ESTRO 2020 Abstract book

S1105 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).

The POTD with 1 cm margin was chosen most frequently: 53 resp. 44 % of fractions for long- and short- coursepatients. (Fig. 2). The POTD with -1 cm margin was never selected. For 12/15 patients, more than one plan was selected, illustrating the daily shape variations of the CTV. For the long-course group, no time-trend in POTD- selection was observed. The average PTV was 821 ± 185 cc for PTV small and 818 ± 180 cc for the POTD approach. Although patients were selected on acceptable CBCT image quality, plan selection was time consuming.

Conclusion For a significant number of rectal cancer patients, reduced PTV margins can safely be used. However, for some patients, this could lead to ventral underdosage of the UM which can either be accepted based on the low recurrences in this area, or mitigated by plan adaptation. Although time consuming, plan selection was feasible for patients with acceptable CBCT quality. In the POTD approach, target coverage increased with comparable irradiated volumes. PO‐1886 Dosimetric impact of groin oedema in inguinal lymph node boosts in locally advanced vulvar cancer S.M.I. Mohamed 1 , L. Fokdal 2 , M. Assenholt 3 , J. Kallehauge 4 , J.HR. Lindegaard 2 , K. Tanderup 5 1 NCI- Cairo University, Radiotherapy, Cairo, Egypt ; 2 Aarhus University Hospital, Oncology, Aarhus, Denmark ; 3 Aarhus University Hospital, Medical Physics, Aarhus, Denmark ; 4 Danish Centre for Particle Therapy, Danish Centre for Particle Therapy, Aarhus, Denmark ; 5 Aarhus University, Clinical Medicine./Medical Physics, Aarhus, Denmark Purpose or Objective Groin oedema during pelvic radiotherapy (EBRT) may have an impact on lymph node (LN) dose. We aimed to evaluate the effect of oedema on dose in patients with inguinal LN boost. Material and Methods Ten patients were screened and the maximum level of oedema during RT was determined. This abstract present the patient with the most inguinal oedema. EBRT was delivered with VMAT: 51.2Gy/32 fx to the elective target and 64Gy/32 fx as simultaneously integrated boost to the primary tumour and to 6 groin LNs (3 right & 3 left). Daily CBCT was acquired and rigid bony registration to the planning CT (pCT) was done. Each boosted LN (CTV) was contoured on CBCT. The body contour was delineated on pCT and on each CBCT. Oedema during EBRT(difference between the body contour of pCT and CBCT) was measured. The oedema was contoured as a separate volume (shell) and included for dose recalculation on pCT. The depth from skin to each LN was named “d”. The