Keywords:
Pelvis, CT, Localisation, Image guided radiotherapy
Authors:
C. Wilson, M. Chao, F. Foroudi, D. Lim Joon, D. Bolton, A. Tan, N. Lawrentshuck, H. Ho, S. Spencer; VIC/AU
DOI:
10.1594/ranzcr2018/R-0094
Methods and materials
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Partial bladder irradiation is aimed at reducing treatment volumes by delivering radiotherapy only to the bladder tumour,
sparing the remaining normal bladder.
This can been delivered either as a single phase of treatment or as a boost phase alone,
following lower dose irradiation of the entire bladder (1-4).
This technique however is limited by both reliable target delineation on CT imaging as well as organ motion and subsequent variations in tumour position (5).
Strategies have subsequently been developed to better delineate the location and extent of the tumour for more localised radiotherapy assisted by online verification of its position prior to each daily treatment.
The use of image guided radiotherapy via daily tumour localisation and verification has demonstrated the ability to significantly reduce the irradiated normal tissue volume without compromising target coverage (6). The most suitable choice of modality for image guidance and verification has thus become a topic of interest.
From September 2015 to July 2017,
four patients with muscle invasive bladder carcinoma were included in the study. All patients were treated with intensity modulated radiotherapy (IMRT) to a total dose of 64 Gy in 32 fractions,
comprising of initial partial bladder radiotherapy (or boost) to a dose of 14Gy in 7 fractions,
followed by radiotherapy to their whole bladder to 50 Gy in 25 fractions.
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Trace IT gel was injected via flexible cystoscopy into the bladder wall at the periphery of the tumour,
prior to CT simulated.
This CT dataset was used to plan the partial bladder component of the radical treatment course (14Gy in 7 fractions).
Delineation of the gross tumour volume was based on the Trace IT fiducial markers as well as the cystoscopy findings.
CBCT was used daily for online imaging and verification included standard bone matching as well as fiducial matching on CBCT.
For each of the daily CBCT scans acquired over the 7 days of partial bladder radiation,
the volume of residual Trace IT gel,
the visibility of gel in Housfield Units and the verification shift in mm was recorded.
In doing so were we able to assess the sustained size and visibility of the gel over the treatment course,
along with the extent of verification shift (mm) in comparison to that of bone matching.