Brachytherapy

Brachytherapy (from the Greek term "brachio" which means short) is the subspecialty of radiation oncology which uses selected radioisotopes and specialized instruments to directly administer radiation to a tumor mass or bed. The radiation source is placed either adjacent to the surface of a tumor mass or bed or inside the mass itself. Depending on the technique employed, the treatment may involve a permanent implantation of a radioactive source (eg, permanent I125 seeds injected into a recurrent nasopharyngeal mass) or may involve temporary exposure after which it is removed (eg, intracavitary insertion of I125 seeds for a localized recurrent nasopharyngeal carcinoma, temporary interstitial catheter implantation for afterloading with Ir192 sources for a neck mass or tumor bed).

Brachytherapy radiation travels only a short distance to the desired target region. Its dose intensity has a rapid falloff with distance according to the

Table 21-4. TYPES OF HEAD AND NECK BRACHYTHERAPY

Permanent

Temporary

LDR I125 implanted into a mass by free hand technique Interstitial implantation of catheters to the tumor bed or mass afterloading using an applicator (eg, recurrent nasopharyngeal lesion) with Ir192 (eg, squamous cell carcinoma of the lip or base of tongue)

LDR I125 Vicrylâ„¢ suture placed into tumor bed Intracavitary insertion of brachytherapy instruments afterloading

(eg, resected neck mass with close or positive margins) with high activity I125/LDR or HDR Ir192 (eg, recurrent nasopharyngeal cancer) Intraoperative radiation therapy (IORT) using HAM applicator applied to the tumor bed using HDR Ir192

LDR = low dose rate; HDR = high dose rate; HAM = Harrison-Anderson-Mick applicator.

inverse square law (I ^ 1/D2) which allows for a sharp decrease in the dosage in the surrounding normal tissue. The dose rate can be low and continuous (low dose rate = LDR with a rate of 40 to 200 cGy/hour) or high and administered in a hyperfrac-tionated fashion (high dose rate = HDR with dose rate > 1,200 cGy/hour). The radiation dose is delivered to a relatively small, well-defined volume.

Radiobiologically, low dose rate brachytherapy treatment is a continuous low dose rate of radiation and is likened to fractionated radiation with an infinite number of small individual doses. This allows redistribution of the tumor cell within the cell cycle, resulting in a greater percentage of malignant cells in the more radiosensitive phases. However, this probably has little clinical consequence. This approach also allows time for reoxygenation of hypoxic cells during the treatment and thus results in an increase in their radiosensitivity. This effect varies tremendously between different tumor types. Repair is the major radiobiological factor in low dose rate brachytherapy. This favors late responding normal tissues relative to tumors. Repopulation occurs but unfortunately benefits tumors.

High dose rate treatments need to be well-fractionated (hypofractionation with only 1 to 3 fractions per week) as this approach is at a greater risk for complications with late responding tissues, particularly since the dose per fraction is usually rather high.

Brachytherapy may be used as the primary treatment or more commonly as a local boost in conjunction with external beam radiation therapy and perhaps surgery. It is usually employed with a curative intent but can be used for highly selected patients needing palliative care. The isotopes most commonly used in head and neck cancer treatments are I125 and Ir192 (Tables 21-4 and 5). The implants can be planar (single plane) or volumetric (more than one parallel plane separated by 1 to 1.5 cm). Pretreatment planning with respect to any surgical or anatomic considerations as well as dosimet-ric concerns is critical.

Head and neck tumor sites commonly considered for possible brachytherapy include the lip, floor of mouth, oral tongue, base of tongue (Figure 21-19), buccal mucosa, tonsillar region, nasopharynx, skull base and neck nodes (Figure 21-20) (Table 21-6). The size and volume of the primary lesion, anatomic extent, topography, adjacent vital organs, any prior therapy, and the medical and psychologic condition of the patient must be critically evaluated when considering a case for possible brachytherapy intervention. In the oral tongue and floor of mouth regions, while T1 and T2 lesions can be treated with brachytherapy alone or with external beam radiation therapy with good therapeutic results, the risk of possible complications from treatment such as soft-tissue or bone necrosis must be weighed against consideration of a primary surgical approach with its low complication rate. In the case where the radioactive sources are in close proximity to the mandibular mucosa and bone, the risk of complications greatly increases and thus this would be a contraindication to brachytherapy. If an implant cannot adequately encompass the tumor region with margin or if the

TABLE 21-5. HEAD AND NECK BRACHYTHERAPY

ISOTOPES

Isotope

Type of Radiation

KeV Energy

Half Life

I125

X-rays

27-32

60 days

Ir192

Gamma rays

340

74 days

TABLE 21-6. COMMON HEAD AND NECK BRACHYTHERAPY PROCEDURES

Site

Stage

Procedures

Lip

T1, T2 (small)

Interstitial implant alone, EBRT + interstitial implant

Oral tongue

T1 (small)

Interstitial implant alone

T1, T2, T3

EBRT + implant

Base of tongue

T1, T2, T3, T4

EBRT + implant

Floor of mouth

T1 (small)

Interstitial implant alone

T1, T2

EBRT + interstitial implant

Buccal mucosa

T1, T2 (early)

Implant alone

T2, T3

EBRT + interstitial implant

Base of tongue

T1, T2,

EBRT + interstitial implant

Nasopharynx

Recurrent T1, T2

EBRT + intracavitary insertion

Neck nodes

Adherent to carotid artery

Surgery with intraoperative planar implant of I125 Vicrylâ„¢ suture permanent seed

implant or placement of afterloading catheters or IORT with HDR Ir192 (add EBRT with all of the above if never irradiated)

implant or placement of afterloading catheters or IORT with HDR Ir192 (add EBRT with all of the above if never irradiated)

Figure 21-19. Base of tongue implant

Figure 21-20. Locally recurrent squamous cell carcinoma in the neck following prior external beam radiation therapy. The mass was resected but was fixed to underlying tissues. A, A permanent low dose rate I125 seed implant was performed. B, A lateral film shows the seed implant.

region is not technically approachable, then brachytherapy should not be performed.

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