Intracavitary Brachytherapy

20.1 Introduction

Intracavitary brachytherapy can be performed using low dose rate (LDR), pulsed dose rate (PDR), or high dose rate (HDR) sources. LDR implants expose the patient care team to more radiation than HDR and require longer hospitalization of the patient and more resources dedicated to source storage and radiation safety measures. There is also an increased risk of error, for example, by selecting the incorrect source strength when retrieving source capsules from storage or implanting sources in the incorrect order. For these reasons, intracavitary brachytherapy has largely moved to HDR delivery using remote afterloaders.

The main issue in converting from LDR to HDR was in establishing equivalent dose fractionation regimens. LDR had a long clinical history with well-established dose fractionation, survival rates, and complication rates. Theoretically, HDR has a lower therapeutic ratio than LDR because of the short duration of the treatments. HDR with computerized treatment planning has the ability to optimize the dose to targets and critical structures, which can overcome this deficit. In fact, studies have shown the equivalence of LDR and HDR.

In evaluating HDR treatments it is sometimes useful to convert to radiobiologically equivalent doses (most commonly the EQD ₂ , or equivalent dose in 2 Gy fractions), though it is important to remember the caveats and assumptions that come with such model-based conversions. Numerous biologically equivalent dose (BED) calculators are available online and as apps; the American Brachytherapy Society (ABS) provides Excel worksheets to convert HDR doses to 2 Gy equivalent doses ( http://www.americanbrachytherapy.org/guidelines/index.cfm ). These worksheets should be used with caution and can support, but not supplant, careful clinical assessment.

20.2 Intracavitary Treatment Sites

Intracavitary brachytherapy takes advantage of anatomic pathways to place radioactive sources directly adjacent to tumor sites. This is less invasive than interstitial implants and can in many cases be done without general anesthesia.

Standard gynecological (gyn) applicators are made from surgical steel with plastics used for the build-up caps and vaginal cylinder segments. Magnetic resonance imaging (MRI)-compatible applicators replace surgical steel with either titanium or carbon fiber; neither of these materials has the same mechanical strength as surgical steel, and therefore they require extra care in handling during sterilization and the procedure.

Gynecological Sites

Endometrial

One common application of intracavitary brachytherapy is the treatment of endometrial cancers. Figure 20.1 shows the anatomy for reference. The ABS consensus guidelines for adjuvant vaginal cuff brachytherapy after hysterectomy contains the most recent society recommendations on this procedure. According to National Comprehensive Cancer Network (NCCN) guidelines, for Stage I patients with no adverse risk factors brachytherapy can be the sole adjuvant therapy after surgery. For Stage I patients with adverse risk factors and Stage II patients, pelvic external beam therapy of 4500 to 5000 cGy is given in addition to brachytherapy. After surgical removal of the uterus, the standard applicator is a segmented cylinder with one central catheter. A standard cylinder set consists of cylinders of different diameters (2 cm to 4 cm) with several segments each and one or two types of cylinder end caps. The physician chooses the appropriate cylinder based on the patient anatomy; the goal is to use the cylinder shape that minimizes any air gaps between cylinder and tissue. In practice the largest diameter cylinder that is comfortable for the patient is used. This has the added benefit of reducing the rate of dose fall-off due to the inverse square law. Figure 20.2 shows a typical cylinder implant. A variation of the standard cylinder is the shielded cylinder, in which tungsten shielding segments of 90°, 180°, and 270° can be inserted. Shielded cylinders have the disadvantage that the CT simulation image for treatment planning cannot be taken with the shields in place because of the large artifacts the shielding would cause. Some shielded applicators have removable shields so that they can be placed after imaging. A solution to the imaging challenges that come with shielded cylinders, the Miami applicator was designed to allow for noncylindrical dose distribution without the use of metal shielding. Its design is similar to a cylinder, but in addition to the central channel, six peripheral channels allow for customization of the dose distribution. Most cylinder and Miami applicator designs allow for the central channel to be replaced with a tandem to treat patients who were medically inoperable and in whom the uterus was not removed. Figure 20.3 shows typical cylinder applicators. A tandem and ovoid applicator can also be used for these patients as described in Section 2.1 . More recently, 3D printing technology has been used to custom-print cylinder applicators, and an inflatable vaginal cylinder has been introduced.