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Conclusion
The clinically estimated
α/β
values for prostate cancer
from an isoeffective L-Q model were inversely related only
to the proportion of patients receiving ADT. This is in
accordance with
in vitro
and
in vivo
studies demonstrating
ADT induced cell cycle arrest and induction of apoptosis in
prostate cancer, both of which results in decelerating the
tumour growth. Thus, ADT-primed prostate cancers cells
could radiobiologically mimic late-responding normal
tissues and display lower
α/β
values. This suggests a
possible selective benefit of ADT in cancer prostate
patients on HRT treatment protocols.
OC-0037 Low dose volume effect is a critical
determinant for radiation-induced lung fibrosis
G. Defraene
1
, M. La Fontaine
2
, S. Van Kranen
2
, B.
Reymen
3
, J. Belderbos
2
, J.J. Sonke
2
, D. De Ruysscher
1,3
1
KU Leuven - University of Leuven, Department of
Radiation Oncology, Leuven, Belgium
2
Netherlands Cancer Institute, Department of Radiation
Oncology, Amsterdam, The Netherlands
3
Department of Radiation Oncology MAASTRO GROW -
School for Oncology and Developmental Biology,
Maastricht University Medical Centre, Maastricht, The
Netherlands
Purpose or Objective
Severe normal lung tissue damage after (chemo)
radiotherapy, presenting as fibrotic changes, occurs in 20%
of patients and often coincides with clinical symptoms.
The mechanism of formation and/or propagation of these
lung infiltrations is not well understood. The aim of this
study was to quantify and explain lung tissue density
increase assessed by CT scans as a surrogate of lung
damage in a dataset in which unusually large prescription
doses resulted in a wide range of lung tissue doses.
Material and Methods
The dataset consisted of 75 stage I-III non-small cell lung
cancer patients from 3 institutions treated in the PET-
Boost trial: a randomized phase II study (NCT01024829).
The randomization arms were a dose escalation to the
primary tumour (PTV
prim
) as a whole (Arm A) and an
integrated boost painted to the 50% FDG PET SUV
max
subregion of PTV
prim
(Arm B), both delivered in 24
fractions. When dose constraints prevented escalation ≥72
Gy, standard treatment of 66 Gy in 24 fractions was
delivered (Arm 0). The planning CT (CT
0
), follow-up CT
(CT
fup
) +/-3 months post RT and planned dose maps (IMRT
or VMAT) corrected to equivalent doses in 2 Gy fractions
(EQD2, α/β=3Gy) were collected. A deformable
registration mapped CT
fup
to CT
0
using a multiresolution B-
spline algorithm with constraints on rigidity and
smoothness to avoid reshaping of infiltrations on CT
fup
. The
median density change in Hounsfield Units (∆HU) within
the ‘lungs minus GTV’ contour was then extracted per
dose bin of 5 Gy from the difference image (HU
fup
-HU
0
).
Prognostic factors of ∆HU were obtained through linear
regression. The studied covariates were CT
fup
timepoint
(T
fup
), total and ipsilateral lung volume (LV
tot
and LV
ipsi
),
mean lung dose, lung volumes receiving 5 Gy, 20 Gy and
40 Gy (V
5
, V
20
and V
40
), mean heart dose (MHD), heart D
max
and PTV
prim
mean dose.
Results
The average density change response curve p er study arm
is depicted in Figure 1. A saturation was only observed
above 60Gy in Arm A. The higher response in A rm 0 above
40Gy suggested that local dose is not the on ly driver of
local ∆HU risk. Prognostic factors of individ ual patient
response at 20-25Gy, 40-45Gy and 60-65Gy were therefore
searched in the combined dataset. Table 1 shows the
significant covariates. Lower LV
ipsi
was highly prognostic of
∆HU in all dose bins. The dosimetric prognostic factors
lung V
5
and MHD could explain the higher ∆HU in Arm 0
(standard RT was only delivered in presence of limiting
constraint(s)): V
5
was on average 78.9%, 64.6% and 62.8%
in Arm 0, A and B, respectively, while MHD was on average
17.9Gy, 13.4Gy and 13.1Gy, respectively. In multivariate
analysis, LV
ipsi
, V
5
and T
fup
were independent prognostic
factors for ∆HU in the 40-45Gy and 60-65Gy dose bins.
Conclusion
This study indicates that the low dose volume effect is
critical for the induction of severe lung fibrosis in high
dose regions. This supports the hypothesis of the
importance of residual lung volumes spared from low dose
for optimal repair within the whole lung.
OC-0038 Patterns in ano-rectal dose maps and the risk
of late toxicity after prostate radiotherapy
E. Onjukka
1
, C. Fiorino
2
, F. Palorini
3
, A. Cicchetti
3
, I.
Improta
2
, C. Cozzarini
4
, C. Degli Esposti
5
, P. Gabriele
6
, R.
Valdagni
7
, G. Gagliardi
1
, T. Rancati
3