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Interstitial Brachytherapy


21.1

Introduction

Interstitial brachytherapy has a long tradition, starting with radium needles being implanted in superficial tumors in the 1930s. The implant systems developed at the time, Manchester and Quimby, still guide the needle placement patterns used in implants today.

Though radium needles where initially used for interstitial implants, other radioactive nuclei were developed over time. Table 8.1 in Chapter 8 contains a list of the most commonly used brachytherapy sources. Low dose rate (LDR) ribbons of 192 Ir or 137 Cs were the most popular devices for interstitial brachytherapy until remote high dose rate (HDR) afterloaders were developed. Compared to LDR brachytherapy, HDR afterloaders reduce the exposure to physicians and staff treating the patient, reduce the time a patient has to spend immobile as an inpatient, and allow more flexibility in customizing treatment plans. In addition, the number of sources that need to be kept, calibrated, and stored securely is significantly reduced in HDR compared to LDR.

The transition from LDR to HDR raised some concerns about the change in cell kill based on the difference in radiobiology between the two methods. Therefore, some centers developed a transitional pulsed dose rate (PDR) delivery method, in which HDR afterloaders were used to deliver fractions of the total dose at preset time intervals, typically 1 hour. As outcome and toxicity data from interstitial HDR have matured, PDR and LDR treatments are being phased out of clinical use.

Interstitial brachytherapy can be used either as monotherapy or as an adjuvant boost to external beam radiotherapy. Its strength lies in delivering highly conformal doses with steep dose gradients to tumors that are not surgically resectable because of the close proximity to critical organs. Interstitial brachytherapy can also be used in patients who are not surgical candidates. These advantages have to be carefully weighed against the risks of the procedure, including risk from local anesthesia, infection, and patient immobilization over the duration of treatment.

21.2

Clinical Targets

Gynecological Targets

This section focuses on diseases treated with interstitial brachytherapy. Earlier stage disease is sometimes treated with intracavitary implants (e.g., tandem and ovoid); more information can be found in Chapter 20 .

Interstitial brachytherapy for the treatment of cervical cancer is used as a last resort if external beam treatment does not reduce the tumor size sufficiently to get satisfactory coverage with an intracavitary implant. There are several options for designing the interstitial implant, including those that are a blend of intracavitary with interstitial components:

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    Tandem & ring (T&R)/tandem & ovoid (T&O) with two to three free-hand interstitial needles

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    T&O applicators with combined interstitial channels

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    Perineal implant with central vaginal cylinder, tandem and concentrically arranged catheter template. A common applicator is the Syed-Neblett template consisting of 12 interstitial catheters around a central cylinder, though other applicators are available.

Though it is more invasive, planning studies suggest a better coverage with the latter applicator. A variety of dose and fractionation schedules are used in interstitial brachytherapy for gynecologic cancers, ranging from 1 to 8 fractions, either daily or twice daily (BID), typically to biologically weighted dose of 75 to 85 Gy.

For vaginal cancer, the American Brachytherapy Society (ABS) consensus guidelines for interstitial brachytherapy for vaginal cancer recommend that patients with bulky disease (≥0.5 cm) be considered for interstitial implants. Because of the rarity of the disease, the consensus guidelines are based on the results of single-institution studies since no randomized prospective trial data are currently available. A cylindrically symmetric perineal implant template with a vaginal cylinder is recommended for this type of implant. The typical needle spacing is not more than 1 cm between needles. Table I of the consensus documents summarizes the dose fractionation schemes used by experts or reported in small single-institution studies. Dose fractionation schemes include 2 Gy × 18 fractions of external beam radiation therapy (EBRT) followed by 6 Gy × 5 fractions of brachytherapy or 1.8 Gy × 28 fractions of EBRT followed by 7 Gy × 3 fractions of brachytherapy.

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