ESTRO 2021 Abstract Book

S1342

ESTRO 2021

The absorbed dose measurements were carried out in 13 patients treated with an afterloader microSelectron v3 Digital (Elekta, The Netherlands) and a vaginal cylinder applicator to irradiate the vaginal cuff. The treatment plans were calculated using Oncentra Brachy TPS v.4.5.3 (Elekta, The Netherlands) after importing the 0.6 mm spaced axial images from a Siemens Somatom go. Open Pro CT (Siemens Healthineers, Germany). The dosimeter was operated without previous calibration for dose. The raw measurements were processed with a Matlab (MathWorks, Massachussets) script to obtain the relative absorbed dose rate and dwell times of each dwell time position and finally, a comparison with the planned values was performed. Results A total of 85 dwell positions were measured. The mean deviation between the treatment plan dwell times and the measured ones is lower than 0.1%. The percentual deviation of the total treatment time is also less than 0.1%, with a maximum deviation of 1.3%. The relative dose deviation ranges between 5.6% and 35.9% with a mean value of 11.5%. Conclusion In vivo PSD determines accurately the dwell times, and it is a quality assurance procedure that allows verification of dose delivery. It is possible to monitor the absorbed dose to a point in the bladder, but the associated uncertainties are considerable. A stronger calibration method and algorithm is under testing

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PO-1620 Quantification of Incidental Clinical Target Volume Coverage in Linear Accelerator Based Lung SABR M. Higgins 1 , S. O'Keeffe 2 , M. Quinn 3 , D. Harper 4 , P. Thirion 4 , J. Armstrong 4 1 St Luke’s Radiation Oncology Network, Radiation Oncology, Dublin, Ireland; 2 St Luke's Hospital, Medical Physics, Dublin, Ireland; 3 St Luke's Hospital, Radiation Oncology, Dublin, Ireland; 4 St. Luke's Hospital, Radiation Oncology, Dublin, Ireland Purpose or Objective In stereotactic ablative body radiotherapy (SABR) for non-small cell lung cancer (NSCLC) an expansion for CTV is not used. Despite this marginal failures are rarely reported. For linac based SABR treatment we analysed incidental dose to a clinical target volume (CTV) to see if it is adequate to control potential microscopic disease (MD). Materials and Methods We reviewed 20 early stage NSCLC (10 adenocarcinoma (adeno) and 10 squamous cell carcinoma (SCC)) SABR plans. All patients were treated with coplanar volumetric modulated arc therapy (VMAT), 4D CT planned with internal target volume (ITV) approach and 5mm PTV expansion. Prescribed doses (PD) were 54Gy/3# (n=18), 60Gy/5# (n=1), and 60Gy/8# (n=1). For adeno we created an 8mm CTV, isotropically expanded from the ITV and a 6mm CTV expansion for SCC. We assessed CTV coverage by determining if 99% of the target volume was covered with 100% or more of the dose (PTV D99 of 100%) equivalent to 50Gy in 2Gy per fraction (50Gy EQD 2 ) using an α/β ratio of 10. Coverage was assessed from the original treatment plan, without any re-optimisation. We also explored CTV coverage relating to PD and target

motion. Results For adeno the CTV 8mm was covered by a median of 100% of the 50Gy EQD 2

. SCC median coverage of CTV

6mm by 50Gy EQD 2

was 100%. However, D99% value does not describe effect of heterogenous fall off as a