Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA
( Accepted 21 December 2005)

Abstract—In this review article, we discuss various technical aspects of image-guided respiration-gated radiation
therapy. We first review some basic concepts related to respiratory gating, including gating window, duty cycle,
residual motion, internal/external gating, amplitude/phase gating, etc. We then discuss 2 implementations of
image-guided respiration-gated treatment, i.e., the Mitsubishi/Hokkaido technique for internal gating and the
MGH technique for external gating. Several existing problems related to respiratory gating, namely external
gating mode (phase vs. amplitude), imaging dose for internal gating, gated treatment for lung cancer without
implanted fiducial makers, as well as gated intensity-modulated radiation therapy issues, are also discussed along
with potential solutions.

Recent technological advances in radiation therapy, such
as intensity-modulated radiation therapy (IMRT), provide
the capability of delivering a highly conformal
radiation dose distribution to a complex static target
volume. However, treatment errors related to internal
organ motion may greatly degrade the effectiveness of
conformal radiotherapy for the management of thoracic
and abdominal lesions, especially when the treatment is
done in a hypo-fraction or single fraction manner. This
has become a pressing issue in the emerging era of
image-guided radiation therapy (IGRT).
Intra-fraction organ motion is caused mainly by
patient respiration, and sometimes by skeletal muscular,
cardiac, or gastrointestinal systems. Respiration-induced
organ motion has been studied by directly tracking the
movement of the tumor, the host organ, radiopaque
markers implanted at the tumor site, radioactive tracer
targeting the tumor, and surrogate structures such as
diaphragm and chest wall. It has been shown that the
motion magnitude can be clinically significant (e.g., of
the order of 2–3 cm), depending on tumor sites and
individual patients.
One category of methods to account for respiratory
motion is to minimize the tumor motion, using techniques
such as breath holding and forced shallow breathing
(such as jet ventilation).1–9 These techniques require
patient compliance, active participation and, often, extra
therapist participation. They may not be well tolerated by
patients with compromised lung function, which is the
case for most lung cancer patients.10 Another category of
the methods accounting for respiratory motion is to allowfree tumor motion while adapting the radiation beam to
the tumor position by either respiratory gating or beam
Respiratory gating limits radiation exposure to the
portion of the breathing cycle when the tumor is in the
path of the beam.10–25 Beam-tracking technique follows
the target dynamically with the radiation beam.26 It was
first implemented in a robotic radiosurgery system (CyberKnife).
27–31 For linac-based radiotherapy, tumor motion
can be compensated for using a dynamic multileaf
collimator (MLC).32–41 Linac-based beam tracking is
still under development. Much effort is needed to overcome
the technical difficulties and to solve quality assurance
issues associated with this technique before it
can be applied to patient treatment. On the other hand,
respiratory gating techniques have been introduced in
clinical routine, although some improvements are still
required. In this review article, we will discuss various
technical aspects of respiration gated radiation treatment.

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