ABSTRACT
Internal target volume (ITV) margins were estimated by evaluating the movement of
mesorectum and bladder during neoadjuvant long-course radiation therapy (RT) for rectal
cancer. In this prospective study, 23 patients with rectal cancer had planning CT
(pCT) and weekly cone beam CT (CBCT) in supine position during preoperative long-course
RT. Mesorectal wall motion was analyzed based on the coordinates of the most anterior,
posterior, left and right points on the pCT and CBCT. Overlap volume (OV) between
the pCT bladder and CBCT mesorectum was generated. Variables that might affect relative
bladder volume (ratio of CBCT to pCT bladder volumes), anterior mesorectal wall position,
and OV were studied. ITV margins were also calculated. In females, smaller OV and
less movement of the upper anterior mesorectal wall were identified, suggesting smaller
ITV margins might be required compared to males. The relative bladder volume did not
change significantly over time and was correlated with OV: the larger the relative
bladder volume, the less the OV. ITV margin of 0.8 to 1.1 cm in right-left direction
is satisfactory. Posteriorly, only 8 to 9 mm margin is required for upper and mid
rectal regions but double of this is required for inferior third. Anteriorly, 1.3
cm margin is adequate for lower and mid rectal regions and 2.4 cm is required superiorly.
An anisotropic ITV expansion of clinical target volume (CTV) for rectal cancer radiotherapy
contouring provides a robust method to encompass the deformation of bladder and mesorectum.
The ITV margin should take into account sex and distance from the anal verge.
Keywords
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Article info
Publication history
Published online: April 15, 2022
Accepted:
March 18,
2022
Received in revised form:
February 27,
2022
Received:
September 28,
2021
Identification
Copyright
Crown Copyright © 2022 Published by Elsevier Inc. on behalf of American Association of Medical Dosimetrists. All rights reserved.
