ESTRO 38 Abstract book

S305 ESTRO 38

respectively.

Material and Methods The settings of standard and image gently protocols of kV CBCT was listed in table 1 (OBI V.2.5). The kV imaging dose to the CT dose index (CTDI) phantom was measured using a calibrated PTW CT chamber. After measurement-based validation, the Halcyon MV CBCT imaging dose (V.1.0) to the corresponding volumes were calculated on Eclipse Treatment Planning System (V.15.1), since Halcyon uses identical 6 MV flattening-filter-free photons for both imaging and treatment, enabling the automated incorporation of the two doses. The 'High Quality' and 'Low Dose' modes uses dose rates of 45 MU/min and 27 MU/min, delivering 10 MU and 5 MU per scan respectively. The weighted CTDI (CTDIw) were computed for both systems. Using various modalities and protocols, the accuracy of correcting a known couch shift (5 mm on three directions) for the head, thorax and pelvis regions of CIRS 1-year, 5- year and RANDO adult anthropomorphic phantoms were compared.

Conclusion ‘Image Gently’ kV CBCT is inadequate for adult pelvis, but is applicable to other anatomies achieving comparable positioning accuracy as standard kV CBCT. Similar registration results were achievable using MV CBCT, but the ‘Low Dose’ mode is most cost-effective for all phantoms than the ‘High Quality’ protocol.

Symposium: Recent insights into adverse cardiac effects from multimodal radiation therapy

Results The CTDIw for the standard ‘Head’, ‘Thorax’, ‘Pelvis’ and ‘Image Gently’ (measured on both CTDI head and CTDI body phantoms respectively) protocols of kV CBCT were 0.45 cGy, 0.54 cGy, 1.93 cGy, 0.11 cGy, and 0.05 cGy respectively. The computed CTDIw for MV CBCT in the CTDI head and body phantoms were 8.45 cGy and 6.38 cGy (imaging length=28 cm, the maximum field of Halcyon), 6.88 cGy and 5.55 cGy (imaging length=16 cm, equivalent to that of Edge kV CBCT) respectively, using the ‘High Quality’ protocol. The exposure of ‘Low Dose’ MV CBCT was approximately halved. Figure 1 displays the deviations of image registrations from the actual couch shifts [mm] on the vertical, longitudinal, lateral directions and the root-mean-square (RMS), as guided by Edge kV CBCT and Halcyon MV CBCT. Only the positioning error of adult pelvis guided by ‘Image Gently’ kV CBCT (1.57 mm) exceeded our clinical tolerance of 1.00 mm. All other registrations were satisfactory: the maximum deviations for the 1-year, 5-year and adult phantoms were 0.22 mm, 0.25 mm, 0.58 mm (standard kVCBCT); 0.27 mm, 0.42 mm, 0.54 mm (‘Image Gently’ kV CBCT excluding adult pelvis); 0.39 mm, 0.37 mm, 0.42 mm (‘High Quality’ MV CBCT); and 0.44 mm, 0.36 mm, 0.52 mm (‘Low Dose’ MV CBCT)

SP-0582 Prediction models for adverse cardiac effects to target optimal cardiac radiation dose distributions in breast cancer patients A. Crijns 1 1 UMC Groningen, Radiotherapy, Groningen, The Netherlands Abstract text Incidental cardiac radiation in breast cancer radiotherapy results in an increased risk of major cardiac events. Darby et al. found that the lifetime excess risk of an acute coronary event [ACE] increases linearly by 7.4% per Gray of the mean heart dose. However, we have validated that the first 9 years following radiotherapy the excess risk of an ACE even increases by 16% per Gray of the MHD. In the Netherlands, the model-based approach is applied to select breast cancer patients for proton therapy. To qualify for proton therapy, breast cancer patients should have a ≥2% decrease in the absolute lifetime risk of an ACE after planning comparison between photon and proton therapy. To calculate the absolute lifetime risk of an ACE, the validated Darby model is used, with the difference that the observed baseline risk of an ACE is based on the numbers of ACEs in females and males in the Dutch population. However, for optimization of cardiac photon dose distributions and better selection of patients for proton therapy validated prediction models are essential, describing the relationship between radiation dose to cardiac substructures and MCEs. Furthermore, knowledge about early subclinical cardiovascular effects (ESCEs) induced by radiotherapy that eventually develop into MCEs is also needed for development of primary and secondary preventive strategies. Currently these issues are addressed in the European MEDIRAD-BRACE and -EARLY