ESTRO 2020 Abstract Book

S883 ESTRO 2020

Purpose or Objective Radiotherapy of left-sided breast cancer may be performed with deep inspiration breath-hold (DIBH) gating to reduce the heart dose. Surface-guided radiotherapy (SGRT) can be used for the gating, but the accuracy is subject to inter- and intrafraction variations in the chest wall position relative to the patient surface. This study provides a detailed investigation of the SGRT error budget and investigates potential improvements by combining SBRT with image-guided radiotherapy (IGRT).

for daily variations was performed during a subsequent BH, and a new reference surface was acquired and used for the SGRT gating. This procedure was needed because a new SGRT reference surface cannot be captured simultaneously with the IGRT imaging. Continuous portal MV images (cine MV) recorded at 7.7Hz during all treatments were registered to DRRs post-treatment to determine the treatment error in beam’s eye view. The MV imaging and SGRT signals were synchronized and the following five error contributions were determined in BEV for each fraction: (1) on-line MV match error, (2) inter-breath-hold shift between setup imaging and couch correction, (3) intra-breath-hold patient motion from start of couch correction to capturing of the new reference surface, (4) difference between actual and intended SGRT DIBH level during treatment, and (5) intrafraction shift of the chest wall relative to the SGRT surface. In addition to our current setup protocol (Scenario A), two alternatives were simulated after correction for error (1) above (to remove operator-dependent errors in the comparison): (B) image- guided SGRT with a daily reference surface captured after imaging (errors 2-5), and (C) image-guided SGRT with the reference surface captured at the time of imaging (errors 4-5). Results The SGRT reported error in general correlated well with the actual error in cine MV images at a fraction, but fraction specific offsets were present due to errors introduced in the setup procedure (Figure 1). Table 1 summarizes the individual error contributions and scenario accuracies. Scenario C led to the best result. All scenarios were statistically different from each other (p<0.05). Conclusion A detailed analysis of the error budget in image-guided SGRT of breast cancer was performed. Two alternative SGRT scenarios were investigated. Image guidance can improve SGRT, but for optimal accuracy the reference surface of the day should be captured simultaneously with IGRT imaging. PO-1620 Reconstruction of intra-fractional real-time prostate movement and its effect on dose distribution L. Järvinen 1 , M. Tenhunen 1 , M. Myllykangas 1 , M. Persson 2 , E. Traneus 2 , O. Sjöberg 3 , P. Ekström 3 1 Helsinki University Hospital, Cancer Center, Helsinki, Finland ; 2 Raysearch Laboratories AB, Raysearch Laboratories AB, Stockholm, Sweden ; 3 Micropos Medical AB, Micropos Medical AB, Gothenburg, Sweden Purpose or Objective Stereotactic body radiation therapy (SBRT) of prostate cancer (PCa) has recently been shown well tolerated in phase I and II trials indicating favorable normal tissue complication probability. An ongoing discussion suggests that the α/β ratio of PCa could be even lower than the α/β ratios of the surrounding organs, which supports use of hypofractionation for increased tumor control probability (TCP). As the number of treatment fractions is reduced, the risk of a TCP compromising geometric miss increases, which is the focus of the present study. Material and Methods The selected five patients with local PCa received 7 Gy twice a week in 5 fractions. Real-time monitoring of prostate motion was manifested using the RayPilot system (Micropos Medical AB, Gothenburg, Sweden), which consists of a treatment table top overlay containing an integrated receiving antenna array, and a wired transperineally implanted electromagnetic transmitter. The target was matched using a CBCT. Treatment was cancelled if radial prostate movement exceeded 3 mm from the CBCT match point (the origin). The dose reconstruction was implemented using the scripting interface of a research version of RayStation 9A treatment planning system (RaySearch Laboratories AB, Stockholm, Sweden) reading 3D-coordinates of a RayPilot

Material and Methods Ten left-sided breast cancer patients received DIBH radiotherapy with tangential fields in 15-18 fractions. An SGRT system (AlignRT, Vision RT) was used for initial patient setup and gating. A kV/MV image pair was acquired during breath-hold (BH) and matched with digitally reconstructed radiographs (DRRs) to determine the chest wall position of the day. The couch correction to account