Appropriate patient instructions can reduce prostate motion
Reinhold Graf
0
Dirk Boehmer
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Jacek Nadobny
0
Volker Budach
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Peter Wust
0
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Department of Radiation Oncology
,
Charite Universitatsmedizin Berlin, Campus Virchow-Klinikum, Augustenburger Platz 1, 13353 Berlin
,
Germany
Background: Interfraction prostate motion must be compensated by increased safety margins. If filling status of rectum and bladder is constant, motion should be reduced. We attempted to reduce interfraction motion errors by proper patient instruction. Method: In 38 patients pairs of radio-opaque fiducial markers were implanted prior to definitive radiotherapy. Patients were positioned either according to skin marks or infrared body marker. We measured prostate displacement, i.e. pelvic bones versus intraprostatic marker position, via ExacTrac (two orthogonal radiographies) in 1252 fractions. Systematic and random setup and displacement errors were determined and safety margins estimated. Results: In our study interfraction prostate displacement is < 1 mm in RL direction, and < 2 mm in AP and SI direction. Systematic errors are slightly below random errors (< 1.5 mm). Positioning according skin marks results in higher inaccuracies of 1.5 - 2 mm in RL and 2 - 2.5 mm in AP/SI direction. Conclusions: In case of appropriate patient instructions (constant organ filling) the positioning via bone fusion requires CTV-PTV margins of 2 mm in RL, 4 mm in AP, and 5 mm in SI direction. Studies without any description of patient instruction found much higher margins of > 1 cm in AP and SI direction.
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Introduction
The doseresponse relationship between long-term PSA
control and radiation dose in the prostate is beyond
controversy and validated by numerous studies and analyses,
among them several randomized trials [1,2]. A minimum
dose of 72 Gy (conventional fractionation) is required
[3], but higher doses are desirable and further increase
the rate and duration of PSA control. However, further
increasing the dose towards 80 Gy elevates the dose in
parts of the rectum and might be associated with late
rectal toxicity [4].
The radiation exposure of the normal tissues
surrounding the prostate, in particular the rectum, is mainly
determined by the CTV PTV safety margins. These
margins can be influenced by the positioning technique
of the patient and all measures to cover the CTV with
the prescribed dose as accurate as possible.
Two major sources of uncertainty have been identified.
Firstly, setup errors describe the variation of bony
landmarks relative to skin marks, utilizing either laser
crosses or infrared body markers. These errors depend
on the diligence of the patient positioning (using certain
positioning devices), the setup-procedure and in
particular of the patients habitus. Employing modern image
guidance such as portal images [5], MV-CT [6] or
conebeam CT [7] in conjunction with bone fusion this kind
of set-up error can be minimized. Secondly, prostate
motion relative to the pelvic bones is the remaining and
dominant error source with variations of the prostate
position either between fractions (interfraction) or
during the irradiation (intrafraction). In order to correct the
isocentre with respect to these displacements
intraprostatic implanted markers are used, in particular metallic
markers (gold, titanium). Marker based corrections have
shown in numerous studies (see discussion), that the
uncertainties caused by the prostate movement might be
significant requiring safety margins > 1 cm. These errors
are severe obstacles to further escalate the dose and
should be reduced. Therefore, most investigators
recommend the use of intraprostatic markers to track the
prostate in order to reduce safety margins.
Prostate position or displacement depend on organ
filling or distension, in particular of the rectum and to a
minor degree of the bladder. Theoretically, prostate
displacement can be reduced and is possibly less important,
if a constant and reproducible organ filling is
maintained. A well-defined reference situation might be an
empty rectum and a bladder filled with a given content
of some hundred millilitres. Some authors mention this
reference filling state, but often this patient-dependent
factor is not specified. Elaborated examinations about
the relationship between patient preparation and
prostate displacement are missing to our knowledge, and
marker implantation is recommended.
However under clinical conditions, marker
implantation is not only an additional invasive procedure (with
some additional risk), but also time-consuming and
expensive. Therefore, the question is reasonable if prostate
motion can be reduced (and to which extent) by
appropriate patient education. In the present study
interfractional prostate motion has been investigated for a
patient group, which has been carefully trained to keep
the organ filling constant.
Patients and methods
From 2005 2010 we offered patients with prostate
carcinoma a definitive radiotherapy with an additional
marker-based image guidance. (...truncated)