Session Item

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).

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.

18 planning CT scans and 450 cone beam computed tomography scans (CBCTs) were analysed.

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.

Various directions of movement were noted caused by the position of the bladder, rectum, bowels and patient position in relation to the CTV. Despite the effect of inter-fractional motion on the position of the CTV, the majority were in the ITV. However, a third of patients CTVs were not representative from the localisation scans, found to be patient specific and this suggests the need for individualised ITV margins for those patients to optimise their treatment further.