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S458
ESTRO 36
_______________________________________________________________________________________________
Purpose or Objective
Intestinal toxicity (IT) may affect the quality of life of
patients (pts) treated with whole-pelvis intensity-
modulated radiotherapy (WPIMRT) for prostate cancer.
The aim of this investigation is to identify quantitative
bowel dose-volume relationships for acute patient-
reported IT.
Material and Methods
A cohort of pts was enrolled in 6 Institutions within a
registered prospective trial. Pts were treated with
conventional or moderate hypo-fractionation to
prostate/prostatic bed and WPIMRT delivering 51.8 Gy
(median dose, range: 50.4-56.1 Gy) to pelvic nodes while
sparing the bowel outside PTV as much as possible. Acute
IT was evaluated by the Inflammatory Bowel Disease
Questionnaire pertaining to the Bowel Domain (IBDQ-B)
filled in by pts at baseline and at mid-point/end of RT.
IBDQ-B includes 10 items (#item) scored on a 7-point scale
(worst symptoms=lower scores).
The 25
th
percentiles of the most severe worsening (Δ)
between baseline and half/end RT were set as end-points.
The correlation between end-points and bowel loops cc/%
DVH/DSH (from V5Gy to V60Gy) as well as selected clinical
parameters was investigated through multi-variable
logistic regression. Goodness of fit was estimated by the
Hosmer Lemeshow test (HL) and the Brier score (BS);
performances of the model were assessed by the
calibration plot. Internal validation was performed by
1000 bootstrap resampling.
Results
Data of 206 pts (80 radical, 79 adjuvant, 47 salvage RT)
were available: 25/109/72 pts were treated with fixed-
fields, rotational and Tomotherapy technique
respectively. A relatively small but significant Δ (p<0.05)
was found for all questions: the median Δ was 2 points for
#1 (bowel movements) and 1 point for #5 (loose stools),
#17 (gas passage) and #24 (urgency to defecate with
empty intestine); Δ was 0 for the remaining 6 items that
were then disregarded in current analysis. No DVH/DSH
parameters were correlated with Δ, except for ΔIBDQ5≤-3
(25
th
percentile, 43/191).
The resulting model after backward selection of variables
(R
2
=0.89, slope:1.037, optimism corrected BS=0.17, Figure
1) included absolute V42Gy and age (protective). Due to
the correlation between DVH variables, three values
representing ‘low’ (V20), ‘intermediate’ (V30) and ‘high’
(V42) dose levels were also considered to define an overall
'DVH-shape” predictor.
When grouping pts according to best cut-off values
assessed by ROC curves (high risk: V20>470cc, V30>245cc,
V42>110cc; low risk: the other pts, figure 2), an
alternative model including high-risk DVH-shape (OR:9.3)
and age (protective, OR:0.94) may be suggested. The
model showed very good calibration (slope:1.003, R
2
=0.92)
and accurate prediction even after bootstrap-based
internal validation (corrected BS=0.16).
Conclusion
The DVH shape of bowel loops is highly correlated with the
risk of acute patient-reported loose stools due to WPIMRT
for prostate cancer. Older age was found to be a
protective factor.
PO-0847 The dose-response curve of post-treatment
FDG-uptake in lung tissue of irradiated NSCLC patients
M. La Fontaine
1
, W.V. Vogel
1
, G. Persson
2
, G. Westman
2
,
B. Reymen
3
, D. De Ruysscher
3
, J.S. Belderbos
1
, J.J.
Sonke
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Department of Radiation Oncology,
Amsterdam, The Netherlands
2
Copenhagen University Hospital- Rigshospitalet,
Department of Radiation Oncology, Copenhagen,
Denmark
3
Maastricht University Medical Centre, Department of
Radiation Oncology, Maastricht, The Netherlands
Purpose or Objective
In NSCLC patients undergoing chemoradiotherapy,
pneumonitis is a common occurrence. While its exact
relationship with radiation dose is unknown, there is
evidence that increasing lung dose leads to increased
levels of pneumonitis. Using inflammation on post-
treatment FDG PET scans as a measure of damage to
normal lung tissue, the aim of this study was to investigate
the relationship between pneumonitis and planned dose in
a radiation dose-escalation trial.