420 Ion chamber dosimetry in non-reference conditions for 15 MV narrow photon fields

$184 gel was used for verification of a 4-field CRT prostate plan and a 7-field IMRT bladder plan (Eclipse). In the CRT plan, normalisation was done by using an average of nine pixels in two homogeneous dose regions of the plan (100 and 60 % of target dose); while a 2.16 dl gel phantom (irradiated with doses of 0.5-2.0 Gy) and corresponding calculations from Eclipse were used for calibration of the IMRT bladder plan. The gel was mixed under normal levels of oxygen, and left to set in room temperature. Magnetic resonance (MR) imaging was used to obtain dose maps (based on the spin-spin relaxation rate R2). Measured and calculated dose distributions were compared using the gamma-method. Results: The gel showed a linear dose response between 0.5 and 2.0 Gy (sensitivity: 1.32-1.47 1/sGy, extrapolated intercept R2(0 Gy): 2.2-2.4 1/s and R2 = 0.9989-0.9998; three experiments). Depth dose curves mostly corresponded well, but in a few samples deviations believed to be due to inhomogeneities in the gel were observed. In volumes of the CRT prostate plan where the planned dose exceeded 60 % of the target dose (2.0 Gy), planned and measured dose agreed across 80.9 % of the volume using the gamma-method with 3% dose difference (DD) and 3 mm distance-to-agreement (DTA), while an agreement in 94.9 % of the volume was found using 5 % and 5 mm. In volumes of the IMRT bladder plan where the planned dose exceeded 60 % of the target dose (1.67 Gy), the agreement between planned and measured dose was 87.7 % with a DD of 3 % and DTA of 3 mm. Using 5 % DD and 5 mm DTA the agreement in this volume was 99.7 %. Conclusion: The gel showed a linear response and gave a very good agreement with planning calculations for the PTVs in an IMRT plan. The gel thus shows promise for use as a relative 3D dosimeter, and will be used for verification of other potential IMRT set-ups (head and neck and prostate cancer). 42O I o n c h a m b e r dosimetry in n o n - r e f e r e n c e conditions for 15 MV n a r r o w photon fields

E. Garcia Monta~o ~,2, F. S~nchez-Doblado 1,3, R. Capote 4 , E. Hermoso ~'3, R. Arrans~'3, P. Andreo 4 ~Dpto. Fisiolog[a M~dica y Biofisica, Facultad de Medicina, Universidad de Sevilla, Spain 2Dpto. F/sica Aplicada I, E.U.I.T.A., Universidad de Sevilla,

Spain 3Hospital Universitario Virgen Macarena, Radiof[sica, Sevilla,

Spain 4International Atomic Energy Agency, Vienna, Austria I n t r o d u c t i o n : Reference dosimetry in small fields, as needed in radiosurgery and intensity modulated radiation therapy, is challenging due to the lack of lateral electron equilibrium. Existing protocols for dosimetry in reference conditions are based on relatively large fields (10x10 cm 2) where electronic equilibrium conditions exist. As an extension of the work by S~nchez-Doblado et al (PMB 2003) this work aims at calculating by Monte Carlo (MC) simulation the conversion factor necessary to determine directly the absorbed dose to water using an IBA-Wellh6fer NAC 007 micro ionization chamber in 15 MV narrow radiation fields. Spencer-Attix (A=10 keV) stopping-power ratios and electron/photon spectra at a reference depth in a water phantom are also obtained. Materials and Methods: The required 15 MV phase-spaces for a Siemens Primus linac were calculated with BEAMnrc to model the linac treatment head and its multileaf collimator. Narrow 5x5 and 2x2 cm 2 and Z-shaped on-axis and off-axis fields have been simulated together with reference 10x10 cm 2 beam. Phase-space data have been used to generate 3D dose distributions in a water phantom by DOSXYZnrc code, which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been obtained using the FLURZnrc code, followed by

Posters Spencer-Attix (A=10 keV) stopping-power ratios calculations by SPRRZnrc code, which have been compared to those, used in the IAEA TRS-398 Code of Practice. Detailed MC simulations of an IBA-Wellh6fer NAC 007 micro ionization chamber were carried out. The chamber has an active volume of 0.007 cm 3, and a central electrode made of copper; it was positioned at 10 cm depth in water, perpendicular to the beam axis (as recommended by the manufacturer for reference dosimetry) at SSD=100 cm. The dose to the air cavity and to water was calculated using the cylindrical CAVRZnrc code for ion chamber simulations. The dose to the air cavity (Dair) was determined in the MC simulation as the energy deposited in the air chamber volume (excluding the electrode) divided by its mass. The absorbed dose to water (Dwater) was derived as the energy deposited in a 1-mm 3 water cylinder, centred at the reference point. The electron and photon transport cut-off energy in the volume surrounding the ion chamber were 1 keV (10 keV for stopping power calculations), and 100 keV in the rest of the water phantom. Results and Conclusions: The TPR2o,10 beam quality of the SIEMENS Primus linac was estimated to be 0.765 for a 15 MV reference beam. For water/air stopping-power ratio, agreement within 0.05% has been obtained for the 15 MV reference 10x10 cm 2 field as quoted in the IAEA TRS-398 Code of Practice. For narrow square fields and IMRT on-axis beamlet, the calculated Sw,airvalues agree with the reference within :E0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). For narrow off-axis 15 MV beamlet the difference in Sw,air was 1.2%, because of the highly degraded average energy corresponding to the off-axis photon field. Following Sempau et a/ (PMB 2004), factors fc,Q to convert the absorbed dose to air into dose in water for the 15MV beams were calculated directly as Dwater/Dair. For the reference field the calculated factor was 1.09:1:0.01. This value was nearly constant within the stated uncertainty for other field sizes down to 2x2 cm 2, including the IMRT 2x2 cm 2 on-axis beamlet. Preliminary value of the correction factor calculated for 15 MV IMRT prostate carcinoma treatment verification decreases by 5% over the reference value. This difference is being investigated. 421 Evaluation of portal d o s i m e t r y for I M R T verification in a clinical e n v i r o n m e n t

A. Vinall~ A. Williams, V. Ra-Bett, G. Azangwe, J. AwotwiPratt Norfolk & Norwich University NHS Trust, Radiotherapy Physics, Norwich, UK An IMRT program has recently been established at the Norfolk & Norwich University Hospital using portal dosimetry for patient Quality Assurance. This has replaced the conventional ion chamber and film verification processes for individual patient QA thus significantly reducing the QA time required. The Department at the Norfolk & Norwich University Hospital is a Varian site with four EX linear accelerators all equipped with Millenium talcs and amorphous silicon electronic portal imaging detectors. IMRT planning is carried out using Varian Eclipse and Helios treatment planning systems. To introduce portal dosimtery initial fine tuning of the mlcs on each machine was carried out to ensure an identical dosimetric leaf separation on all machines. Each EPID was calibrated individually to give 1 Calibrated Unit (CU) per 100MU for a 10x10 field at SSD = 100cm. Output factors for different field sizes were measured on each portal imager and found to be in good agreement with each other. For each field of every IMRT patient a portal dose predicted fluence was created which was compared to the acquired portal dosimetry image taken at the linac. Each field was also delivered to a PTW 2D ion chamber array and additional measurements were made with a pinpoint ion chamber in a