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ESTRO 36 2017
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individualized PTV margins around the SV based on MRI
information.
Material and Methods
We have obtained CT, T2 weighted (T2w) MRI, dynamic
contrast enhanced (DCE) MRI and diffusion weighted
imaging (DWI) MRI for 21 high-risk prostate cancer patients
with an intact prostate. All patients completed their
radiotherapy treatment course of 78Gy in 39 fractions. Our
clinical standard margins of 5mm, 5mm and 7mm in the
anterior-posterior, left-right and cranio-caudal direction,
respectively, were applied to the SV delineation for
preparing the clinical plans. MRI scans were then further
examined retrospectively. All three MRI sequences were
delineated by a radiation oncologist and a suspicious
volume on T2W confirmed by an overlap with one of the
functional scans (DCR and/or DWI) was assumed to be
macroscopically involved, see Figure 1. For patients with
confirmed suspicious volume in the SV, we applied a PTV
with a margin of 11mm (PTV11mm) to the clinical
delineation of SV to account for the mobility of the SV
when registering on implanted markers within the
prostate as suggested by [2]. Coverage of the PTV used in
the clinical dose plan (PTV5mm) and the PTV required for
reliable SV coverage (PTV11mm) were extracted from the
dose-volume histogram (DVH) of the clinical dose plan.
Results
For 14 out of 21 patients had a T2w suspicious volume
confirmed either by DWI or DCE, with 6 cases having
suspicious volumes in all three MRIs. The 14 patients had
median (range) of dose to 98% of the PTV volume (D98%)
of 73.1Gy (57.6-75.6) and 45.9Gy (39.3-61.7) for PTV5mm
and PTV11mm, respectively. In four cases D98% of
PTV5mm was below the clinical recommendation of
70.2Gy while all 14 patients had D98% of PTV11mm below
70.2Gy, see Figure 2.
Conclusion
Coverage of the SV to the prescription dose is not
guaranteed when using tight margins and highly conformal
radiotherapy despite daily match on fiducials in the
prostate. This is a particular concern as studies found a
higher rate of failure with highly conformal radiotherapy
when comparing to rectangular fields [3], yet uniform
wide margins to SV will substantially increase the dose to
the rectum. With the current study we used multi-
sequence MRI to yield risk-adapted margins to increase
coverage of SV for selected patients, while maintaining
standard, tight margins for patients without suspicious MRI
findings.
[1]
Morlacco
A,
et
al.
Eur
Urol.
http://dx.doi.org/10.1016/j.eururo.2016.08.015,2016.
[2] Boer J, et al,Int J Radiat Oncol Biol Phys, vol 86, No 1,
pp 177-182, 2013.
[3] Heemsbergen W, et al. Radiother Oncol, 107, 134-139,
2013.
EP-1694 DW-MRI as a predictor of tumor response to
hypofractionated stereotactic boost for prostate cancer
D. Pasquier
1
, A. Hadj Henni
2
, E. Tresch
3
, N. Reynaert
4
, E.
Lartigau
1
, O. Colot
2
, N. Betrouni
5
1
CENTRE OSCAR LAMBRET- CRISTAL UMR CNRS 9189,
Academic Department of Radiation Oncology, Lille,
France
2
CRISTAL UMR CNRS 9189, LILLE University, Villeneuve
d'Ascq, France
3
CENTRE OSCAR LAMBRET, Department of Biostatistics,
Lille, France
4
CENTRE OSCAR LAMBRET, Department of Medical
Physics, Lille, France
5
INSERM 1189 ONCO-THAI, LILLE University, Lille, France
Purpose or Objective
To evaluate the feasibility of diffusion-weighted MRI (DWI)
as an early biomarker in patients receiving
hypofractionated stereotactic boost for intermediate risk
prostate cancer
Material and Methods
Patients with intermediate risk prostate cancer were
included in a multicenter CKNO phase II trial. During the
first part of the treatment, 23 fractions (2 Gy/session)
were delivered over 42 days maximum using 3DCRT.
During the second treatment part, hypofractionated
stereotactic boost (3 fractions of 6 Gy) was delivered each
other day. Median follow up was 26.4 months (range:13.6
- 29.9). Multiparametric (mp) MRI was realized before (M0)
and at regular follow-up intervals after radiotherapy (6, 9,
12, 24, 30, 36 and 48 months). This ancillary study
included the 24 patients treated in our center (3T MRI).
None of them presented with recurrence so apparent
diffusion coefficient (ADC) and ktrans variations were
correlated to PSA kinetics. GTV was delineated by
experienced radiologist and radiation oncologist on
baseline and follow-up mpMRI. Mean tumor ADC values (b=
0, 1000 and 2000 s/mm2) were normalized to obturator
muscle (nADC). Spearman’s coefficient correlation and
non-parametric Wilcoxon Mann-Whitney tests were used
for continuous and categorical variables respectively.
Results
GTV was visualized on MRI in 12 patients. Mean nADC
improved from 1.14x10
-3
mm
2
/s (min 0.49 max 1.53) to
1.59x10
-3
mm
2
/s (min 1.15 max 1.94) between M0 and
M24. The nADC variation between M0 and M12 was
correlated with PSA at M18. The nADC increase between
M0 and M12 was larger in patients with PSA < 1ng/mL at
M18 (p=0.034) (Table). No other correlation was found.
Ktrans variation was not associated with PSA kinetics.