K Cheong

Feasibility study of radiophotoluminescent glass rod dosimeter postal dose intercomparison for high energy photon beam

A radiophotoluminescent glass rod dosimeter (GRD) system has recently become commercially available. In this study we evaluated whether the GRD would be suitable for external dosimetric audit program in radiotherapy. For this purpose, we introduced a methodology of the absorbed dose determination with the GRD by establishing calibration coefficient and various correction factors (non-linearity dose response, fading, energy dependence and angular dependence). A feasibility test of the GRD postal dose intercomparison was also performed for eight high photon beams by considering four radiotherapy centers in Korea. In the accuracy evaluation of the GRD dosimetry established in this study, we obtained within 1.5% agreements with the ionization chamber dosimetry for the (60)Co beam. It was also observed that, in the feasibility study, all the relative deviations were smaller than 3%. Based on these results, we believe that the new GRD system has considerable potential to be used for a postal dose audit program.

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.

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.

Development of IMRT Treatment Planning System

Development experience of IMRT treatment planning system focused on the objective function and optimization method was presented. Beamlet based fast dose calculation algorithm was implemented to calculate Dij matrix and a quadratic objective function based on DVH volume constraint was implemented. It was solved with a steepest descent method and the optimized intensity map could be obtained. For the verification of the algorithm, several test phantoms and sample plan was tested. As a result, all the dose volume histogram satisfied the treatment prescriptions and the high dose regions maintained the shape of PTV and did not violated OAR. In conclusion, it could incorporate clinical treatment prescriptions into optimization module and generate corresponding dose distributions within reasonable time.

MO‐EE‐A1‐05: Direct Aperture Optimization of Intensity Modulated Arc Therapy

Purpose:Intensity modulated radiation therapy has been a very popular and effective treatment technique for the treatment of prostate, head & neck and liver etc. Meanwhile, another innovative treatment technique, intensity modulated arc therapy, was developed to complement some drawbacks of IMRT like long treatment time and low MU efficiency. Since the IMAT completes the treatment just within one or two rotations, it is not easy to get optimized leaf sequences in a deterministic way. In this study, we tried to get optimized IMAT treatment plan by genetic algorithm. Method and Materials: First, the start/end positions of MLC leaves at each rotation angle with 10° interval were selected as optimization variables and encoded into genetic chromosomes. They experience genetic operations such as generation, selection, crossover, mutation and reproduction and the most optimized solution remains in the end of iteration. The constraint of maximum leaf speed was included in these operations. And the fitness of each population was evaluated by DVH volume constraint based objective function. IMAT dose distribution was calculated as a weighted sum of MLC shape at each angle and related Dij matrix similar to IMRTdose calculation. The algorithm was implemented in our treatment planning system and the dose distributions and DVHs of single and double gantry rotation cases were compared. Results: IMAT plan gave comparable results with conventional IMRT even with single gantry rotation and there was not significant improvement in double gantry rotations. Genetic algorithm required about 3,000 generations to reach optimized value due to its stochastic nature. Conclusion: It was possible to optimize IMAT plan with genetic algorithm and the results are optimized MLC leaf sequences readily deliverable in general linear accelerators. It can be an efficient method to solve IMAT optimization problem despite of relative slow convergence.

SU‐FF‐T‐35: A Monte Carlo Study for Evaluation of Tissue Heterogeneity Effect of Inversely Optimized Intracavitary High Dose Rate Brachytherapy Plan

Purpose: To investigate tissue heterogeneity effect of an intracavitary high dose rate brachytherapy plan which is optimized by the inverse planning option. Method and Materials: A CT and VOI based brachytherapy plan for an intra‐uterine and cervix cancer treatment was made using inverse planning simulated‐annealing (IPSA) module provided by Plato TPS. The microSelectron‐HDR Ir‐192 afterloader and 3 catheters (2 ovoids and 1 tandem) were used for the treatment. For a verification of the plan result, EGSnrc based dosxyznrc Monte Carlo code was employed. Patient CT data was converted to egsphant data by the ctcreate code and each source position and weight from the plan data was used for a Monte Carlo simulation as well. Ir‐192 source was approximately modeled by 3.6 mm length and 0.6 mm diameter and it was assumed to be located at the center of the catheters, while parallel to the z‐axis at each position because of restricted freedom of source modeling. The microSelectron‐HDR Ir‐192 spectrum data could be obtained from the EGSnrc code, however, number of history was determined empirically based on known air‐kerma and dose rate value. Conventional point A and B doses and DVHs of PTV, bladder and rectum were evaluated for a quantitative comparison. Results: Significant decrease of dose at point A and B more than 10% was observed in the Monte Carlo simulation. PTV coverage by 100% isodose surface was degraded while rectum and bladder mean dose was reduced up to 15%. Resultant dose rate decrease was also observed. Conclusion: Conventional brachytherapydose calculation method could not appropriately consider tissue heterogeneity effect and inverse planning optimization could invoke serious erroneous result. Therefore inverse planning optimization should be carefully implemented for a HDR brachytherapy planning.

SU-FF-T-109: The Dependence of IMRT Plans On the Maximum Number of Segments

Purpose: Pinnacle3 provides an direct aperture based step‐and‐shoot IMRT optimization algorithm, so called direct machine parameter optimization (DMPO). This algorithm is known to result in better plan quality with less monitor units than the previous two‐step optimization. Here, we present the dependence of the IMRT plan quality on the maximum number of segments. Method and Materials: Pinnacle3 (ver 7.4f) with a DMPO option was used for the IMRT plan of the five hypopharyngeal cases. The planning system had been commissioned for a Varian 21‐EX with 120 multileafs upon which larger fields are split into smaller ones due the MLC span limitation. PTVs were contoured for 66 Gy, 59.4 Gy and 56.1 Gy with 33 fractionations. Critical organs delineated were as follows: cord, brain stem, inner ears, parotid glands, and cavity for mucosa. Three‐fold dose shaping rings encompassing the PTVs were also added and seven beams were used. Plans were generated for the number of segments between 20 to 80 and evaluated based on the plan with 80 segments. Results: As the number of segments decreased, the plans generally showed a) a tendency of adopting smaller monitor units, b) a wider low dose region, and c) a more inhomogeneous target dose distribution. However, the IMRT plan for the hypopharyngeal case showed similar DVHs and dose distributions down to 40 segments, which amounts to three segments per beam in average after beam splitting. Conclusion: With the DMPO option, unexpectedly smaller number of segments were found to be enough for the clinically acceptable IMRT plan. We plan to extend this study to nasopharyngeal and maxillary cases. With the evaluation of the plan quality along with the segments, more economical IMRT plan and beam delivery would be possible.

SU‐GG‐T‐134: Dosimetric Verification of Dose Calculation Algorithms for IMRT Treatment Plan

Purpose: The purpose of this study was to determine the accuracy of dose calculation in penumbra region from two different commercial IMRTtreatment planning systems (TPS). The accuracy of IMRT calculations with a convolution/superposition and a pencil‐beam algorithm was tested using commission data with the modified values from Gaussian fitting approaches the real doses to correct for the spatial response of finite‐sized ionization chamber against measurement. Method and Materials: The IMRT head and neck phantom used in this study was housed in a custom‐designed package for efficient evaluation of the measured doses with different materials and various detectors. The Varian 21EX linear accelerator with 6 MV beam was used. The Pinnacle and the Ecilpse TPSs were calculated based on commissioning data that included beam profiles collected with a 0.125 cm3ionization chamber. We have modified the commissioning data by a Gaussian function of an ionization chamber kernel to the real profile. Dose measurements made by ionization chamber, and glass dosimeter positioned within the phantoms target insert were compared with the calculated doses. Results: The differences for these algorithm results in average PTV doses were within 1.0%. Ionization chamber results showed approximately 1.7% better agreement than the glass dosimeters and the differences between measured and calculated doses were more than 3.0% for both algorithms. However, calculations using these algorithms after it were re‐commissioned from Gaussian fitting gave better agreement with measurements of IMRT field. A main reason of these results was attributed to detector size effects in the commissioning data. Conclusion: These results show that accurately measuring the penumbra region improves the accuracy of the dose calculations predicted by the TPS and thus is important to choose an appreciate detector.