M Wolfe

SU‐FF‐T‐147: Improved Calibration Method of EDR Films for IMRT‐QA

Purpose: Due to film processing variation and the optical density not being linear with the dose, a full calibration has to be obtained for every IMRT‐QA with film. We are proposing the application of a linearization method and the use of a single dose calibration EDR film for relative and absolute dose measurement in IMRT‐QA. Method and Materials: The linearization method was studied for the EDR films using 6, 10 and 15 MV photon beams. The films were developed in a Konica film processor, then scanned with a Vidar VXR‐16 scanner and analyzed using RIT‐114 version 4.1. A standard 30‐point calibration curve was obtained with the 6 MV beam. Using the method developed in a previous paper, a curve fitting was obtained for a sigmoid expression modulated by a 3rd degree polynomial: ApparentDose = b (1+ a 1 x + a 2 x 2 + a 3 x 3 ) [ log ( m )− log ( m − x )] ; where x is the net optical density, m is the net saturation density, b, a1, a2 and a3 are parameters of the model. Results: A value of 1870.1945 was obtained for b that is a parameter related to the dose unit. The net saturation density was 3.5587 for our system. The parameters a1, a2 and a3 are, respectively, −0.4551, 0.1167, −0.0134. For every IMRT‐QA a spreadsheet is used to obtain a 70 point calibration curve for RIT, using only one exposed film developed together with the composed and enface films, obtaining <5% uncertainty. Conclusion: Some softwares are available to make film dosimetry in IMRT QA less cumbersome, however daily calibration still remains a time consuming procedure. Absolute dosimetry requires a full calibration every time film is used. The linearization method presented here is being used in our department for isodose comparison and for absolute measurement at calculation point. The overall time is reduced while obtaining uncertainty comparable to the existing methods.

Consistency of vendor-specified activity values for 192Ir brachytherapy sources

A long-term comparison was done between the manufacturer-stated ¹⁹²Ir activity and the measured ¹⁹²Ir activities determined with a well-type ionization chamber. Sources for a Nucletron Micro Selectron high-dose-rate (HDR) unit were used for this purpose. The radioactive sources reference activities were determined using a PTW well-type ionization chamber traceable to the National Institute of Standards and Technology Primary Calibration Laboratory. The measurements were taken in a period of 56 months with 17 different radioactive sources. The manufacturer stated activities were taken from the source calibration certificate provided by the manufacturer. These values were compared with the measured activities. The results have shown that both the percentage deviation of the monthly control measurements with the well-type chamber and the ratio between the measured activities to the manufacturer-stated value lie within ± 2.5%. These results were compared with similar published data and with uncertainty level (3% of the mean and 5% maximum deviation from mean) for brachytherapy sources calibration recommended by the AAPM. It was concluded that a threshold level of ±2.5% can be used as a suitable quality assurance indicator to spot problems in our department. The typical ±5% uncertainty as provided by the manufacturers may be tightened to ±3% to be more in line with published AAPM reports.

SU‐GG‐J‐50: Comparing IGRT Shifts with MVision and CT On Rails Systems Using Anthropomorphic Phantom

Purpose: To evaluate the IGRT repositioning shifts determined by MV Cone Beam CT and by CT on rails using an anthropomorphic phantom. Method and Materials: Two IGRT systems installed in the same Linac room were studied. The CT based IGRT has been used in our Siemens Primus since 2004. Recently, a MV conebeam CT (Siemens MVision) was implemented with the objective to reduce the therapist time to obtain the IGRT shifts. The shifts and the corresponding total time were evaluated using an anthropomorphic PIXY® phantom. CTs of the phantom were obtained for head‐and‐neck, pelvis and lung and treatment planning was generated in XIO‐CMS for each site. The set of images, structures and plans was sent to both IGRT stations. Bbs were placed on the phantom surface at the isocenter axis, as a reference for the daily CTimages. For each plan the MVCBCT was obtained three times to determine its reproducibility for 5 and 8 MU protocols. The registration for MVCBCT was done automatically and verified with manual registration. The table was then flipped to the CT position and the images acquired. In this case the registration is done manually and the isocenter shift is obtained through the bbs position. The procedures ware repeated for all sites. The total time to obtain the shifts and the image quality were also registered. Results and Discussion: The reproducibility of the MVCBCT for different protocols was within 2 mm. The maximum difference for MVCBCT and CT was 4 mm. The MVCBCT has poor image for soft tissue, and the fusion registration is based on mutual information. The total time to obtain the shifts is about 3 min for the new system and 7 to 8 min for the CT based system. This study excludes any deviation due to patient rotation and internal changes.

SU‐GG‐T‐25: Intercomparison Between Nucletron Plato and Simuplan Systems for Ir‐193 HDR Brachytherapy Treatment Planning

Purpose: To compare two commercial TPS systems for 2D HDR planning. Method and Materials: The Nucletron Plato System (NPS) version BPS13.7 is a well established TPS for Brachytherapy procedures and has been used clinically for more than 20 years. New softwares have been released for 2D and 3D calculation using new optimization methods. In this work we intercompare the dose at prescription points calculated by the NPS and by Simuplan Planning System (SPS) version 8.2d, with the same dwell position times. The input data for both systems was done minimizing the geometric error in the final reconstruction. Plannings were done in both systems for several configurations, varying the length of active path (point source, 2, 3, 4 and 5 cm). For each active path calculation distances were changed from 10 to 30 mm from the catheter). The same prescription points were defined in both systems. The optimized plans were generated with the SPS system, adjusting the dwell times aiming a maximum dose difference around ±2% among the prescription points. The total treatment time was verified by manual calculation. The dwell times obtained by SPS system were input into NPS system, to compare the dose at the prescription points. The error at the source and points coordinate positions was evaluated for both systems. Results and Discussion: The dose difference at the prescription points, changing the active length ranged from 0.24% to 0.66%. Keeping the length fixed and varying the treatment distances, these differences ranged from 0.25% to 0.65%. The overall average difference was 0.45%±0.19%. The error in positioning the source and the points was 0.21 mm for SPS and 0.61mm for NPS. All the manual calculations were within ±5% difference.

SU-FF-J-20: An Evaluation of User Variability for Image-Guided Radiation Therapy (IGRT) Shift Determination

Purpose: To evaluate the Image‐Guided(IGRT) shift variability calculated by different users using the same patient data and same IGRT process. Method and Materials:IGRT was performed using the Siemens Primatom. This system consists of a Siemens Primus Linear Accelerator and a Siemens Emotion diagnostic CT on rails. Patients have a daily, pretreatment CT scan taken that is transferred to the Siemens Coherence Workstation where the daily CT is fused with the original treatment planningCT. The fusion is performed so that the daily internal organ or critical structure position can be determined and the organ/structure can be aligned to give the same position relative to the isocenter that was determined from the treatment plan. A shift in the x, y and z directions are made to facilitate this alignment. Four users retrospectively calculated the shifts required for an IGRT patient using the same CT data set and the same IGRT process. The treatment area for the patient was the prostate gland and a total of 39 daily shifts were performed. Results: The maximum variation on any one day was 0.90cm in the right/left direction, 1.10cm in the superior/inferior direction and 0.93cm in the anterior/posterior direction. The averages of the maximum daily variations were 0.39cm, 0.54cm and 0.55cm respectively. Conclusion: There are multiple systems that will perform IGRT but the advantage to using CT‐on‐rails systems is the high soft tissue resolution you get from the diagnostic CT sets. The higher resolution allows the user much more information that can be included in the shift evaluation. Our results show the user variability to be acceptable but when implementing this type of IGRT, user variability must be considered.