ESTRO 2022 - Abstract Book

S581

Abstract book

ESTRO 2022

Conclusion Although the dose to PCM was significantly reduced when applying dose constraints during the optimization process, this was not translated into a significant reduction in NTCP. The patient material was however limited and we recommend further analysis in a larger patient cohort where outcome data on swallowing dysfunctions are included.

MO-0646 Do planning scans represent inter-fractional planning margins in adaptive cervix EBRT?

A. Krishnan 1 , R. Butt 2 , D. Megias 1

1 Radiotherapy, Mount Vernon Cancer Centre, East & North Hertfordshire NHS Trust, Northwood, United Kingdom; 2 Radiotherapy , Mount Vernon Cancer Centre, East & North Hertfordshire NHS Trust, Northwood, United Kingdom Purpose or Objective To evaluate whether the CTVs (clinical target volumes) derived from the empty and full bladder CT (computed tomography) and MRI (magnetic resonance imaging) localisation planning scans account for the inter-fractional CTV position during EBRT (external beam radiotherapy). Materials and Methods A retrospective audit was carried out on 18 adaptive cervix radiotherapy patients, both movers and non-movers, in a single centre cohort study. All patients had two CT planning scans (empty and full bladder) with an MRI and received 45Gy in 25 treatment fractions using daily online image guided radiation therapy (IGRT). Non-movers were defined as no significant change in position of the uterus between the empty and full bladder CT planning scans. Movers were defined as significant changes to uterus position between the empty and full bladder CT planning scans. The direction of CTV movement was noted (neutral, anterior/ posterior, superior/inferior/lateral) and whether the CTV was within the ITV (internal target volume). Of those CTVs not found to be within ITVs the pattern of direction was noted. A Fisher’s exact test was used to analyse any association between the movers and non-movers and whether the CTV was within the ITV. The majority of CTVs were within the ITV for non-movers and movers at 82% and 69.5% respectively. The association between the mover and non-movers and the CTV being inside and outside the ITV was found to be statistically significant (The two-tailed P value < 0.01). 6/18 patients who’s CTVs were outside the ITV for more than 10 fractions the inter-fractional positions of the CTVs were noted to be patient specific. A variety of CTV movements were observed in all directions. The patterns of direction in those CTVs not within the ITV were patient specific and had a systematic trend in a consistent translational direction apart from 2 patients whose CTVs directions varied. Results 18 planning CT scans and 450 cone beam computed tomography scans (CBCTs) were analysed.