Taejin Hwang

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.

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.

SU-F-T-315: Comparative Studies of Planar Dose with Different Spatial Resolution for Head and Neck IMRT QA

PURPOSE To quantitatively investigate the planar dose difference and the γ value between the reference fluence map with the 1 mm detector-to-detector distance and the other fluence maps with less spatial resolution for head and neck intensity modulated radiation (IMRT) therapy. METHODS For ten head and neck cancer patients, the IMRT quality assurance (QA) beams were generated using by the commercial radiation treatment planning system, Pinnacle3 (ver. 8.0.d Philips Medical System, Madison, WI). For each beam, ten fluence maps (detector-to-detector distance: 1 mm to 10 mm by 1 mm) were generated. The fluence maps with larger than 1 mm detector-todetector distance were interpolated using MATLAB (R2014a, the Math Works,Natick, MA) by four different interpolation METHODS: for the bilinear, the cubic spline, the bicubic, and the nearest neighbor interpolation, respectively. These interpolated fluence maps were compared with the reference one using the γ value (criteria: 3%, 3 mm) and the relative dose difference. RESULTS As the detector-to-detector distance increases, the dose difference between the two maps increases. For the fluence map with the same resolution, the cubic spline interpolation and the bicubic interpolation are almost equally best interpolation methods while the nearest neighbor interpolation is the worst.For example, for 5 mm distance fluence maps, γ≤1 are 98.12±2.28%, 99.48±0.66%, 99.45±0.65% and 82.23±0.48% for the bilinear, the cubic spline, the bicubic, and the nearest neighbor interpolation, respectively. For 7 mm distance fluence maps, γ≤1 are 90.87±5.91%, 90.22±6.95%, 91.79±5.97% and 71.93±4.92 for the bilinear, the cubic spline, the bicubic, and the nearest neighbor interpolation, respectively. CONCLUSION We recommend that the 2-dimensional detector array with high spatial resolution should be used as an IMRT QA tool and that the measured fluence maps should be interpolated using by the cubic spline interpolation or the bicubic interpolation for head and neck IMRT delivery. This work was supported by Radiation Technology R&D program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (No. 2013M2A2A7038291).

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‐720: Maintaining Prostate IMRT Plan Quality Using the Fraction of Rectum Overlapping the PTV

Purpose: It is clinically meaningful to generate the optimal intensity‐modulated radiation therapy (IMRT) treatment plans within the restricted time with equally good quality for different patients, which is planner‐dependent. We developed a planner‐independent method for a clinical decision of prostate IMRT plan using the rectal complication probability with the volume of rectum overlapping the PTV. Methods: Eighteen prostate IMRT plans were retrospectively analyzed. The prescription dose in the all the planning target volumes (PTVs) was normalized with 76 Gy in 2 Gy fractions. We found a correlation between the fraction of rectum overlapping the PTV and the rectal normal tissue complication probability (NTCP), which is the predictable NTCP curve, which means the lower bound of the rectal NTCP. The plans beyond the predictable NTCP criteria were generated again with seven 10 MV beams from the Varian Clinac iX linear accelerator. The beams were arranged coplanar at: 0, 50, 100, 140, 220, 260, and 310. IMRT plans were generated using direct machine parameter optimization in the Pinnacle3. Results: All the regenerated plans came to be included in the predictable NTCP criteria. In our patient data, two plans were re‐planned: the NTCP for patient 7 was changed from 10.9 to 8.8 and the other NTCP for patient 14 was from 15.6 to 12.9. The rectal NTCP is clinically more meaningful than the mean dose of rectum. Conclusion: This model can predict the rectum NTCP before IMRT planning, to control the IMRT plan quality during planning. Hence we can maintain prostate IMRT plan quality using the fraction of rectum overlapping the PTV.

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%.

SU‐E‐T‐168: Variations in Dose Response of Gafchromic EBT2 Film Scanned in Transmission and Reflection Mode

Purpose: 2D planar dosimeter, radiochromic film has commonly been used to perform patient quality assurance (QA). It is our aim to investigate the properties of new EBT2 film scanned in transmission and reflection scanning mode. Method: We compared the transmission and reflection scanning mode of EBT2 (Lot No. A07160901) film with an EPSON expression 1680 pro flatbed scanner. Films were cut up into 3×3cm, 2 and irradiated from 20cGy to 700cGy using a 1 0 × 10cm, 2 field size with 6MV and 10MV photons for calibration. The red channel was confirmed to provide the greatest sensitivity and was used for all measurements. We investigated the effect of an absolute dose and film orientation using film pieces which were delivered 100cGy, 300cGy, and 500cGy at the depth of maximum dose in a phantom. The EBT2 films were irradiated with 6MV and 10MV photons for intensity modulated radiotherapy(IMRT) QA and compared with the films which have left under different room light conditions. Result: A maximum dose variation was −9.7% in transmission mode and 6.6% in reflection mode respectively. The maximum standard deviation in a region of interest (ROI) was 2.2% in transmission and 3.6% in reflection mode. The transmission scanning readings show strong film orientation dependency compared with reflection readings. In addition to, readings in transmission scanning mode are more sensitive to room light than that in reflection mode. Conclusion: Transmission scanning mode is conventionally used for the purpose of patient QA using EBT2 film. However, the mode is sensitive to the orientation of the film nd room light. Reflection scanning mode has the long term stability of the film readings and a relatively low orientation effect on EBT2 film. On the other hand, the large standard deviation of absolute dose in reflection mode is a drawback in providing high‐precision dosimetric information.

SU‐GG‐T‐578: Radiation Treatment Plans Based on the 4D CT Phased Images Generated by Interpolation

Purpose: One of the drawbacks of the four dimensional (4D) CT scan is the larger scan dose to the patient than that of the conventional scan. We present the generation of the phased CTimages from two inhale and exhale images via deformable registration, and compare the treatment plans made on both the maximum intensity projection (MIP) images from the original and interpolated phased images.Method and Materials: Under free breathing, CTimages for a lungcancer patient were obtained using the cine mode with the associated respiratory signals. The periodic whole images were sorted into 10 phased bins. For the artificial intermediate phased images, the end‐inhale and end‐exhale images were deformably registered. The resulting deformation vector field was modified for the appropriate deformation and applied to the inhale images, resulted in the target phased images. The same beam configurations were applied to both MIP images constructed from the original and generated phased images, respectively. Both plan results were compared based on the gamma index. Results: When compared with the original CT scanned MIP images, generated MIP images were acceptable for the planning and gated treatment, such that 95% pixels passed the gamma evaluation (2%/2‐mm in local). Conclusions: The MIP images for the gated treatment can be obtained from two end phased images using the deformable registration. In application, patient training is necessary for the natural tidal volume maintenance when the patient stops breathing at end‐inhale/exhale positions. Goggles displaying the respiratory signal of patient itself are helpful. CT scanners with multi‐detectors are necessary for the short breath‐hold period of the patient. This technique could reduce the additional radiation dose to the patient from the 4D CT scan aiming the gated treatment.