Brachytherapy is a special procedure in therapeutic radiology that utilizes the irradiation of a target with
radioactive sources placed at short distances from the target. Typically, the sources are implanted in the
target tissue directly (interstitial brachytherapy) or are placed at distances of the order of a few millimeters
from the target tissue, in body cavities such as the uterus, mouth, etc. (intracavitary brachytherapy),
or externally on structures such as the eye, skin, etc., (surface plaques or molds). Brachytherapy generates
highly conformal dose distributions in a target volume because radioactive seeds are implanted
directly within or in the vicinity of the target tissue. For example, in a typical interstitial brachytherapy
implant, 50 to 100 radioactive seeds, each about the size of a rice grain, are implanted in the tumor using
image-guided implantation techniques such as ultrasound, computed tomography (CT), or fluoroscopy,
which allow the physician to place radioactive seeds precisely at desired locations with minimal invasiveness.
For these applications, low-energy, photon-emitting radionuclides such as 125I (27 keV) and 103Pd
(21 keV) are preferred because these sources provide adequate coverage of tumor when used in a grid
of about 1 cm spacing and produce minimal exposures to distant organs in the patient, to the hospital
personnel performing the procedure, and to the family members and friends of the patient after he/she
is released from the hospital with the radioactive seeds in place. Most brachytherapy procedures today
are performed in one-day surgery suites without the need for hospitalization. These factors and the depth
dose characteristics make brachytherapy a very cost effective and patient friendly procedure compared
to 3-D conformal radiotherapy (3DCRT) or intensity-modulated radiation therapy (IMRT), which also
produce highly conformal dose distributions. A key advantage of 3DCRT or IMRT over brachytherapy
is that it is noninvasive. However, both 3DCRT and IMRT are very sensitive to patient localization and
setup errors because of high dose gradients at the periphery of the target volume. Therefore, the target
must be placed at the right position with a precision of about a millimeter relative to the linear accelerator
(linac) daily over a course of 5 to 6 weeks of 3DCRT and IMRT, and the clinical target volume
expanded by 5 to 10 mm for intrafraction movement. In contrast, brachytherapy requires a single visit
to a one-day surgery clinic or 3 to 5 visits in an outpatient clinic for high dose-rate (HDR) intracavitary
brachytherapy. Unlike 3DCRT and IMRT, brachytherapy is far more forgiving of localization and target
motion errors because the implanted sources of radiation in an interstitial implant move with the target.
Thus, brachytherapy solves a critical problem of 3DCRT and IMRT, which is that they have a potential
to “miss the target very precisely” unless they are implemented with extreme precision. This high-precision
requirement makes 3DCRT and IMRT very expensive, labor intensive, and technically complex.
For these practical reasons and other important radiobiological reasons related to continuous low doserate
(LDR) irradiation, brachytherapy remains a valuable treatment modality for selected cancers despite
the current trend of widespread adoption of 3DCRT and IMRT in radiotherapy. In this chapter, an
overview of the rich spectrum of various brachytherapy procedures is presented so that the students have
an appreciation of the overall field.