ESTRO 38 Abstract book

S945 ESTRO 38

consistently higher than that with a J20 LSF (see Table 1). The average LSF yielded reconstructed leaf opening times that lay between J7 and J20 LSFs for all jaw openings.

Conclusion Thickness of the topical agent influenced surface dose, regardless of metallic composition or the base ingredient, even when applied in a very thin layer. It is necessary to consider appropriate application and timing in cases where topical agents are applied to the radiation treatment site. EP-1752 Impact of Leaf Spread Function on Fluence Reconstruction from Exit Detector Signals in TomoTherapy H.H.F. Choi 1 , T.Y. Lee 1 , Y.W. Ho 1 , W.K.R. Wong 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective The onboard exit detector of TomoTherapy has been proposed as a tool to detect errors in fluence delivery during treatment. Many fluence reconstruction methods require a deconvolution of the collected signal with a leaf spread function (LSF). Measuring the LSF for all leaves and all jaw openings will yield the most accurate results, but may be time-consuming and unnecessarily complicated. Conversely, relying on a single LSF for the deconvolution of all data may be an oversimplification. This study aims to investigate the impact of the leaf spread function as measured with different jaw widths and different multileaf collimator (MLC) opening. Material and Methods During the projections 6000-6599 in the TomoTherapy Quality Assurance (TQA) DailyQA module, blocks of eight neighbouring leaves were opened successively with a narrow jaw opening (J7, corresponding to a width of 1 cm at isocentre). The LSF was then taken to be the signals received in the exit detector channels corresponding to the fifteen unopened leaves next to the block of opened leaves. The central and edge LSFs were measured when the block of opened leaves was located at the centre and the edge of the beam, respectively. This was repeated for a similar MLC opening pattern with a wider jaw opening (J20, corresponding to a width of 2.5 cm at isocentre) in projections 7000-7599. Finally, a third “average LSF” was constructed by averaging the J7 and J20 LSFs. This was to observe the effects of jaw opening on LSF profiles. Next, the exit detector signals were collected for three standard plans (TomoPhant) supplied by the manufacturer, with the couch retracted from the bore. Fluence was then reconstructed using the J7, J20 and average LSFs for the TomoPhant plans. The deconvolution and fluence reconstructions were done with MATLAB. Results The central and edge LSFs were found to be very similar (see Figure 1). The J20 LSF exhibited a slower drop-off than that measured with J7. The leaf opening times calculated from deconvolving with a J7 LSF were

Conclusion The position of the opened leaf was not found to affect the measured LSF profile; however, different jaw openings were found to influence the fluence reconstruction. Measuring LSF for all clinically applicable jaw widths may yield a more accurate reconstruction. EP-1753 A dual detector system for in-vivo dosimetry: transit dose verification and error identification O. Brace 1 , S. Alhujali 1 , S. Deshpande 2 , P. Vial 2 , P. Metcalfe 1 , M.L.F. Lerch 1 , M. Petasecca 1 , A.B. Rosenfeld 1 1 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia ; 2 Liverpool and Macarthur Cancer Therapy Centres, Department of Medical Physics, Sydney, Australia Purpose or Objective This work tests the ability to perform real time transit dosimetry and MV imaging with a dual detector system for radiotherapy. The system’s ability to accurately measure water equivalent dose, predicted by the TPS for a heterogeneous phantom will be validated. Further the dual detector will be used for the identification of dose errors though co-registration of dose and imaging profiles. Material and Methods The dual detector comprises of a silicon diode array detector (MP) and standard a-Si EPID. The MP is between two sheets of 5 mm thick water equivalent material and mounted directly above the EPID. The dual detector was placed at 150 cm SSD and aligned beneath the LINAC couch. The lung phantom has at its centre a 2 cm diameter water equivalent sphere to represent a tumour volume, surrounded by lower density lung equivalent material. With the target positioned at isocentre an ELEKTA LINAC was used to deliver 250 MU with a 6MV beam for a 5x5 cm 2 field. Transit dose was compared to both TPS calculations and measurements taken with the EBT3 film substituted for the MP in the dual detector system. Two deliberate dose errors were also introduced. Firstly a 5.2% increase of monitor units (263 MU) was delivered. Additionally, with the nominal 250 MU delivery the target was laterally shifted 7 mm from isocentre. EPID images taken were co- registered to MP profiles through physical markers present in the EPID images. Prior to transit measurements the MP