Me-Yeon Lee

Low Initial Human Papilloma Viral Load Implicates Worse Prognosis in Patients With Uterine Cervical Cancer Treated With Radiotherapy

PURPOSE To evaluate whether human papillomavirus (HPV) viral load measured in cervical smear and HPV type 18 are associated with radiotherapy outcomes in uterine cervical cancer. PATIENTS AND METHODS HPV DNA: was semiquantitatively measured in the cervical smears of 169 radiotherapy patients. HPV viral load was classified as low or high according to median HPV DNA titer and examined for its prognostic value. The multivariable Cox proportional hazards model was used to adjust for covariates. A relapse-predicting model was constructed to classify three risk groups for disease-free survival (DFS), which were used for internal validation. RESULTS Patients with lower HPV viral load showed worse DFS in univariate analysis. HPV type 18, younger patient age, stage group, nodal status, histologic grade, and histologic type were other prognostic factors for poor DFS. Among these factors, all except stage group were associated with HPV viral load. Multivariate analysis showed the strong influence of HPV viral load for poor DFS. The prognostic model developed using our outcome data performed well in predicting the risk of relapse. CONCLUSION Our data suggest that HPV viral load is a strong independent prognostic factor for DFS. HPV type 18 showed a significant relationship with poor radiotherapy outcome in univariate analysis, but not in multivariate analysis.

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.

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%.

Evaluation of dosimetric accuracies of gated step-and-shoot IMRT using Dynalog data

Gated intensity modulated radiation therapy is often used for a lung cancer treatment. We investigated dosimetric accuracies of delivered radiation from each segment due to lag effect of beam on and off by a gating system varying dose rate from 100 to 600 MU/min. 7 ports step-and-shoot lung IMRT study which is planned with the direct machine parameter optimization (DMPO) option in Pinnacle3 and the respiration signals acquired from the Real-time position management (RPM) system of the home-made moving phantom were tested. We analyzed Dynalog file since all information of MLC including leaves position and delivered dose is recorded in that file. The discrepancies between the planned MUs and delivered MUs increased along with an increase of dose rate, even more than 0.5 MU. However there was no significant additional error due to gating system comparing with ungated cases. MLC controller communication delay time was more serious for both cases.

Dose Distribution Depending on the Number of Control Points in Forward-Planned Intensity-Modulated Radiation Therapy for Breast Cancer

Dose uniformity was estimated depending on the number of control points in forward-planned intensity-modulated radiation therapy for breast cancer.

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‐GG‐J‐165: Reconstruction of Missing Data On Kilovoltage and Megavoltage Cone‐Beam CT Images for Adaptive Radiation Therapy

Purpose: Kilovoltage (kV) or megavoltage (MV) cone‐beam CT(CBCT) technique become popular nowadays for patient setup verification and dose recalculation aiming image‐guidedradiation therapy. In case of kV CBCT, field‐of‐view (FOV) is large enough to cover whole thoracic or pelvic region at detector off‐set mode while length of image set is limited and MV CBCT, vice versa. In this study, we dealt with missing data issue; compensation of missing part with kV CTimages used for treatment planning. Method and Materials: First, obtained kV or MV CBCTimages were registered with the planning kV CTimages respectively. For kV CBCT,CT slices beyond the obtained volume were filled with planning CT data. For MV CBCT, overlapped regions in the kV CTimages with the MV CBCTimages after registration were substituted with the MV CBCT data. To reduce statistical discrepancies of pixel numbers between CBCTimages and kV CTimages and to minimize dose recalculation error, pixel number mapping and spatial filtering were employed. Varian Clinac iX and Siemens Primus linacs were used for CBCTimage acquisition. Image registration and dose recalculation were implemented using Pinnacle3 ver 8.0d. Proposed algorithm were tested for chest and pelvic regions of a humanoid phantom and patients. Isodose comparison and gamma index were adopted as evaluation criteria. Results:Dose calculation result with the reconstructedCTimage set agreed with calculation done with kV CT meaningfully within 2% on a phantom but error was up to 5% for clinical cases. Discrepancies were mainly due to anatomic changes and incomplete match of boundaries at the interface of two image set. Conclusion: The reconstruction algorithm showed a possibility of adaptive radiation therapy using CBCTimages but image quality of CBCT data needs to be improved more. Research sponsored by MOST (Korean government).