ESTRO 2020 Abstract Book

S1051 ESTRO 2020

PO-1885 Margin evaluation and Plan-of-the-day exploration for rectal tumors E. Kneepkens 1 , K. Waizy 1 , E. Hagelaar 1 , J. Cnossen 1 , J. Theuws 1 , P. Van Haaren 1 1 Catharina Hospital Eindhoven, Radiotherapy, Eindhoven, The Netherlands Purpose or Objective Due to bladder and rectum filling variability, PTV margins required for rectal tumors locally reach up to several cm. However, the required margins are often not applied clinically, because the risk of recurrence after neoadjuvant treatment is relatively low and because a balance between OAR sparing and target coverage is desired. Following the introduction of daily CBCT for rectal cancer patients, we evaluated whether our current clinical margins could be reduced and whether a plan-of-the-day (POTD) approach would be feasible. Material and Methods CBCT scans of 10 patients treated with short-course RT (5 x 5 Gy, all fractions) and 5 patients with long-course RT (25 x 2 Gy, 13/25 fractions) were evaluated. Online matching of CBCT-images was performed on bony anatomy. Retrospectively, the coverage of the mesorectum and relevant lymph node regions by the PTV was evaluated by two observers (1 RTT, 1 medical physicist resident). The PTV margins varied in the ventral part of the upper mesorectum (UM, cranial to the base of the bladder), which was 2 cm for the PTV large and 1.5 cm for the PTV small . Dorsal and lateral margins were 0.7 cm, cranial and caudal margins were 1 cm. Additionally, for each fraction a selection was made out of 5 POTD contours with a ventral margin for the UM varying from -1 to +3 cm (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

Results PTV small

did not cover the CTV for on average 23% resp. 16% of the fractions for the short-course patients, and the long- course patients. There was a large variability among patients, ranging 0-60% of the fractions in both groups. For 2/5 long-course patients and 6/10 short-course patients, PTV small always covered the CTV. PTV large only improved coverage for 3/115 evaluated fractions. Whenever the CTV was not fully covered, it was at the ventral part of the UM.