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S64
ESTRO 36 2017
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Purpose or Objective
Clinical treatment planning of proton therapy assumes
that the
relative biological effectiveness
(RBE) of protons
is uniformly 10% higher than photons. However, the energy
deposition of protons is considerably different from
photons resulting in an increased
linear energy transfer
(LET) at the distal end and laterally of the proton beam
Bragg Peak. In addition, the distal end of proton beams is
often located in or very close to normal tissues / organs at
risk. This gives rise to concern of an increase in the
biological effects of the normal tissues. We are currently
initiating a normal tissue complication probability
modelling project on outcomes following proton therapy
of prostate cancer. As a first step, we have in this study
investigated the dose-averaged LET
d
distributions in the
normal tissues (rectum and bladder) for spot scanning
proton therapy of prostate cancer using the two most
common field arrangements for these patients.
Material and Methods
Spot scanning proton therapy plans (3 mm spot size with 1
mm spacing) were created in the treatment planning
system PyTRiP on CT scans of seven prostate cancer
patients (without rectal balloons or spacers). The CTV
included the prostate gland, while CTV–to–PTV margins of
4 mm axially and 6 mm in the superior and inferior
directions were used. The PTV was planned to receive a
dose of 78 Gy (RBE). Treatment plans with two opposing
lateral fields (90°/270°) and with two lateral oblique
fields (70°/290°) were created for all cases. Dose and LET
d
distributions were calculated in PyTRiP for all plans and
visualised as dose and LET
d
volume histograms
(DVHs/LET
d
-VHs).
Results
The LET
d
distribution patterns for both the lateral
opposing and the lateral oblique fields were overall similar
for the seven patients. For the two lateral opposing fields
the median (range) of the mean LET
d
in the prostate was
2.1 keV/µm (2.0–2.2 keV/µm) while somewhat higher LET
d
was found in the high-dose areas outside of the prostate
(Fig. 1). For the rectum, 6% (median; range 4–13%) of the
volume had an LET
d
higher than the mean LET
d
in the
prostate, compared to 27% (median; range: 8–36%) for the
bladder. For the lateral oblique fields the mean LET
d
in
the prostate was 2.1 keV/µm (2.0–2.1 keV/µm) while the
LET
d
more than doubled compared to the plans with
lateral opposing fields (Fig. 2). For the rectum, 15%
(median; range 12–24%) of the volume had an LET
d
higher
than the mean LET
d
in the prostate, compared to 23%
(median; range: 2–35%) for the bladder.
Conclusion
High LET
d
volumes outside the target were seen in both
rectum and bladder when treating with spot scanning
proton therapy. In particular, the rectum was subject to
high LET
d
volumes when irradiated with lateral oblique
fields compared to lateral opposed fields.