3.10 Brachytherapy Principles, HDR, LDR & Isotopes

Radiobiological Principles of Brachytherapy

Brachytherapy (derived from the Greek word brachys, meaning short-distance) involves placing sealed radioactive sources directly inside (interstitial), within body cavities (intracavitary), or on the skin surface (surface mold) adjacent to a tumor volume.

       [ Inverse Square Law ]                  [ Clinical Implantation Modes ]
 Dose Rate ∝ 1 / r²                     • Interstitial: Seeds / Needles directly in tissue
 High tumor dose + Steep dose falloff   • Intracavitary: Tandem & Ovoids in body cavities
 Spares adjacent normal tissues         • Surface Mold: Custom plaque on skin / eye

The Inverse Square Law Advantage

Brachytherapy relies on the Inverse Square Law ($I \propto 1/r^2$). Because radioactive sources are embedded directly inside or against the tumor, the dose rate is extremely high within the tumor but drops off exponentially with distance ($r$). This produces unparalleled normal tissue sparing.

Dose Rate Categories

• Low Dose Rate (LDR): $0.4\text{ to }2.0\text{ Gy/hour}$. Delivered continuously over several days (e.g., permanent prostate seeds, historical inpatient tandem & ovoids).
• High Dose Rate (HDR): $> 12.0\text{ Gy/hour}$ (typically $100 - 300\text{ cGy/min}$). Delivered in brief outpatient fractions lasting several minutes using a computerized remote afterloader.

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Clinical Implantation Techniques

1. Interstitial Brachytherapy: Radioactive sources (seeds, wires, or hollow needles) are inserted directly into tumor tissue (e.g., permanent prostate seed implants, temporary breast implants, soft tissue sarcomas, base of tongue).
2. Intracavitary Brachytherapy: Applicators housing radioactive sources are placed inside anatomical body cavities near the tumor (e.g., Tandem and Ovoids / Tandem and Ring for cervical carcinoma, vaginal cylinder for endometrial cancer).
3. Surface Mold (Plaque) Brachytherapy: Radioactive sources are affixed to custom acrylic molds or eye plaques placed directly over superficial lesions (e.g., Iodine-125 plaques for ocular melanoma, custom skin molds).

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Classical Dosimetry Systems: Manchester and Paris

1. Manchester (Paterson-Parker) System

Developed for LDR implants to deliver a uniform dose (within $\pm 10%$) across a target plane. Employs non-uniform source distribution, placing higher radioactive activity near the margins to counterbalance central dose falloff.

                    Uterine Cavity
                         ||  <-- Central Tandem
                         ||
             Point A --> *  (2 cm Superior, 2 cm Lateral to Cervical Os)
                        /||\
        Point B --> *  / || \  * <-- Point B (5 cm Lateral to Midline)
                      (  ||  )
                     [Ovoid] [Ovoid]

Manchester Gynecologic Prescription Points

• Point A: Located 2 cm superior to the external cervical os along the tandem, and 2 cm lateral to the uterine tandem. Represents the anatomical intersection of the uterine artery and ureter. Point A is the primary prescription point for cervical brachytherapy.
• Point B: Located 2 cm superior to the os and 5 cm lateral to patient midline (3 cm lateral to Point A). Evaluates dose to obturator lymph nodes and pelvic side wall.

2. Paris System

Designed primarily for interstitial wire and line source implants (e.g., Iridium-192 wires). Mandates uniform source activity per unit length and uniform parallel source spacing. The prescription dose is specified as 85% of the basal dose (the average dose calculated at midpoints between radioactive sources).

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Physical Characteristics of Radioactive Isotopes

ARRT exam note: Official Radiation Therapy content specifications state that only basic brachytherapy concepts are covered (including dose to surrounding tissue and radiation protection). Specific procedures and isotope characteristics are not covered. Use any isotope table below as clinical context only—not as a memorization checklist for the exam.

Radioactive source decay follows the exponential formula:

$A(t) = A_0 e^{-\lambda t} = A_0 \left( 0.5 \right)^{\frac{t}{t_{1/2}}}$

Where $A_0$ is initial activity, $\lambda = \frac{\ln 2}{t_{1/2}}$ is the decay constant, and $t_{1/2}$ is physical half-life.

• Iridium-192 ($^{192}\text{Ir}$): Standard isotope for HDR remote afterloaders. A single high-activity (10 Ci / 370 GBq) seed attached to a drive cable moves through catheters. Replaced every 90 days (~3 months) due to its 73.83-day half-life.
• Iodine-125 ($^{125}\text{I}$): Standard low-energy isotope for permanent prostate seed implants ($t_{1/2} = 59.4\text{ days}$).
• Palladium-103 ($^{103}\text{Pd}$): Short half-life isotope ($17.0\text{ days}$) delivering a higher initial dose rate, preferred for high-grade or fast-growing prostate malignancies.

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AAPM TG-43 Brachytherapy Dosimetry Formalism

AAPM Task Group 43 establishes the standard mathematical calculation for dose rate $\dot{D}(r, \theta)$ at point $(r, \theta)$ around a brachytherapy source:

$\dot{D}(r, \theta) = S_K \cdot \Lambda \cdot \frac{G_L(r, \theta)}{G_L(r_0, \theta_0)} \cdot g_L(r) \cdot F(r, \theta)$

Where:

• $S_K =$ Air-Kerma Strength of the source (in units of $U = \mu\text{Gy}\cdot\text{m}^2/\text{h}$).
• $\Lambda =$ Dose Rate Constant in water at reference point ($1\text{ cm}, 90^\circ$), in $\text{cGy}/(\text{h}\cdot U)$.
• $G_L(r, \theta) =$ Geometry Factor accounting for spatial activity distribution (line vs point source model).
• $g_L(r) =$ Radial Dose Function accounting for tissue absorption and scatter along the transverse axis.
• $F(r, \theta) =$ Anisotropy Function accounting for self-attenuation within encapsulation at oblique angles.

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Remote Afterloader Safety Protocols

HDR remote afterloaders propel a single high-activity (10 Ci / 370 GBq) $^{192}\text{Ir}$ seed attached to a flexible drive cable through transfer tubes into implanted catheters under computerized stepper motor control.

NRC Mandated Safety Features

1. Emergency Manual Retract Crank: Mechanical hand crank mounted on the afterloader to manually draw the source back into the shielded tungsten safe if power or motor fails.
2. Door Interlocks: Automatically retracts the source instantly into the safe if the treatment room door opens.
3. Radiation Area Monitor: Independent wall-mounted Geiger monitor with battery backup confirming source retraction.
4. Emergency Equipment: Lead storage container (pig), long-handled forceps, and wire cutters kept inside the vault at all times.