ESTRO 2020 Abstract book

S1088 ESTRO 2020

account for post-operative or radiotherapy-induced swelling of the breasts. Results With our new technique the PTVs were covered with at least 95% dose to 95% of PTV volume, while the mean doses to the heart and lungs remained low, see table 1. Per fraction 10 breathholds were needed (including CBCT scans) with a maximum time of 25 seconds each, which was feasible for these patients. Figure 2a and 2b show examples of dose distributions. Note that the beams connected to the left isocenter also contribute to the dose in the right breast and vice versa. Position verification with CBCT is performed in a central position between both breasts. If necessary a couch correction is performed. After this the couch is shifted to the left and right isocenter strictly according to plan. The robustness was tested by moving 1 isocenter 5 mm in different directions. PTV coverage dropped but was at least 90% of the prescription dose to 90% of PTV volume. Conclusion We developed a robust VMAT technique with two isocenters for irradiation of both breasts simultaneously, which creates treatment plans with adequate PTV coverage and low dose to the OARs . This technique can be used with or without breathhold, with or without simultaneously integrated boost and with or without regional treatment.

Conclusion A VMAT class solution for patients with esophageal carcinoma was developed with the AP module of Pinnacle. Two different beam setups are used depending on tumor location. Dose to OARs decreases (table 1). As a result of a higher CI, lower HI with similar coverage of the PTV the plan quality of the esophageal treatment plans has improved with the new VMAT-AP treatment technique. Using a VMAT technique results in giving a low dose to a larger volume of tissue. The long term effect of this is not completely clear yet. With our class solution no significant increase in lung V5 was seen. This technique has been implemented in our institute. PO‐1857 A robust two isocenter VMAT technique for bilateral breast cancer. P. Van Kollenburg 1 , M. Wendling 1 , L. Abbenhuis 1 , H. Meijer 1 , P. Westhoff 1 1 Radboudumc, oncology, Nijmegen, The Netherlands After introduction of the ESTRO guidelines for delineation of the CTV breast and lymph node regions (ref. B. Offersen et al), we concluded that our irradiation technique in patients with bilateral breast cancer could be improved in terms of PTV coverage and dose to organs at risk (OARs), while maintaining an acceptable number of breathholds. Purpose The purpose of this study was to design a technique for irradiation of both breasts simultaneously, that can be used with or without breathhold, with or without simultaneously integrated boost and with or without regional irradiation. The treatment plans should be robust, conformal and homogeneous, with adequate coverage of the target volume and optimal sparing of the OARs. Material and Methods Treatment plans were created in the Pinnacle 3 treatment planning system version 16.0.2 (Philips Healthcare, Fitchburg, WI, USA), using 8 coplanar 6 MV VMAT arcs of approximately 90° with 2 isocenters. Figure 1a and 1b show the beam configuration. Fifteen patients with bilateral breast cancer were planned and treated with this new technique (for details see table1). All patients underwent a CT scan with 3 mm slice thickness. Contoured OARs were lungs, heart (ref. M. Feng et al) and, in case of regional treatment, thyroid and esophagus. CTVs were delineated in accordance with ESTRO guidelines, a PTV margin of 7 mm was used. A virtual planning contour exterior of the breasts was created to ascertain beam aperture width sufficient to Purpose or Objective Background