ESTRO 2020 Abstract Book

S1052 ESTRO 2020

geographical shift of the LNs from pCT to CBCT (center of mass) was measured. Nine CBCTs evenly distributed across the treatment (including 1st and last fraction) were analysed. Dose recalculation was done on pCT taking volume of oedema and the location of the 6 LNs on each CBCT into account. Fractional “delivered” CTV D98 was extracted for each LN on each CBCT (in total 6x9=54 recalculations). Total “delivered” dose was estimated by averaging D98 across all 9 CBCTs. Planned dose was compared to “delivered” dose from two plans with 10mm and 5mm PTV margin, respectively. Results The registered level of oedema during EBRT was 5mm (1 - 13mm) across all 10 patients screened. The patient with maximum bilateral inguinal oedema during EBRT had a swelling of 5 mm (2-13mm). Table 1 shows swelling, LN depth, and CTV D98 for each LN and for the 10mm and 5mm PTV margin plans.Total median (range) delivered CTV D98 in % across the 6 LNs was 98(94- 99) and 97(96-98) for 10mm and 5mm PTV plans respectively. No statistical difference was found between 10mm and 5mm PTV plans (p=0.1).The difference between planned and delivered dose in single fractions was <5% when the LN depth was >2mm and the LN shift was <9mm. In LNs located at depth ≤2mm, the dose decreased up to 6%. In CBCTs with LN shifts ≥9mm (5mm PTV plans) and ≥14mm (10mm PTV plans) the dose decreased up to 12% and 9%, respectively (Fig 1).

Conclusion Total delivered dose was acceptable for all LNs with 5mm and 10mm PTV plans even for the patient with considerable swelling (up to 13mm). For swelling <10mm, the fractional dose difference was <5% for LNs which were not located in the dose build up region (d≤2mm). In fractions with considerable LN displacement, the dose could drop by up to 12%. Therefore, early and persistent swelling could potentially have relevant dosimetric impact. Monitoring by RTTs of swelling on daily CBCT could be relevant. Furthermore, our results motivates for testing if margin reduction from 10mm to 5mm PTV margin of inguinal LN boosts could be possible in vulvar RT. PO-1887 Reproducibility of Geometric QA in MR-RT C. Krog 1 , G. Grimnisdottir 1 , H.D. Nissen 1 1 Vejle Hospital, Department of Medical Physics, Vejle, Denmark Purpose or Objective We are planning to introduce MR-only workflow for pelvic cancers based on Philips’ MRCAT. Geometric fidelity is of utmost importance when moving from CT-based delineation to MR-only delineation. Here, we investigate the geometric performance of our scanner and variations related to daily set-up using a vendor-supplied phantom. Material and Methods Using a Geometric QA Phantom on an Ingenia 1.5 T MR-RT- scanner (both: Philips Healthcare, Best, The Netherlands), a series of QA measurements were made over six weeks. Three times a week, three sets of measurements were taken. The measurements were performed on Tuesday mornings and Thursday mornings and afternoons. First, the RTT positioned the phantom on the MR-RT couch top over marker H1, further guided by two strips of tape on the couch top. The Ingenia system’s lasers were used to place the phantom at the scanner’s isocentre. Philips’ geometric fidelity QA scan sequence was run, yielding data from the isocentre, ± 6 cm, ± 13 cm, and ± 20 cm distance from the isocentre. Second, the RTT repositioned the phantom using the Ingenia system’s lasers and repeated the geometric fidelity scan. Third, the RTT removed the phantom rack from the couch top, removed the geometric phantom from the phantom rack, and then set it up again as before to produce the final set of measurements. Data were analysed with Philips' automatic geometric QA algorithm and in-house python scripts.