Part 6: Treatment Delivery Accessories

Bolus

• Often made from materials that simulate human tissue, such as water-equivalent gels, paraffin wax, or commercially available flexible rubber-like substances.

• It is applied directly to the patient’s skin and conforms to the body’s surface.

• Effectively increases the dose at shallow depths by making the radiation behave as if the skin is closer to the source of radiation, often used for superficial tumors.

Wedge Filters

~ Typically made from dense materials like steel, brass, or lead, chosen for their ability to absorb and attenuate radiation.

~ Has a sloped or angled shape, which causes radiation beam to have a gradient in intensity across the treatment field.

~ The thickness varies across its length, modulating the dose delivered to the patient.

Dynamic Wedge

~ Computerized field shaping to modify dose intensities by changing the dose distribution

~ Uses the secondary collimators or jaws just above the MLCs

~ Creates a 15, 30, 45, or 60 degree hard wedge dose distribution by controlling the jaws and the dose rate.

Hard Wedges

~ Attenuates the beam to help push dose away or towards a structure.

~ 15, 30, 45, or 60 degree changes the dose distribution in the patient

~ Slides into accessory mount; x is width, y is length; heel to toe portion is y

Beam Hardening

When the radiation beam passes through something, lower-energy photons are absorbed or attenuated in the material and higher energy photons pass through the material. This changes the average energy of the photon beam by making it a higher energy. This happens with Bolus, hard wedges, and compensators but NOT dynamic wedges

Compensators

~ Made from materials such as aluminum, brass, or acrylic, which attenuate radiation while maintaining precision in dose shaping.

~ Designed to fit the contours of a patient’s body or specific anatomical structures

~ They are customized to adjust the beam intensity so that the dose delivered accounts for tissue thickness and irregularities, ensuring uniform dose distribution

Transmission Factor

~ Measurement of the percentage of the radiation beam that passes through an object or material.

~ This needs to be calculated when using beam modifying devices.

~ Dose Rate/Initial Dose Rate (w/o modification)

Cerrobend Block and Electron Cutouts

~ Used to block photons (thicker) and electrons(thinner)

~ Composed of bismuth, lead, tin, and cadmium

~ Used often before MLCs; now rarely used

~ Slides into the accessory mount; they can be very heavy

Primary Collimator (Jaws)

~ Gives the beam its first shape

~ Largest field is 40cm x 40cm projected at isocenter

Multileaf Collimator (MLCs)

~ 80-160 leaves, 120 being common; more leaves = finer beam modulation & precise shaping

~ Located below secondary collimation

~ Use computer controlled metal leaves that adjust to create the field shape required for treatment, ensuring precision in radiation delivery

~ Each leaf operates on independent motors

Penumbra

The areas of partial dose beyond the sharp edges of the field, a common challenge in radiation therapy, and occurs at the edges of the leaves.

Electron cones

~ Primarily used for superficial treatments, such as skin cancers or shallow tumors. Electrons don’t penetrate as deeply as photons, making them ideal for treating conditions near the surface of the body.

~ Metallic cone shaped devices that attach to the linear accelerator

~ Primary function is the collimate the electron beam to match the treatment area.

~ Cutouts are often placed at the end of these to create a field that matches the tumor shape exactly

Snout

~ Used in conjunction with beam nozzle systems in proton or ion therapy

~ Serves as a guide for the beam, defining the treatment field, similar to how electrons are used in electron therapy

~ Holds range shifters, apertures, and compensators

~ Acts as an extension of the beam line and is placed close to the patient to minimize beam spread and maintain precision in targeting

Surface-Guided Radiation Therapy (SGRT)

~ Involves the use of 3D surface imaging systems to monitor a patient’s position before and during RT

~ Ensure that the patients setup is consistent with the treatment

~ It requires initial setup which is used as a 3D reference. This 3D reference is compared before each treatment to the patient’s surface scan.

~ Ensures the patient is properly aligned prior to treatment. It monitors movement during treatment (intrafractionated) where therapists can track and determine if the radiation beam needs to be paused for re-alignment.

Couch Indexing

~ A procedural approach to placing immobilization devices on the treatment couch

~ The treatment table has marked areas that a car (indexing bar) can be secured

~ The indexing bar has two raised prongs that an immobilization device is fixed to.

~ This helps with treatment accuracy because the device is in the same spot everyday, instead of sliding up or down the table, which in turn allows therapists to accurately record table parameters

~ This helps catch set up errors

Thermoplastic Masks

~ Used for brain and head & neck treatments mainly

~ Material becomes malleable when heated and hardens as it cools

~ Placed in a water bath or oven to a certain temperature

~ Once soft it is molded to patient’s head, neck, and possibly shoulders.

~ After a few minutes the material cools and hardens creating a custom made mask for the patient that can be worn for each treatment session

~ Attached to the treatment table using a frame that is indexed

Vacuum Cushions (Vac-Lok bags)

~ Used for all areas of the body but seem most often for thorax, abdomen, pelvis, and extremities

~ A vacuum sealed bag filled with small polystyrene beads

~ Like a bean bag, it’s placed under the area being treated and the patient is positioned

~ The air is then removed from the vacuum bag, creating a fixed mold around the patient

~ Can be indexed on the treatment table and used for every treatment

~ After the patient has completed all treatments the bag can be disinfected and air can be restored ready to be used for the next patient.

Wing Boards & Breast Boards

~ Used for thorax treatment, either lung, or breast

~ Generally made of carbon fiber

~ The board can have an angle or not and their are arm rests and adjustable handles to provide comfort and immobilization

~ These boards can be indexed and while they have movable pieces they are not custom made for each patient.

Knee & Foot Supports

~ Can be a position aid or an immobilization device depending on device and treatment location

~ Used to stabilize the lower back and hold the lower extremities in a certain position. Can also be used solely for comfort

~ Usually a preformed supportive cushion design that can but typically is not indexed

~ Universal form that is not custom

Belly Boards

~ Used for patients in the prone position for abdomen, pelvic patients especially rectal

~ Indexed on the treatment table and the patient is also adjusted on the board to a particular number

~ The goal is to have the belly (small intestines) fall into the open area in the board and away from the treatment field, since the small bowel is a dose limiting structure

Bite Blocks and Mouthpieces

~ Used for head & neck treatments to immobilize the jaw or tongue, ensuring that internal structures stay in place.

~ Can be generic or custom-made from dental impression material or soft plastics

~ These devices can be reused throughout the treatment course

Body Casts (Alpha Cradle)

~ Used for thorax, abdomen, pelvis, extremities

~ Polyurethane foam that expands and hardens when activated

~ Patient lies on a bag filled with two liquid components that, when mixed, begin to foam and expand

~ As foam expands, it conforms to the patient’s body, creating a custom-molded cast

~ The foam harden to form a rigid support, perfectly shaped to the patient’s body. This cast is reused throughout the treatment course to ensure precise, reproducible positioning.

~ NOT reusable

Orthogonal Images

~ A set of two images taken at 90 degrees apart from each other