ESTRO 36 Abstract Book

S773 ESTRO 36 _______________________________________________________________________________________________

rectum/bladder, no relevant discrepancies were detected in SBRT patient. The results are supplied in quasi real- time, with IVD tests performed and displayed after only 1 minute from the end of arc delivery. Figure 1 shows the SOFTDISO user interface.

The considerable Δ meas, calc values could arise from inaccurate beam modelling in the build-up region; with the modelling less accurate for MLC fields. For MLC fields,Δ meas, calc increased with increasing SSD, perhaps due to an underestimation of the scatter contribution from MLC fields at extended SSD. The jaw fields gave rise to a greater dose than the MLC fields. The calculation underestimated Δ jaw, MLC with a build-up screen, while it overestimated it without a build- up screen. Doses were greater with than without a build-up screen in all the scenarios investigated. Δ b,no b was not considerably different between the calculation and the measurement. Δ b,no b was greater for 18X than for 6X .

Conclusion The present EPID-based IVD algorithm provided a fast and accurate procedure for SBRT-VMAT delivery verification in clinical routine, with results obtained 1 minute after each arc delivery. This strategy allows physics and medical staff to promptly act in case of major deviations of dose delivery. EP-1449 The effect of a build-up screen on superficial dose in total body irradiation L.S. Fog 1 1 Rigshospitalet, The Clinic of Oncology, Copenhagen, Denmark Purpose or Objective In total body irradiation (TBI), a build-up screen is typically positioned between the linac and the patient to reduce the build-up effect in the patient skin. With the implementation of step and shoot TBI (SS TBI), the dose conformity is considerably improved compared with TBI delivered with open fields. Thus, the delivery of an accurate skin dose becomes pertinent. We measured and calculated skin dose for a range of TBI conditions. Material and Methods The dose was measured in a 20 cm thick solid water phantom using a NACP parallel plate chamber, and a PTW Unidos electrometer. The energy response from the chamber contributed no more than 5% to the measurement uncertainty (Phys Med Biol. 2001 Aug;46(8):2107-17). The dose was measured at a depth of 1 mm in the phantom; for a range of SSDs (340-440 cm); for 6 and 18MV; and for open jaw fields (used in conventional TBI treatments) and MLC defined fields (used in SS IMRT); and with and without a lucite build-up screen of 16 mm thickness, placed 20 cm from the phantom. The MLC fields were created with a 3 cm distance from the phantom edge to the field edge when projected to the isocentre. The chamber was calibrated by measurements under standard reference conditions. The doses were calculated using Eclipse™ (Varian Medical Systems, Palo Alto), AAA algorithm, v.13.6, with a 1 mm calculation grid. The difference between measured and calculated doses Δ meas, calc , between jaw and MLC fields Δ jaw, MLC , and with and without build-up screenΔ b,no b were determined. Results For jaw fields, Δ meas, calc is reduced from 0-22% to 0-9% when using a build-up screen (table 1). For MLC fields, Δ meas, calc increases from 10-31% to 4-48% when using a build-up screen. With the exception of the scenario with jaws and a build- up screen, Δ meas, calc increased with SSD.

Conclusion The presence of a build-up screen increases superficial phantom dose. However, differences of up to 48% exist between calculated and measured doses at the phantom surface. These differences generally increase with SSD and depend on beam energy and field type (jaw vs MLC) in a complex way. The modelling of scatter from MLC fields at large SSDs appears to be a particular challenge. EP-1450 Implementation of dosimetry equipment and phantoms in clinical practice of light ion beam therapy. L. Grevillot 1 , J. Osorio 1 , V. Letellier 1 , R. Dreindl 1 , A. Elia 2 , H. Fuchs 3 , A. Carlino 4 , S. Vatnitsky 1 , H. Palmans 5 , M. Stock 1 1 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria 2 EBG MedAustron GmbH / University of Lyon France, Medical Physics, Wiener Neustadt, Austria 3 EBG MedAustron GmbH / Medical University of Vienna, Medical Physics, Wiener Neustadt, Austria 4 EBG MedAustron GmbH / University of Palermo Italy, Medical Physics, Wiener Neustadt, Austria 5 EBG MedAustron GmbH / National Physical Laboratory UK, Medical Physics, Wiener Neustadt, Austria Purpose or Objective QA equipment (water phantoms, films, ionization chambers, anthropomorphic phantoms, etc.) is generally delivered and accepted based on certificates provided by the manufacturer and only minimum testing is performed. At MedAustron, advanced acceptance testing procedures of the QA equipment were additionally developed and implemented by medical physicists. QA equipment passing