ESTRO 37 Abstract book

S964

ESTRO 37

Purpose or Objective To obtain S cp

diameter, and irradiations in flatten-filter free beams or other non-uniform beams are therefore subject to volume averaging and EPR spectrometer cavity sensitivity correction. In this work, we report on a simple model that can provide volume and cavity sensitivity correction factors for improved output factor measurements in small MV photon beams. Material and Methods The x-ray beam was delivered by an Elekta Versa HD linac with an Agility MLC160 radiation head. Symmetric and square field sizes (FS) for 6 MV flattened filter free (FFF) beams were considered. The data were acquired at SSD=90 cm, depth 10 cm. The alanine pellets were the standard Harwell/NPL type (Ø4.83×2.80 mm). The pellets were placed in water with a latex sleeve to protect against water. A Bruker EMX-micro EPR spectrometer equipped with an EMX X-band high sensitivity resonator was used to read out the dose deposited in the alanine pellets. The beam profiles were measured using single detectors in a PTW MP3 water tank. A rotational symmetric Gaussian horizontal beam profile and exponential decaying depth dose profile in the vicinity of the pellet was fitted to the measured profiles. Cavity sensitivity correction functions found in literature (Anton et al. 2015) and in the technical manual of the Bruker spectrometer were used. Results The fit of the beam profile in three dimensions was based on two parameters: the variance for the Gaussian profile and the gradient of the depth profile. The parameters in turn were both changing as function of FS. Using the fitted beam profiles, an analytical model was developed for the calculation of volume and cavity sensitivity correction factors k V and k S for given FS (see Table 1). Table 1: Calculated volume correction factors k V , sensitivity correction factors k S (Anton/Bruker), temperature, volume, and sensitivity corrected output factors (OF) with SD being one standard deviation (SD) are displayed for the 6 MV FFF beam as function of the field size FS. Conclusion The cavity sensitivity correction was found to influence the alanine measurements in the range of 3 to 17 ‰ for the smallest field size depending on the cavity sensitivity correction function applied. A simple analytical expression for the correction factors as function of field size and the radius of the sensitive volume of the detector was obtained. The method presented here would be applicable for other detector geometries and beam energies. The cavity sensitivity correction was found to be small. The differences in the cavity sensitivity corrections functions (Anton/Bruker) might arise from the method used in detection of the cavity correction function and the applied spectrometer. EP-1795 Scp measurement of a 5 mm diameter cone using a scanning chamber method. A. Prado 1 , R. Díaz 2 , E. Cabello 2 1 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain 2 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain

values for a 5 mm diameter cone using a scanning chamber method (SCM) utilizing different number of scanning positions. Material and Methods Measurements were performed on a 6MV Varian Unique to which a 5 mm diameter cone was attached. A PTW UNIDOS electrometer along with a Pin Point 31014 ion chamber were employed. A Blue Phantom 2 (Iba Dosimetry) water tank of 50 x 50 x 50 cm 3 was utilized. A stereotactic diode (Iba Dosimetry) was used to obtain the 5 mm cone profile, essential to perform this methodology. The SCM is based on the superposition principle, according to which a beam can be expressed as a sum of smaller beams. This multiple beam configuration along with a static detector position can be shown to be equivalent to a configuration in which the beam is still and the detector changes its position. A grid of measurement points centered in the cone field was created, varying the number of points in the grid and the distances between them. Hence, the total dose from the multiple beam field is expressed as: axis beam dose, D(r,d max ) is the central axis dose and r is the cone field size. Measurements were performed using different beam number (NB) and distinct neighboring beam distances (NBD). The sum of all field contributions is D tot . The f(x i ) factor for each detector position is the value of the measured profile normalized to unity at distance x i of its maximum. D(r,d max ) was computed for each case through equation (1). S cp is defined as the ratio of D(r,d max ) to D(10,d max ), which was measured with the same Pin Point ion chamber. In addition, an S cp value was obtained measuring D(r,d max ) and D(10,d max ) directly and calculating the ratio between them so as to compare it with the SCM value. Results Table 1 shows S cp values obtained for each configuration. Table 1: Results obtained for different configurations of number of beams and neighboring distance. S cp results are consistent from 441 to 9 beams (discrepancies with respect to the average value lower than 1%). For 5 beam configuration this difference is greater (6.3%). This suggests that a further reduction on the number of beams could yield a higher value and, in consequence, a worst result for the S cp . The average value compared with the direct measurement value (0.509) entails a 22% increase. Conclusion The SCM diminishes the influence of volume averaging effect on the final S cp result. This can be seen in the 22% increase achieved with respect to direct measurement value. Obtained results agree quite well with data published in the literature, so they are considered as satisfactory. Although the SCM could be a bit tedious to perform, it clearly provides an excellent result for the S cp value of such a narrow beam. where N is the number of beams, f(x i ) is the ratio of the dose at distance x i from the beam center to the central