Inter and intra-fractional stability of rectal gas in cervical cancer patients
Shortall, J. Vasquez Osorio, Eliana Cree, Anthea Dubec, Michael Chuter, Robert McWilliam, Alan Kirkby, Karen J Mackay, Ranald I Van Herk, Marcel
Shortall, J.
Vasquez Osorio, Eliana
Cree, Anthea
Dubec, Michael
Chuter, Robert
McWilliam, Alan
Kirkby, Karen J
Mackay, Ranald I
Van Herk, Marcel
Citations
Altmetric:
Abstract
Purpose or Objective
Due to the Electron Return Effect during Magnetic
Resonance guided Radiotherapy (MRgRT), rectal gas during
pelvic treatments can potentially result in hotspots in the
rectal wall. Determining the clinical impact of this effect
on rectal toxicity requires estimation of inter and intrafractional
stability of rectal gas. We investigate local inter and intra-fractional changes of rectal gas in cervical
cancer patients.
Material and Methods
Six cervical cancer patients underwent four scanning
sessions (~20 minutes each). Within each session, T2-w
MRIs were taken at seven time-points (TP, mostly ~3
minutes apart). The rectum on each image was delineated
by a clinical oncologist. Slices where rectal delineations
were present across all TPs in a scan session were selected
for analysis. The length of the rectum was normalised,
going from 0 (most cranial) to 1 (most caudal).
The area of gas within the rectum delineations was
identified on each slice using thresholding and recorded
(ATPX) {X = 1,…,7} (fig. 1). The (ATPX) for averaged over all
TPs was used to calculate the local inter-fractional local
group Mean (M), systematic (Σ) and random (σ) error. To
determine the intra-fractional changes, the difference in
gas area between TP2, TP3, … TP7 vs. TP1 was found on
each slice (ΔATPX) and used to calculate the local M, Σ, σ
and σf, but now for intra-fractional motion.
Results
Figure 2 shows the local statistics (M, Σ, σ (and σf)) for
inter (panel A) and intra (panel B) fractional changes
across all patients. Both inter and intra-fractional changes
are larger in the cranial aspect of the rectum, and gas
appears more stable at the caudal end. Further, interfractional
changes (~2cm2/slice) are larger than intrafractional
changes (~1cm2/slice).
The intra-fractional changes (~1cm2/slice) are small
compared with the absolute area of gas in each slice,
which is up to 10cm2/slice in the patient in figure 1. This
indicates that gas remains relatively stable during a 20
minute period.
Conclusion
We are the first to statistically evaluate stability of rectal
gas in cervical patients. Inter-fractional gas changes are
larger than intra-fractional gas changes. I.e. gas is appears more stable at the caudal end. Further, interfractional
changes (~2cm2/slice) are larger than intrafractional
changes (~1cm2/slice).
The intra-fractional changes (~1cm2/slice) are small
compared with the absolute area of gas in each slice,
which is up to 10cm2/slice in the patient in figure 1. This
indicates that gas remains relatively stable during a 20
minute period. Changes are more likely to occur at the cranial aspect of
the rectum than the caudal end. This work will feed into
assessment of the total dosimetrical effect of rectal gas
during MRgRT. However, in practical MRgRT the relevant
time period may exceed our measurement period.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Shortall J, Vasquez Osorio E, Cree A, Dubec M, Chuter R, McWilliam A, et al. PO-1651: Inter and intra-fractional stability of rectal gas in cervical cancer patients. Radiotherapy and Oncology . 2020 Nov;152:S906–7.