12
Planning CT scan will be taken with patient in immobilization and in
treatment position. For three-dimensional conformal radiotherapy
(3DCRT), slice thickness of 5 mm may be taken; for intensity modu-
lated radiotherapy (IMRT), slice thickness of at least 3 mm or less is
required. The planning CT should include whole thorax and abdo-
men and extend from cricoid bone to lumbar spine (L4). Arterial
phase contrast will help in delineating mediastinal vessels and the
celiac axis. Breath holding may be used to reduce organ motion with
breathing or four-dimensional CT may be used to accurately assess
organ motion and allow estimation of internal margin required.Gross tumor volume (GTV) is defined as all known gross disease as
defined by imaging and from clinical and endoscopic information.
Preoperative CT scan, PET/CT scan will help define the radial
extent of tumor and any involved lymph nodes. Endoscopy and
barium swallow can define the mucosal and longitudinal extent of
tumor. Endoscopic ultrasound may detect paraesophageal nodes
and assess the extent of tumor through the esophageal wall. Image
fusion of planning CT with PET scan or diagnostic CT scan can be
performed to allow better delineation of tumor. GTV of the pri-
mary (GTV-P) and any enlarged/hypermetabolic lymph nodes
(GTV-N) should be localized separately.
Clinical target volume (CTV) includes margins to cover the
subclinical involvement around the GTV. For the primary CTV
(CTV-P), the proximal and distal margins added to GTV-P can
be 3–4 cm proximally and 2–3 cm distally depend on how much
organs at risk (OARs) are irradiated. The radial margin is
1–1.5 cm, which will include prophylactic paraesophageal nodal
irradiation. In the postoperative setting, the preoperative GTV
(the primary tumor bed) will become the CTV. The CTV will
also include the anastomosis, remnant stomach, any site of resid-
ual disease, and the lymphatic drainage at risk including parae-
sophageal, paracardial, and celiac nodes. In case the CTV is close
to anatomical structure such as vertebrae, aorta, or liver, these
normal structures can be excluded from the CTV if there was no
evidence of involvement by tumor.
CTV for enlarged lymph nodes (CTV-N) is defined by a margin of
1–1.5 cm around GTV-N in all directions. If prophylactic lymphatic
irradiation is required, the celiac artery should be traced on planning
CT and added on a margin of 1–1.5 cm and added to CTV-N [ 13 ].
The combined volumes of CTV-P and CTV-N will be the final
CTV for formulation of the planning target volume (PTV). PTV
will provide a margin around the CTV to compensate for variations
in daily treatment setup. If four-dimensional CT is performed, an
additional internal target volume with margins as estimated from
organ motion can be added before expansion of margins for
PTV. The margin for PTV will depend on the immobilization tech-
nique and RT machine used. This is usually at least 1 cm for three-
dimensional conformal radiotherapy (3DCRT) and can be reduced3.2 Simulation
3.3 Target Volumes
Dora L. W. Kwong and K. O. Lam