Abstract Book

S944

ESTRO 37

Lateral isotropy Charge was measured around the source, symmetrically, in for points: at 0º, 90º, 180º and 270º. This was repeated for every applicator, and without applicator. Results Depth dose distribution Maximum difference between measurements and data from vendor was 5% from 4 to 20 mm. In 3 mm we had a 7% difference. Transference function Difference between measurements and data from vendor was within 4% in every case. Lateral isotropy Compared with each other, differences in measurements around the source were within 5%. Conclusion Depth dose distribution According to the vendor, measured depth dose at 20 mm must be within 5.3% compared to the data supplied by the vendor. Thus, we had a good agreement from 4 to 20 mm. Next to the source (3 mm) a higher difference was found; this is justified because of the steep dose gradient. Transference function Transference function must be within 10% (calculated from the uncertainty of the depth dose distribution) so measured data is in good agreement. Lateral isotropy According to the vendor, lateral anisotropy can reach a difference of 5%. Measurements are in good agreement too. EP-1761 Dosimetric impact of the Double Shell Positioning System for cranial stereotactic radiotherapy A. Erogluer 1 , A. Bel 1 , J. Sijbrands 1 , N. Van Wieringen 1 1 Academic Medical Center, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective To limit intra-fraction movement in intracranial stereotactic radiotherapy mask type immobilization devices are used to fixate the patient during treatment. The Double Shell Positioning System (DSPS), is a new type mask from Macromedics (The Netherlands). The DSPS, see figure 1, contains a top and a bottom thermoplastic part which is held together with quick fasteners (QF). Fixation to the table is realized by a carbon fibre (CF) cradle, which contains 6 supporting columns and is locked in place on the table. Both the QF and the columns could affect the dose distribution within the patient. The purpose of this study is firstly to determine the dosimetric impact of the DSPS for a variety of plans; secondly, to evaluate if the treatment planning system models the dose attenuation by DSPS accurately. Material and Methods A polystyrene slab phantom was created in the shape of a head. In one of these slabs, holes were drilled to fit a diamond detector (PTW micro diamond type 60019). The thermoplastic top and bottom parts of the DSPS are shaped around the head phantom. CT scans are made of the phantom with and without the DSPS mask. Three sets of plans were created (details in table 1): 1) Simple rectangular static fields perpendicular through the QF or column; 2) Simple 360° arcs without modulation, each arc partly passing through the QF or column; 3) Patient plans of 4 clinical cranial stereotactic cases with the isocenter and table rotation adapted in such a way that all arcs go through a QF or column, an adjustment to mimic a worst case situation. The dose was calculated using Oncentra Treatment planning (v4.5.2) for all plans on both CT scans, i.e. with and without DSPS. Measurements were performed with and without the DSPS. The attenuation by the DSPS is determined for both

planning and measurement by taking the ratio of the dose for the situation with and without DSPS. Results Table 1 shows the results for the three plan sets. The largest measured attenuation is caused by the QF, 7.3% and 5.6% at 6 and 10 MV respectively. For simple rectangular fields through the columns, the attenuation is <1.0%. For 360 ⁰ ARCs without modulation, the measured attenuation is < 1.3%. The attenuation for the clinical plans varies between 0.6 and 2.9%. The planned attenuation agrees well with the measurement. The difference is < 1%. Conclusion The QF causes large dose attenuation, but the overall attenuation of the DSPS mask is limited for clinical situations. For clinical stereotactic plans, a maximum attenuation of 2.9% was measured. The planned attenuation is in accordance with our measurements with differences in plan attenuation < 1% for all clinical cases.

EP-1762 Pre-treatment quality assurance for flattening filter free dynamic arcs : a detector comparison L. Parent 1 , M. Goubert 1 , F. Husson 2 1 Institut Universitaire du Cancer Toulouse - Oncopole, Ingénierie et physique médicale, Toulouse, France 2 Dosisoft, Scientific R&D, Cachan, France Purpose or Objective Pre-treatment quality assurance (QA) with flattening filter free (FFF) beams is challenging because of the high dose rate and the required spatial resolution as small fields are usually used. Our current clinical practice is to