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The Use of RapidArc™ in the Treatment of Prostate Cancer
objective for each parameter. If intensity modulation with fixed- Figure 1: Typical Dose Distribution for a
beam and dynamic sliding window delivery (dynamic IMRT) is
Prostatic Patient with RapidArc in Axial,
Coronal and Sagittal Views
considered as a benchmark, the reported values correspond to an
improvement of at least 10% in sparing organs at risk. These findings
are comparable to those previously reported for prostate treatment
with RapidArc
6
and for cervical treatments (a treatment with strong
similarities in terms of organs at risk and geometrical arrangement
with respect to target mass).
7
The clinical relevance of the
dosimetric parameters indicated above has to be properly assessed
through detailed analysis of treatment outcome and associated
toxicity, if any. Nevertheless, as often proved in the past, dosimetric
improvement from the physical viewpoint contains the seeds for
clinical advances.
Similarly to dosimetric advances, RapidArc looks highly promising
Figure 2: Typical Dose–Volume Histograms for a
in the management of prostate treatment with radiation because of Prostatic Patient Treated with RapidArc
its logistic efficiency. Our data show that a single fraction of 2Gy
can be delivered within 74 seconds (the time needed for a full
gantry rotation). This time is significantly faster than that achievable
today with standard IMRT, where a single fraction in the prostate,
normally delivered with five to seven beams, takes approximately
10 minutes because of multiple instances of machine set-ups
creating delays during delivery. These times are relevant from many
points of view. Besides efficiency and throughput considerations for
the department, fast delivery time enhances the chances of
accurate treatment for the patient. This is achieved through
minimisation of the patient’s discomfort on the treatment couch
reducing the risk of voluntary movements (e.g. muscles in the
Red = target volume; yellow = bladder; brown = rectum; green and white = femoral heads.
glutei) and minimisation of deformation and displacement of
internal organs.
Table 1: Preliminary Comparison Between RapidArc and
Objective Dosimetric Parameters
The first effect can be reduced with proper patient education and
training (not easily achievable under the psychological stress during Objective RapidArc
radiation therapy); however, the second issue is almost completely
Bladder volume >50Gy <40% 37.2±13.7
independent of human control. It has been proved that, for example,
Rectum volume >50Gy <35% 25.3±5.5
the bladder under normal conditions continuously fills at a significant
Target volume >95% of dose prescription >95% 96.4±1.5
speed and that, over a period of 20 minutes, it can increase beyond
tolerable levels. Similarly, global displacement of target mass was Image Guidance for RapidArc
shown to be greater than 3mm after six to seven minutes in more Image-guided radiotherapy (IGRT) is a fundamental pillar of the safe
than 15% of patients.
8
management of advanced treatment modalities such as RapidArc.
IGRT can be used to measure and correct target and critical structure
RapidArc technology also incorporates a complementary feature that positional errors immediately prior to or even during treatment
could be significantly beneficial for prostate patients. In fact, a delivery. One of the most recent achievements of IGRT is the
significant fraction of these patients present with single or bilateral introduction in routine practice of in-room kV imaging tools capable of
metal implants for femoral head or hip replacement. Normally, photon acquiring planar 2D images, fluoroscopic sequences and cone beam
beams should not be directed through any metal implant to avoid CT (CBCT) scans with almost radiological quality. The so-called
undue generation of low-energy scattered secondary radiation – On-Board Imager
©
(OBI) became operational at IOSI at the same time
highly dangerous for the implants and for the surrounding tissues as RapidArc, during autumn 2008. Since then, all patients who have
and undue enhancement of the beam intensity to compensate for the undergone RapidArc treatment have received a sequence of image-
extreme attenuation of photon intensity caused by the metal. guidance procedures to assess the accuracy and reliability of their
daily radiation positioning.
Both of these effects are detrimental to the quality and safety of
treatment. RapidArc, as mentioned, allows the generation of The IGRT protocols, still under investigation at IOSI, include two
‘doseless’ or ‘avoidance’ sectors, i.e. sub-arcs, defined by the users levels of guidance. With a higher frequency, patient positioning is
where the beam intensity is dropped intentionally to zero to avoid verified by means of two planar views acquired orthogonally: one
direct entrance through metallic structures. Figure 3 shows an with a kV beam, one with an MV beam. The set-up is depicted in
example of this application, which enhances treatment efficiency and Figure 4 and is the most efficient approach to image guidance for
safety. The patient presenting with bilateral femoral head replacement RapidArc. In fact, with this type of set-up the machine is rotated to
was irradiated with two avoidance sectors to exclude the implants, the treatment start position before imaging. The patient is properly
clearly visible on the computed tomography (CT) images. positioned and imaging arms are extracted to acquire 2D data sets.
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