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S922
ESTRO 36
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radiosensitivity
before
the
start
of
RT.
Additional sets of imaged-derived parameters will be
investigated and further cell-lines be measured to identify
relations with radiosensitivity for the development of a
multiparametric prediction model for personalized RT in
HNSCC.
EP-1689 Gleason driven dose painting based on ADC
MR imaging
E. Grönlund
1
, S. Johansson
2
, T. Nyholm
1
, A. Ahnesjö
1
1
Uppsala University, Medical radiation sciences, Uppsala,
Sweden
2
Uppsala University, Experimental and clinical oncology,
Uppsala, Sweden
Purpose or Objective
To investigate a Gleason driven dose painting approach for
high risk prostate cancer patients based on outcome for
conventional treatments, and using apparent diffusion
coefficient (ADC) MR images for dose prescription.
Material and Methods
We based our retrospective study on a total of
N
=122 high-
risk prostate patients treated with radiotherapy, with
inclusion criteria to have a pre-treatment PSA<60 µg/L and
biopsies analyzed a Uppsala University Hospital. The 5-
year local tumor control probability was estimated with
Kaplan Meier analysis to TCP
obs
=94.7% (CI 86.4-98.0%). The
PSA inclusion condition was used to exclude patients with
possible pre-treatment spread. The homogeneous
treatment dose
D
h
was estimated to 91.6 Gy EQD
2
based
on α/β=1.93 for the given proton boost (20Gy in 4
fractions, RBE=1.1) and photon dose (50 Gy in 25
fractions). All patients underwent androgen deprivation
therapy. We parameterized the populations dose-response
TCP
pop
(
D
) with a logistic function with the parameter
γ
50
=2.01 and
D
50
chosen so that TCP
pop
(
D
h
)= TCP
obs
. The
patients’ biopsy statements were used to construct
simulated prostates with voxelized distributions of
Gleason scores
G
varying per voxel.
Voxel specific dose-response functions TCP
vox
(
D
,
G
) were
derived with the logistic parameters γ
50,eff
and
D
50
(
G
) set
so that the average TCP
pat
for all patients equals TCP
obs
at
D
h
, and the average slope for the patients TCP
pat
equals
the slope for TCP
pop
(
D
) at
D
h
. Hence, the voxel specific
dose-response functions are be described by
TCP
vox
(
D
,
G
)=1/(1+(
D
50
(
G
)/
D
)
4γ50,eff
),
where
D
50
(
G
) and γ
50,eff
, for
D
=
D
h
, reconstructs
TCP
vox
(
D
h
,
G
<6)=
C
and
TCP
vox
(
D
h
,
G
≥6)=
C
-
k
×(
G
-6).
For
G
<6 TCP
vox
was set to not vary with Gleason scores
since ADC-MRI likely not distinguish
G
<6 from normal
tissue. We used 3 different values of
C
, a high value
C
high
=1
resulting in zero desired dose for
G
<6 voxels, a low value
C
min
resulting in a homogeneous dose distribution (
k
=0),
and an intermediate
C
im
for a certain minimum dose.
ADC images for a high-risk patient were translated into a
3D-map of Gleason scores based on results published by
Turkbey et al. We used the above functions for dose
painting to minimize the average dose while keeping the
TCP
pat
equal to that for a homogeneous dose of
D
h
.
Results
For the
C
high
scenario the average dose decreased by 9 Gy
(max dose 98 Gy). For the intermediate
C
im
scenario the
average dose decreased by 2 Gy with doses in the range of
74 to 98 Gy. Fig. 1 shows resulting Gleason score to TCP
mappings normalized for a 50cc prostate while Fig. 2
shows a dose painted prostate for the
C
im
scenario.
Fig 1.
TCP vs Gleason scores comprising a 50cc prostate
volume and corresponding dose-response functions for the
intermediate
C
im
scenario.