Lumpectomy followed by external-beam irradiation of the whole breast is the standard of care for most early-stage breast cancers [10]. Despite excellent outcomes in terms of both local control and cosmesis [11, 12], the typical 6-week duration of the treatment may be problematic to some patients, and a program of accelerated partial breast irradiation (APBI) has been developed. APBI may be delivered using multicatheter implants [13] or by placing an applicator in the lumpectomy cavity [14]. In both instances, imaging is used for placement of the catheters as well as applicator reconstruction and image-based treatment planning. Target definition for treatment differs from most conformal treatment planning because the tumor has been removed with negative margins and the target of the radiation is potential microscopic disease in the surgical cavity. Clips may be placed at the time of lumpectomy to help delineate the region of concern. Inflatable balloon applicators seek to fill the cavity and target the tissue within a specified distance of the balloon surface.

The development of prostate-specific antigen (PSA) tests in the 1980s led to the detection of many prostate cancers when they were in the early stage and confined to the gland. These patients are appropriately treated with local therapies including brachytherapy [15]. Brachytherapy treatment outcomes after 5–10 years of follow-up are comparable to those of surgery. Early-stage disease may be treated with permanent implants of low-energy sources such as ¹²⁵I, ¹⁰³Pd, and ¹³¹Cs under transrectal ultrasound guidance [16–18]. The implant process involves every aspect of image-guided brachytherapy [5]. Implants are planned based on volumetric imaging of the prostate, and the implant is image-guided to match the plan. Post-implant follow-up includes a dosimetric assessment of the implant using CT for image-based brachytherapy evaluation as seen in Fig. 26.4. The approach has been further developed to include MR imaging [19, 20] as well as dosimetry-guided [21] implant procedures including adaptive treatment planning.

Gynecologic malignancies are well treated with brachytherapy, and local control improves when external-beam radiation is supplemented by brachytherapy [22–24]. The vagina and uterus allow intracavitary placement of applicators and are relatively tolerant of radiation. Implants are temporary using afterloaded sources, and treatments may be delivered by means of low-dose-rate (LDR) sources, delivering dose continuously over the course of days; pulsed-dose-rate (PDR) afterloaders, delivering dose in 10-min intervals hourly; or high-dose-rate (HDR) afterloaders, delivering a few fractions of radiation separated by at least 24 h. Historically these implants have used imaging for evaluation, but there is a recent trend towards image-based conformal plans, and there are recommendations for the use of MR imaging [25, 26]. There are efforts to move beyond image-based planning towards image-guided implants [8].

Fluoroscopy is a vital component of many brachytherapy procedures. Planar x-ray images provided a means of verifying placement and/or size of the applicator or provided a general sense of the distribution of brachytherapy needles or sources in the region of the target. While not well suited to distinguishing soft tissues, the difference in attenuation between soft tissue and dense materials makes x-rays well suited to visualizing brachytherapy applicators once placed within the patient. Contrast or radio-opaque material may be used to indicate the location of sensitive structures, such as bladder and rectum in cervix treatments, in the vicinity of the applicators. Geometric distortions make fluoroscopy images inappropriate for dose calculations. Fluoroscopy is useful to guide the placement of brachytherapy catheters and may be used as a supplement to ultrasound imaging in permanent prostate implants to visualize implanted sources within the patient.

Ultrasound is widely used in brachytherapy procedures for both qualitative and quantitative guidance. Ultrasound offers a non-ionizing imaging modality appropriate for guiding the safe placement of brachytherapy applicators in internal structures. Abdominal transducers are appropriate for the placement of applicators through the cervix, and vascular ultrasound allows appropriate placement of the brachytherapy catheters with respect to vascular lesions.

With some additional equipment, ultrasound can become a volumetric imaging modality. Ultrasound-guided prostate implants use manual or motorized movement of a transrectal ultrasound device to generate an imaging dataset for image-based treatment planning and provide real-time image guidance of needle placement to ensure that the sources are deployed according to the treatment plan [5].

CT imaging is based on the ability of x-rays to distinguish soft tissue from denser materials but is susceptible to imaging artifacts from traditional brachytherapy applicators. CT provides a spatially accurate volumetric dataset appropriate for treatment planning. Manufacturers are now producing brachytherapy applicators that are compatible with CT. Limited soft tissue contrast is a significant problem for image-guided brachytherapy because it is so frequently used in the pelvis. Generally found outside the procedure room, CT provides a means to calculate radiation dose distributions after permanent sources have been placed (image-based evaluation) or plan dose around applicators used for temporary brachytherapy (image-based treatment planning).

The soft tissue contrast of MR makes it an extremely appealing imaging modality for brachytherapy. The endometrium, cervix, and prostate are the most common sites for brachytherapy, and in such cases the brachytherapy dose is constrained by neighboring radiosensitive structures that MR can visualize well. MR is subject to geometric distortions, though the relatively small extent of brachytherapy implants may make the distortions acceptable. The GEC-ESTRO recommendations [25] suggest MR for brachytherapy of the cervix. MR imaging for brachytherapy is challenging because metallic or rigid plastic objects are routinely placed in the region of interest, and the optimal pulse sequences differ between visualizing soft tissue and applicators. The use of applicators raises concerns for the use of MR, and care must be taken that all devices are both safe for use in a high magnetic field and compatible with the acquisition of an acceptable MR image. Current trends are for greater use of MR in image-based treatment planning either alone or in conjunction with CT by means of image registration. MR is not generally used for image-guided procedures other than at a few major academic medical centers.

Traditionally imaging has been used to produce a model of the geometry of the implanted sources or applicators. The radiation dose delivered by the sources could then be calculated in the coordinate system of the applicator. Radiation dose is reported in terms of an isodose distribution and the dose to reference points representative of anatomic points of interest. Even in ultrasound-guided permanent prostate implants [28], the prototypical image-guided brachytherapy procedure, the implant is evaluated based on post-implant imaging (Figs. 26.1 and 26.4) when there is no ability to affect the implant other than returning to the procedure room to implant additional sources if underdosed regions are detected.