A convolution model for obtaining the response of an ionization chamber in static non standard fields.

PURPOSE: This work contains an alternative methodology for obtaining correction factors for ionization chamber (IC) dosimetry of small fields and composite fields such as IMRT. The method is based on the convolution/superposition (C/S) of an IC response function (RF) with the dose distribution in a certain plane which includes chamber position. This method is an alternative to the full Monte Carlo (MC) approach that has been used previously by many authors for the same objective. METHODS: The readout of an IC at a point inside a phantom irradiated by a certain beam can be obtained as the convolution of the dose spatial distribution caused by the beam and the IC two-dimensional RF. The proposed methodology has been applied successfully to predict the response of a PTW 30013 IC when measuring different nonreference fields, namely: output factors of 6 MV small fields, beam profiles of cobalt 60 narrow fields and 6 MV radiosurgery segments. The two-dimensional RF of a PTW 30013 IC was obtained by MC simulation of the absorbed dose to cavity air when the IC was scanned by a 0.6 × 0.6 mm(2) cross section parallel pencil beam at low depth in a water phantom. For each of the cases studied, the results of the IC direct measurement were compared with the corresponding obtained by the C/S method. RESULTS: For all of the cases studied, the agreement between the IC direct measurement and the IC calculated response was excellent (better than 1.5%). CONCLUSIONS: This method could be implemented in TPS in order to calculate dosimetry correction factors when an experimental IMRT treatment verification with in-phantom ionization chamber is performed. The miss-response of the IC due to the nonreference conditions could be quickly corrected by this method rather than employing MC derived correction factors. This method can be considered as an alternative to the plan-class associated correction factors proposed recently as part of an IAEA work group on nonstandard field dosimetry.

SU-E-T-02: Radiobiological Comparison of Two Treatment Schemas in IMRT Prostate Radiotherapy.

PURPOSE: Different fractionation schemas are used for the treatment of prostate with IMRT. Due to the low α/ß of prostate hypofractionated schemas are achieving more importance. The purpose of this study is to compare two IMRT techniques from a radiobiological point of view in the treatment of high risk prostate cancer, the sequential IMRT technique (SIMRT) and a simultaneous integrated boost IMRT treatment (SIBIMRT). METHODS: 20 patients were selected and planned using both treatment strategies to give an EQD2 of 82 Gy to prostate, 70 Gy to vesicles and 46 Gy to lymph nodes, using the linear quadratic model LQ supposing an α/ß prostate=1.5 Gy. Dose volume histograms of prostate, rectum and bladder were calculated. Tumour Control Probabilities (TCP) and Normal Tissue Complication Probabilities (NTCP) were also calculated. As the prostate α/ß is not known exactly the TCP has been calculated for different a/bprostate values ranging from 1.5 to 10 Gy. NTCPs of bladder and rectum were calculated also for α/ß oar values ranging between 1 to 15 Gy. RESULTS: Both treatment strategies were found to be equivalent in terms of prostate ptv TCP and EQD2 for low α/ß prostate=1.5 Gy. In the case of high α/ß prostate values, 3 Gy and 10 Gy, EQD2 and TCP for prostate ptv are lower for the SIBIMRT technique. For rectum and bladder, if a/boar ranges between 2 and 15 Gy EQD2 values are higher for the SIMRT treatment schema. NTCP for bladder and rectum is higher for the SIMRT technique when α/ß OAR >= 2 -3 Gy. CONCLUSIONS: In this study a comparison between two treatments strategies have been done. Our results show that a hypofractionated technique for high risk prostate cancer reduces, or equals, EQD2 and NTCP for bladder and rectum while maintaining the TCP of prostate for the typical α/ß values in use.

WE-E-213AB-01: Medical Physics Challenges for Implementation of New Technologies in External Beam Radiotherapy.

The AAPM has signed two formal Educational Exchange Agreements with the Spanish (SEFM) and the Russian (AMPR) medical physics societies. While the primary purpose of the Agreements is to provide educational opportunities for young medical physicists, the Agreements also contemplate holding joint sessions at scientific congresses. The purpose of this professional AAPM/SEFM/AMPR Joint Symposium is to explore the challenges that medical physicists in the three countries face when new external beam radiotherapy technologies are introduced in their facilities and to suggest potential solutions to limitations in testing equipment and lack of familiarity with protocols. Speakers from the three societies will present reviews of the technical aspects of IMRT, Arc EVIRT (IMAT/VMAT/Rapid Arc), SRS/SRBT, and IGRT/Adaptive radiotherapy, and will describe the status of these technologies in their countries, including the challenges found in tasks such as developing anatomical and biological dose optimization techniques and implementing QA management, risk assessment and patient safety programs. The SEFM will offer AAPM and AMPR members the possibility to participate in collaborative proposals for future research bids in UE and USA based on an ongoing Spanish project for adaptive radiotherapy using functional imaging. A targeted discussion will debate three propositions: the cost/benefit ratio of IGRT, whether IMRT requires IGRT, and the use of non-ionizing radiation technologies for realtime monitoring of prostate IGRT. For these debates, each society has designated one speaker to present and defend either "For" or "Against" the proposition, followed by discussion by all participants. The Symposium presentations and the country-tailored recommendations drawn will be made available to each society for inclusion in their websites. The WGNIMP, the AAPM Work Group charged with executing the AAPM/SEFM and AAPM/AMPR Agreements, will follow up on the commitments made by the AAPM.Di Yan's research on adaptive radiotherapy has been financially supported by: 1) NIH Research Grants, 2) Elekta Research Grants 3) Philips Research GrantConflicts of interest for Cedric X Yu: 1) Board Member of Prowess, Inc., 2) Shareholder of Xcision Medical Systems, LLC, 3) Inventor on patents licensed by Varian Medical Systems, Inc. LEARNING OBJECTIVES: 1. Describe fundamental aspects for four advanced radiotherapy techniques: IMRT, IGRT, SBRT, and adaptive radiotherapy. 2. Review technical and professional challenges for implementation of advanced techniques as a function of resources and capabilities available within each scientific society: AAPM, SEFM, and AMPR. 3. Discuss and plan a proposal for an international trial on IMRT/IGRT based on functional imaging. 4. Debate important implementation aspects of IMRT and IGRT according to country-specific resources.

SU-E-I-83: Detectability Limits of a New Positron Emission Mammography in Relation to Tumour-Size, Tumour-To-Background Ratio and Activity Concentration.

PURPOSE: Positron emission mammography (PEM) improves spatial resolution and sensitivity, making it suitable for early breast tumours detection. The aim of this study is to evaluate the limits of a dedicated breast PET in terms of tumour-size, tumour-to-background activity concentration ratio (TBR) and activity concentration. METHODS: A dedicated PEM is evaluated. To characterize the device, we use a phantom of 15 cm of diameter containing 6 inserts of inner diameters 18, 15, 11, 8, 5 and 3 mm. To evaluate the detectability limits images are acquired by varying the TBR from 10 to 2 and without background activity concentration. In all the studies the activity concentration for the 6 inserts is the same (3.7 kBq/ml). To asses the activity concentration limit, the SBR is maintained fixed and acquisitions at different times are performed. RESULTS: When there is not background, all the inserts are visible in the PEM. Increasing the background decreases the detectability. With a SBR of 10, the smaller insert is still visible. For TBR between 10, inserts >= 5 mm can be seen With a TBR of 2, only inserts with a diameter higher than 5 mm are visible. When the TBR is fixed, decreasing the activity concentration, decreases the capability of detectability. CONCLUSIONS: The results suggest that PEM can be used for diagnosis of small lesions when TBR is higher than 2. Further clinical studies need to be carried out in order to validate these results.

SU-E-T-514: Implementation and Commissioning of a Micro Multileaf Collimator in Pinnacle Treatment Planning System.

PURPOSE: To model and validate, in a Pinnacle treatment planning system, a Brainlab micromultileaf collimator mounted on a Primus Siemens accelerator. The objective is to take advantage of the collapsed cone convolution algorithm and the ability of this system modelling rounded leaf- end MLC's. METHOD: The micro multileaf collimator was modelled using fixed accelerator jaws with a value of 9.2×9.2 cm2 . Profiles and depth dose curves for a wide range of square fields at SSD of 100 cm and depths of 1.5,5,10 and 20 cm were measured using a Scanditronix stereotactic SFD diode. Output factors were measured using a stereotactic unshielded diode for field sizes from 0.6×0.6 cm2 to 3×3 cm2 . For wider fields a Scanditronix ic15 ionization chamber was used. EDR2 films were used to measure adjacent fields in the transverse and longitudinal direction. The film measurements were compared to Pinnacle calculations to model and validate the leaf tip radius, leaf offset calibration values and tongue and groove width. RESULTS: Pinnacle calculations and measurements agree within 2% or 2mm except for the tails of largest fields where differences are <3.5%. Comparison of film measurements and Pinnacle calculations give the optimal value for leaf tip radius of 15 cm and for tongue and groove width of 0.04 cm. CONCLUSIONS: Pinnacle models a Brainlab micromultileaf collimator mounted on a Siemens Primus accelerator with acceptable results for clinical treatments.