Abstract
Dose-wall histograms (DWHs) have been used as alternatives to dose-volume histograms
(DVHs) for hollow organs, with the rationale that the dose delivered to the interior
of a hollow organ would be unrelated to the level of radiation damage. The purpose
of this study is to conduct a statistical comparison of dose statistics for both walled
and solid structure contours for both bladder and rectum in the treatment of intermediate
risk prostate cancer with volumetric arc therapy (VMAT). Ten intermediate risk prostate
cases were randomly selected. Rectum and bladder were first contoured as solid structures,
and then the corresponding wall structures were generated using either a slice-by-slice
cropping (2D method), or with a full 3D cropping tool (3D method). Each case was then
inverse planned using a 2-arc VMAT technique. Two plans per case were created, 1 with
a hypofractionated treatment and 1 with a standard fractionated treatment. DVHs were
calculated for solid structure contours, and DWHs were calculated for the walled structure
contours generated using 2D and 3D contouring tools. A nonparametric Spearman statistic
correlation test was used to compare a large number of relevant dose histogram points,
and to establish the relationship between dose statistics for walled and solid structures.
Several notable relationships were observed. Maximum rectal dose was strongly correlated
between the solid structure and both the 2D-generated (Spearman's correlation rs = 0.988, p < 0.01) and 3D-generated (rs = 0.952 p< 0.01) wall structures. This indicates that the rectal hot spot occurred in or near
the wall for all cases, suggesting that both structure types give similar maximum
dose information for rectum. Maximum bladder dose was not significantly correlated
between solid structures and the 2D (rs = 0.596, p= 0.069) and 3D-generated (rs = 0.681, p= 0.03) counterparts. This suggests that the maximum dose is not consistently in or
near the bladder wall. This favors the use of bladder wall contours when considering
bladder toxicity, with the maximum dose to the wall potentially being more relevant
radiobiologically. This analysis was extended to many other relevant points on the
rectum and bladder histogram curves. Where correlations are strong, equations of best-fit
are presented. This work establishes several statistically-significant relationships
between bladder and rectum DVHs and DWHs for VMAT irradiation of intermediate-risk
prostate cancer. This information may be used to inform contouring requirements for
clinical trial design as well as for standard patient care.
Keywords
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Article info
Publication history
Published online: September 24, 2019
Accepted:
August 18,
2019
Received in revised form:
August 15,
2019
Received:
February 22,
2019
Identification
Copyright
© 2019 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.
