Brachytherapy delivers radiation from sealed radioactive sources placed inside or immediately adjacent to a tumor, rather than from an external beam. The spaces that house it — high-dose-rate (HDR) afterloader suites, low-dose-rate (LDR) shielded inpatient rooms, source storage/hot labs, and applicator preparation areas — are governed less by the megavoltage-photon physics that drives a linear-accelerator vault and more by the gamma energy of specific isotopes, the regulatory regime of the U.S. Nuclear Regulatory Commission (NRC) or an Agreement State, and the operational reality that a live radioactive source is being moved, inserted, and recovered by hand or by machine in a clinical environment. This article covers how those rooms are programmed, shielded, ventilated, and equipped, and where they differ from the external-beam vaults documented elsewhere in this Part.

Brachytherapy modalities drive the room program

The room you build depends almost entirely on the delivery modality the cancer program intends to offer. The three broad families have materially different space, shielding, and licensing implications.

• High-dose-rate (HDR) remote afterloading. A single high-activity sealed source — almost universally iridium-192 (Ir-192), occasionally cobalt-60 — is driven out of a shielded afterloader unit on a cable, through transfer tubes, into applicators positioned in the patient, then retracted. Treatments are short (minutes), delivered in a dedicated shielded room, and the patient leaves the same day. HDR is the dominant modern modality and is the room most cancer centers prioritize.
• Low-dose-rate (LDR) and permanent-seed implants. Lower-activity sources (e.g., iodine-125 or palladium-103 seeds for prostate; cesium-137 historically for gynecologic LDR) are placed in or near the tumor. Permanent seed implants are often done in an OR or procedure room and the patient is discharged. Temporary LDR implants historically required a shielded inpatient room where the patient stayed for hours to days while the sources were dwelling — a programmatic and shielding burden that has pushed most programs toward HDR.
• Pulsed-dose-rate (PDR) and electronic / miniature-x-ray brachytherapy. PDR mimics LDR biology using an HDR-style stepping source on a pulsed schedule. Electronic brachytherapy uses a miniature x-ray source (no radioactive material, no NRC source license) and can substantially relax shielding and storage requirements — an increasingly relevant option for skin and some intra-operative applications.

A realistic brachytherapy build therefore starts with a written modality and isotope list from the radiation oncology physicist and medical director, because that list determines whether you need an NRC/Agreement-State materials license, a shielded source safe, a hot lab, an inpatient LDR room, or merely a procedure room with an electronic applicator.

The HDR afterloader suite is the anchor space

For most contemporary programs the central space is a dedicated HDR treatment room. Functionally it resembles a hybrid of a procedure room and a small imaging room, with shielding sized to the isotope rather than to a megavoltage beam.

Typical components of the suite:

• Treatment room large enough for a treatment table/couch, the afterloader unit, applicator and transfer-tube routing, imaging access (mobile C-arm, in-room fluoroscopy, or proximity to CT), anesthesia/sedation support for gyn and other applicator placements, and clear staff egress away from the source path.
• Shielded control area / console alcove with a viewing window (leaded glass) or closed-circuit camera, intercom, and the afterloader console, positioned so staff are never in the room during source dwell.
• Door interlock and area radiation monitor that pause/retract the source if the door is opened, plus an independent in-room area radiation monitor visible to staff before entry.
• Source storage — the afterloader itself contains the shielded source safe; programs that exchange sources on a quarterly schedule (Ir-192 has a ~74-day half-life and is typically replaced every 3–4 months) need a defined, secured source-exchange workflow and a place to stage the old/new source for transport.

Because HDR rooms are smaller and the gamma energy of Ir-192 is far lower than a 6–18 MV photon beam, shielding is dramatically less massive than a LINAC vault — often achievable with reinforced masonry, high-density block, or modest poured concrete and leaded doors rather than the multi-foot monolithic pours and engineered mazes an external-beam vault requires. The shielding still must be designed by a qualified medical physicist to NRC/state dose limits, but the structural and cost profile is in a different category from the onc linac vault and maze discussed elsewhere in this Part.

Shielding is isotope-specific and physicist-designed

Brachytherapy shielding follows the same governing radiation-protection framework as external beam — the National Council on Radiation Protection & Measurements (NCRP) reports for design methodology, and the dose limits of NRC 10 CFR Part 20 (or the equivalent Agreement-State regulations) — but the inputs are entirely different.

Key design principles:

• Energy and source strength, not beam workload. The barrier calculation is driven by the isotope's gamma energy and the air-kerma strength and use pattern of the source(s), not by a linac's primary/scatter/leakage components. Ir-192's average photon energy (~380 keV) attenuates in far less material than megavoltage photons.
• No primary-beam barrier and no maze in the LINAC sense. Because the source radiates isotropically rather than as a collimated beam, there is no "primary barrier" wall and no neutron-capture concern; the maze/labyrinth engineering and the heavy shielded vault door of a LINAC are generally unnecessary. A conventional leaded or shielded swing door with an interlock usually suffices.