. Reflection scanning mode can offer a potential advantage for film dosimetry in radiotherapy, although transmission scanning mode is still recommended for dosimetry as it provides better uncertainty results.

Comparison of film dosimetry techniques used for quality assurance of intensity modulated radiation therapy

PURPOSE Accurate dosimetry is essential to ensure the quality of advanced radiation treatments, such as intensity modulated radiation therapy (IMRT). Therefore, a comparison study was conducted to assess the accuracy of various film dosimetry techniques that are widely used in clinics. METHODS A simulated IMRT plan that produced an inverse pyramid dose distribution in a perpendicular plane of the beam axis was designed with 6 MV x rays to characterize the large contribution of scattered photons to low dose regions. Three film dosimetry techniques, EDR2, EDR2 with low-energy photon absorption lead filters (EDR2 WF), and GafChromic® EBT, were compared to ionization chamber measurements as well as Monte Carlo (MC) simulations. The accuracy of these techniques was evaluated against the ionization chamber data. Two-dimensional comparisons with MC simulation results were made by computing the gamma index (γ) with criteria ranging from 2% of dose difference or 2 mm of distance to agreement (2%/2 mm) to 4%/4 mm on the central vertical plane (20×20cm2) of a square solid water phantom. Depth doses and lateral profiles at depths of 5, 10, and 15 cm were examined to characterize the deviation of film measurements and MC predictions from ionization chamber measurements. RESULTS In depth dose comparisons, the deviation between the EDR2 films was 9% in the low dose region and 5% in high dose region, on average. With lead filters, the average deviation was reduced to -1.3% and -0.3% in the low dose and high dose regions, respectively. EBT film results agreed within 1.5% difference on average with ionization chamber measurements in low and high dose regions. In two-dimensional comparisons with MC simulation, EDR2 films passed gamma tests with a 2%/2 mm criterion only in the high dose region (γ⩽1, total of 63.06% of the tested region). In the low dose region, EDR2 films passed gamma tests with 3%/3 mm criterion (γ⩽1, total of 98.4% of the tested region). For EDR2 WF and GafChromic® EBT films, gamma tests with a 2% /2 mm criterion (γ⩽1) in the tested area was 97.3% and 96.8% of the tested region, respectively. CONCLUSIONS The EDR2 film WF and GafChromic® EBT film achieved an average accuracy level of 1.5% against an ionization chamber. These two techniques agreed with the MC prediction in 2%/2mm criteria evaluated by the gamma index, whereas EDR2 without filters achieved an accuracy level of 3%/3 mm with the decision criteria of agreement greater than 95% of the tested region. The overall results will provide a useful quantitative reference for IMRT verifications.

Dosimetric characteristics of linear accelerator photon beams with small monitor unit settings

Several studies on the effect of tumor cell killing by dose rate variation have implied that the use of a shorter treatment time is more favorable for intensity modulated radiation therapy (IMRT). Aiming at step-and-shoot IMRT with higher dose rates, the stabilities of beam output and profiles with small monitor unit (MU) settings were investigated for various dose rates. With the use of a Varian 21EX (Varian Medical Systems Inc., Palo Alto, CA), static and step-and-shoot IMRT beam output along with profiles were measured by use of an ion chamber and a two-dimensional diode array detector as a function of monitor units and dose rates. For a static case, as the MU approached 1, the beam output increased up to 2% for 300 MU/min and 4.5% for 600 MU/min, showing a larger overdose as the dose rate increased. Deterioration of the beam symmetry and flatness were also observed as the MU decreased to 1 monitor unit. For the step-and-shoot IMRT case, a large dosimetric error of more than 10% was also detected with the use of a small MU segment. However, no definite correlation with the dose rate was observed due to the combined beam start-up effects by the grid pulse and finite communication time between the machine console and multileaf collimator (MLC) controller. For step-and-shoot IMRT with higher dose rates, beam output and beam profile stability with small MU needs to be checked, and adequate MU limitation where segments are not allowed need to be reflected in the step-and-shoot IMRT planning.

Enhancement of megavoltage electronic portal images for markerless tumor tracking

Abstract Purpose The poor quality of megavoltage (MV) images from electronic portal imaging device (EPID) hinders visual verification of tumor targeting accuracy particularly during markerless tumor tracking. The aim of this study was to investigate the effect of a few representative image processing treatments on visual verification and detection capability of tumors under auto tracking. Methods Images of QC‐3 quality phantom, a single patients setup image, and cine images of two‐lung cancer patients were acquired. Three image processing methods were individually employed to the same original images. For each deblurring, contrast enhancement, and denoising, a total variation deconvolution, contrast‐limited adaptive histogram equalization (CLAHE), and median filter were adopted, respectively. To study the effect of image enhancement on tumor auto‐detection, a tumor tracking algorithm was adopted in which the tumor position was determined as the minimum point of the mean of the sum of squared pixel differences (MSSD) between two images. The detectability and accuracy were compared. Results Deblurring of a quality phantom image yielded sharper edges, while the contrast‐enhanced image was more readable with improved structural differentiation. Meanwhile, the denoising operation resulted in noise reduction, however, at the cost of sharpness. Based on comparison of pixel value profiles, contrast enhancement outperformed others in image perception. During the tracking experiment, only contrast enhancement resulted in tumor detection in all images using our tracking algorithm. Deblurring failed to determine the target position in two frames out of a total of 75 images. For original and denoised set, target location was not determined for the same five images. Meanwhile, deblurred image showed increased detection accuracy compared with the original set. The denoised image resulted in decreased accuracy. In the case of contrast‐improved set, the tracking accuracy was nearly maintained as that of the original image. Conclusions Considering the effect of each processing on tumor tracking and the visual perception in a limited time, contrast enhancement would be the first consideration to visually verify the tracking accuracy of tumors on MV EPID without sacrificing tumor detectability and detection accuracy.

Electron dose distributions caused by the contact-type metallic eye shield: Studies using Monte Carlo and pencil beam algorithms

A metallic contact eye shield has sometimes been used for eyelid treatment, but dose distribution has never been reported for a patient case. This study aimed to show the shield-incorporated CT-based dose distribution using the Pinnacle system and Monte Carlo (MC) calculation for 3 patient cases. For the artifact-free CT scan, an acrylic shield machined as the same size as that of the tungsten shield was used. For the MC calculation, BEAMnrc and DOSXYZnrc were used for the 6-MeV electron beam of the Varian 21EX, in which information for the tungsten, stainless steel, and aluminum material for the eye shield was used. The same plan was generated on the Pinnacle system and both were compared. The use of the acrylic shield produced clear CT images, enabling delineation of the regions of interest, and yielded CT-based dose calculation for the metallic shield. Both the MC and the Pinnacle systems showed a similar dose distribution downstream of the eye shield, reflecting the blocking effect of the metallic eye shield. The major difference between the MC and the Pinnacle results was the target eyelid dose upstream of the shield such that the Pinnacle system underestimated the dose by 19 to 28% and 11 to 18% for the maximum and the mean doses, respectively. The pattern of dose difference between the MC and the Pinnacle systems was similar to that in the previous phantom study. In conclusion, the metallic eye shield was successfully incorporated into the CT-based planning, and the accurate dose calculation requires MC simulation.

SU-E-J-67: Evaluation of Breathing Patterns for Respiratory-Gated Radiation Therapy Using Respiration Regularity Index

PURPOSE Despite the importance of accurately estimating the respiration regularity of a patient in motion compensation treatment, an effective and simply applicable method has rarely been reported. The authors propose a simple respiration regularity index based on parameters derived from a correspondingly simplified respiration model. METHODS In order to simplify a patients breathing pattern while preserving the datas intrinsic properties, we defined a respiration model as a power of cosine form with a baseline drift. According to this respiration formula, breathing-pattern fluctuation could be explained using four factors: sample standard deviation of respiration period, sample standard deviation of amplitude and the results of simple regression of the baseline drift (slope and standard deviation of residuals of a respiration signal. Overall irregularity (δ) was defined as a Euclidean norm of newly derived variable using principal component analysis (PCA) for the four fluctuation parameters. Finally, the proposed respiration regularity index was defined as ρ=ln(1+(1/ δ))/2, a higher ρ indicating a more regular breathing pattern. Subsequently, we applied it to simulated and clinical respiration signals from real-time position management (RPM; Varian Medical Systems, Palo Alto, CA) and investigated respiration regularity. Moreover, correlations between the regularity of the first session and the remaining fractions were investigated using Pearsons correlation coefficient. RESULTS The respiration regularity was determined based on ρ; patients with ρ<0.3 showed worse regularity than the others, whereas ρ>0.7 was suitable for respiratory-gated radiation therapy (RGRT). Fluctuations in breathing cycle and amplitude were especially determinative of ρ. If the respiration regularity of a patients first session was known, it could be estimated through subsequent sessions. CONCLUSIONS Respiration regularity could be objectively determined using a respiration regularity index, ρ. Such single-index testing of respiration regularity can facilitate determination of RGRT availability in clinical settings, especially for free-breathing cases. This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Ministry of Science, ICT and Future Planning (No. 2013043498).

SU‐E‐T‐307: A Dummy Eye Shield for Electron Treatment Planning

Purpose: An electron treatment for the eyelid is routinely executed with the radiation shield accessory. However, the incorporation of the shield into the dose calculation is not satisfactory, mainly because of the metal artifacts from the shield. Here, we present the use of an acrylic dummy eye shield for tungsten shield and evaluate the procedure and accuracy. Methods: Both the 2 mm thickness tungsten eye shield and the acrylic eye shield machined with the same size were used for the CT scan and beam delivery. Considering the tungsten, steel knob and thin aluminum cover, Monte Carlo simulation and the Pinnacle (version, 8.0m; Phillips Medical Systems, Madison, WI, USA) plan were made for the electron 6 MeV deliveries. The results were confirmed with the metal oxide semiconductor field effect transistor(MOSFET)detectors and Gafchromic EBT2 film measurements. Results: From the MC and EBT2 film measurements, both the maximum dose below the tungsten shield and the eyelid dose above the shield agreed respectively to each other within 1.7%. The Pinnacle plan based on the CT number modified images of the acrylic shield showed the maximum dose agreed with the MC within 0.3%; however, the eyelid dose were under calculated by 19.3%. Conclusions: Using the dummy eye shield for the electron treatment plan on the Pinnacle, contouring of the tungsten shield body and steel knob only, and the following density adjustment was sufficient for the evaluation of the dose distribution below the eye shield. The eyelid dose, however, was underestimated by 19.3%.

Improvement in dose homogeneity in a patient with sinonasal cancer using nasal accessory

Many treatment regimens for sinonal malignant neoplams remain empirical and controversial because of a lack of sufficient clinical data. This article discusses the considerations in the radiation treatment of the malignancies that arise in the nasal cavity. When the nasal accessory was inserted in patient’s nose, the equivalent uniform dose (EUD) increased from 104.7% to 106.5 % and the homogeneity index (HI) was drastically improved from 1.05 to 0.41 using nasal accessory. Moreover, delivered dose to organs at risk (OARs) tends to decrease. For sinonasal cancer patients nasal accessory improved homogeneity of target and maximum doses were decreased for critical organs.

Electron arc planning on the commercial radiation treatment planning system

A commercial radiation treatment planning system, Pinnacle3, has been prepared for the electron arc treatment with a Varian machine. For this purpose, a new physics machine was commissioned for the exclusive electron arc therapy. The electron arc plan was made with multiple static beams with fixed interval. The phantom measurements were executed with a MOSFET and EBT films. The resulting plan shows an impressive dose distribution, however, in-vivo dosimetry for a few representative points should be proceeded before treatment to be sure of the calculation accuracy.

SU‐FF‐T‐448: Siemens Primus Linear Accelerator Commissioning for IMRT Verification Using PMCEPT Monte Carlo Code and GAFCHRONICTM EBT Film

Purpose: To implement a dose calculation system that accurately predicts the dose delivered by Siemens Primus 6 MV linear accelerator and to check the IMRTtreatment planning system calculation with an independent calculation system. Method and Materials: The BEAMnrc and the PMCEPT Monte Carlo code were used to simulate Siemens Primus 6 MV photon beams. The BEAMnrc was used to generate phase space (phsp) files at 100 cm source‐to‐surface distance for several beams, defined by the jaw and MLCs. Symmetric square beams ranging from 1 x 1 to 20 × 20 cm2 were calculated to simulate pyramid density shape. The left hand side non symmetric rectangular beams of 3 × 15, 4.5 × 15, and 6 × 15 cm2 and the right hand side non symmetric rectangular beams of 6 × 15, 4.5 ×15, and 3 × 15 cm2 were calculated to simulate inverse‐pyramid density shape. The PHSP files were subsequently used as the input files for the PMCEPT code. The commissioning was verified against EBT film, ion chamber, diode detector measurements for a solid water phantom. Results: A solid water phantom of 30 × 30 × 30 cm3 with cell size of 0.1 × 0.1 × 0.2 cm3 was used for the PMCEPT simulations. For each beam, the MC calculated central axis depth dose and lateral profiles were compared with those of experiments. The agreement between calculated and measured dose distributions was within 2%. For both pyramid and inverse‐pyramid density shapes, superposed‐lateral profiles show agreement better than 2%, even in the region of penumbra where gradient varies sharply. Conclusions: Our results suggest that the PMCEPT code can be used to calculate IMRT dose distributions as well as to calibrate detectors with great accuracy. Moreover, it can be used as a double check system for IMRT QA and treatment planning.