ESTRO 38 Abstract book

S1157 ESTRO 38

treatment. In the remaining patients the dosimetric benefit from DIBH was not considered sufficient to opt for this technique or (only 1 patient) they were not able to perform DIBH at the treatment sessions. Mean DIBH inspiration threshold was 1.7 l (range 1.0‐ 2.7 l). The results are reported in table 1. The percentage of plans with MHD inferior to the threshold of 5 Gy was 84.3 % for the DIBH technique, and 54.1 % for the FB plans. MHD DIBH /MHD FB and V25V DIBH /V25V FB distributions were significantly different (p < 0.001, paired T‐test).

resolution across the central area of the array (15 x 15 cm 2 ) is 5 mm. Dose linearity was characterised between 1 MU and 500 MU under reference conditions ( 10 x 10 cm 2 field) at maximum dose‐rate for all clinical MV photon energies (i.e. 6MV and 10MV in addition to flattening filter free 6FFF and 10FFF). Dose‐rate response was characterised for all available dose‐ rates. The field size dependence of the array was assessed for all energies for square field sizes ranging from 1 x 1 cm 2 through to 25 x 25 cm 2 . Dose linearity and dose‐ rate response measurements were compared to that of a Farmer chamber whilst the field size dependence was compared to that of an IBA CC13 small volume ionisation chamber. PS‐QA plan delivery time was compared to the SunNuclear ArcCheck solution. Deliberate errors (1.8%) were introduced by beam steering into the radial and transvers angles of the Varian linac to check if the array could detect an error. Results The dolphin exhibited a linear response to dose. The dose‐rate response of the Dolphin was comparable to that of the Farmer chamber (<1%) with the most significant dose rate dependence observed for the 6 MV delivery (0.87 ± 0.2%; mean ±2SD). Both the Dolphin and the CC13 exhibited field‐size dependence below 5 x 5 cm 2 (i.e. a difference of 3% from the response at 10 x 10 cm 2 ). The maximum difference between the response of the Dolphin and the CC13 was observed at 1 x 1 cm 2 for the 6FFF delivery (i.e. CC13 under responded by 21% in comparison to the 10 x 10 cm 2 field whilst the dolphin under responded by 43%). PS‐QA time was reduced from 15 minutes on average for ArcCheck to 5 minutes on average for Dolphin. The Dolphin identified flatness and asymmetry in the beam for a 6 MV and 6 FFF delivery. Conclusion The characteristics of this array make this detector suitable for PS‐QA and machine QA, with the array demonstrating significant time savings. EP‐2095 SBRT of prostate with integrated boost of Dominant Lesion. A crowd‐knowledge based planning study A. Savini 1 , S. Cilla 1 , M. Esposito 1 , E. Moretti 1 , E. Villaggi 1 , S. Russo 1 , M. Stasi 1 , P. Mancosu 1 1 SBRT Working Group, AIFM - Italian Association of Medical Physics, Italy, Italy Purpose or Objective Stereotactic Body Radiation Therapy (SBRT) with large dose per fraction has gained increasing popularity in treatment of prostate cancer due to its low α/β ratio (e.g. 1.5 Gy). Furthermore, a dose escalation to the dominant intra‐prostatic lesion (DIL) has the potential to increase biochemical control whilst avoiding the enhance in side effects associated with whole gland dose escalation. There is a lack of consensus on optimization strategies and appropriate plan quality metrics for SBRT plans delivered in simultaneous integrated boost (SIB). The aim of this study was to evaluate and ultimately reduce the variability of planning strategies. A crowd‐knowledge approach was adopted, where teacher/student hierarchy is not defined a‐priori, but hierarchy is dynamically determined by the results. Material and Methods Four radiation oncology centers experienced in performing SBRT participated to this study. The case with pre‐ delineated volumes was distributed among participating centers using an ad‐hoc web‐platform. Two plans were optimized. A preliminary homogeneous plan with a prescribed dose to the target of 35 Gy in 5 fractions. A second plan with prescribed dose to the target of 35 Gy in 5 fractions and 50Gy integrated boost to the DIL. Constraints on target coverage and dose to nearby organs at risk were set a priori. Plans were collected in DICOM‐RT format. A grouped analysis was performed using crowd‐

Conclusion The use of DIBH in a routine clinical setting significantly reduced mean dose to the heart and the volume of the left ventricle that received 25 Gy, thus confirming that DIBH is an efficient technique that can be strongly recommended to lower the risk of ischemic heart disease after breast cancer radiotherapy. EP‐2094 Machine QA Time Efficiency Savings with IBA Dolphin Detector S. Loughlin 1 , R. Lally 2 , A. Reilly 2 1 NWCC, Department of Physics, derry, United Kingdom ; 2 NWCC, Radiotherapy Physics, derry, United Kingdom Purpose or Objective Machine availability is a precious commodity in a radiotherapy department when the burden of measurement based PS‐QA and machine QA are considered, therefore vital that all links in the PS‐QA and machine QA chains are streamline and accurate. Within our portfolio of dosimetry equipment, it was identified that further savings in time could be made by using the IBA Dolphin detector for measurement based PS‐QA and machine QA. Material and Methods Investigations were carried out on Varian TrueBeam linear accelerators (linacs) using the Dolphin detector and an appropriate chamber traceable to the National Physical Laboratory (NPL) in the UK. The Dolphin is a 2D detector array consisting of 1513 vented air ionisation chambers arranged across an area of 40 x 40 cm 2 . The spatial