469 / 1082
S454
ESTRO 36 2017
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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.
Material and Methods
42 patients with inoperable, stage II-III NSCLC were
treated with (chemo)radiotherapy as part of the
(NCT01024829) PET-boost trial. Patients received
escalated doses (≥ 72 Gy) in 24 fractions consisting of
either a homogenous boost to the PTV, or an
inhomogeneous boost to FDG avid (≥50% SUV
max
) areas.
Patients whom could not be boosted received 66 Gy in 24
fractions. All patients received an FDG PET/CT scan 3
months post-treatment, which was registered to the
planning CT. The lung contours minus the GTV were
compared between the SUV on FDG PET and the planning
CT dose. The planning CT dose was adjusted to equivalent
doses in 2 Gy fractions, assuming an α/β = 3 Gy, and then
binned per 5 Gy increments. The SUV was averaged over
all patients per dose bin and a dose response sigmoid was
fit (SUV vs Gy ) to determine upper and lower asymptotes,
as well as the EC50. The linear portion of the sigmoid fit
(17.6%-82.4% asymptotes difference) was applied to
individual patients for linear fitting, yielding the
correlation coefficient and the SUV response to an
increase in dose (slope). All values were reported as
median[interquartile range (IQR)]. All fits were considered
significant with an alpha of 0.05.
Results
A positive relationship was found between SUV and post-
treatment dose. A sample patient with post-treatment
grade 1 pneumonitis is shown in figure 1. The sigmoidal fit
(figure 2) over all patients was significant (chi-squared =
0.01) with an EC50 at 39 Gy, and lower and upper
asymptotes at 0.60 SUV and 1.31 SUV, respectively. The
linear portion of the sigmoidal fit (15- 60 Gy) was found to
be significantly (p ≤ 0.05), highly linear in individual
patients with a median correlation coefficient of
0.93[0.79-0.97]. Four patients did not have significant
linear fits (p-values ranging from 0.05 to 0.13). The
median SUV response per increase in dose (slope) was
heterogeneous with a median of 0.0083[0.005-0.018]
SUV/Gy, implying a 55 Gy to 200 Gy increase needed per
SUV increase within the IQR.
Conclusion
Strong linear and sigmoidal relationships were found
between post-treatment SUV and planned dose. These
results suggest that increasing dose leads to a highly linear