Pre-simulation
Whole Brain Radiation Therapy (WBRT) is commonly treated with a prescription of 30 Gy in 10 fx or 20 Gy in 5 fx. WBRT is typically a last resort for the palliation of symptoms due to multiple brain metastases. WBRT patients are commonly scheduled on short notice and have a quick turn-around.
You will not have to import any ancillary images for this plan.
Simulation
Patients are simulated in the supine position. The patient will lie down on the table, and either grasp the pegs by the sides of the table or rest their hands on their abdomen. The therapists will align the patient and place a plastic headrest underneath the head/neck for support.
The therapists will have prepared a warm thermoplastic mask in an oven. This mask is elastic when it is warm, allowing it to conform to the anatomy of the patient. The mask allows for a reproducible and fast setup. It is particularly important that the mask properly secures the chin as it will prevent neck flexion. This thermoplastic mask is stretched over the head and attaches firmly onto the table. They will gently press on the mask with their fingers to conform the mask securely to the shape of the head. The mask then hardens as it cools, locking in the patient’s position.
Once the mask hardens, the therapists will place three BBs on it. There will be one anterior BB and two lateral BBs, ideally located on the same axial plane as the center of the brain. The CT scan will be done with a slice thickness of 2.5-3 mm, and the images will then be pushed to the treatment planning system (TPS). After the simulation, the BBs will be removed and markings will be made in their places for which the therapists will align the lasers to during treatment setup.
Contouring
Create a structure set that includes the following structures:
- Body
- Brain
- “Eye LT” and “Eye RT”
- “Lens LT” and “Lens RT” (high resolution recommended)
- Spinal Cord
The body contour in the area of the nose may need to be fixed as Eclipse may either leave parts of the nose missing or leave holes in the nasal cavity.
The physician will not contour any volumes and will instead set apertures in External Beam Planning.
Planning Setup
Refer to the link below for a broad setup overview:
WBRT is treated using two lateral opposing fields (around 90° and 270°) set to the CT Isocenter. There is no need for a shift. Energies used are commonly 6X to 15X. Certain physicians may request that the gantry of the lateral fields are rotated to match the anterior field edge. This will allow for easy blocking and minimize scatter to the eyes/lens.
Once the fields are set up, starting with one field, create MLC leaves and use the fit to structure function. Input the following parameters and press ok:
- Select the target structure to be the “Brain”.
- Select circular margin, collimator coordinate system, leaf edge-contour meet point = outside, closed leaf meeting position = center, and only check “optimize collimator jaws.
- Give a 1.5 margin with this tool. This 1.5 CM margin will be for penumbra to ensure full dose to the brain.
After using this tool, you should see that the field size, on the axial view and the BEV, have been adjusted to include the entirety of the “Brain”. Manually create a 2 CM superior and a 1 CM anterior/posterior margin on the brain by adding these numbers to the appropriate X/Y field size. These manual margins are created to provide adequate coverage to the entirety of the brain. However, there is still no actual margin despite the field expansion as the MLC leaves are still positioned 1.5 CM from the brain. In the BeV, move the MLC leaves out of the field from the level of the frontal sinus to the superior field edge for both the anterior and posterior field edge.
Some physicians prefer to bring the treatment fields’ inferior border down to cover C1 & C2.
The physician will typically modify the MLC position and field borders. The physician will commonly use the MLC to block the lens and also tighten the leaves to the brain located inferiorly to the eyes. Do NOT modify physician set apertures.
WBRT Planning
After the physician is finished setting apertures, place your reference point in the center of the brain and calculate. Manually move the reference point to a location that gets you the best coverage possible. Ideally, 100% of the brain volume receives as close to 100% of the prescription dose (100/100) but this is not always realistic.
To lower the hotspot:
- Adjust the weighting of the fields.
- Use the field-in-field technique to block the 110% hotspot. Refer to Basic Field-in-Field Guide.
- Settle for a lesser brain coverage such that beam weighting and/or using a field-in-field will reduce the hotspot to 110%, i.e. 95% of the brain receives 100% of the prescription dose.
A different method of achieving optimal coverage without continually moving the reference point is by normalizing to a target volume. As the reference point will not be the prescription point unlike the previous method, it may be placed anywhere within the brain for now. In the dose prescription tab, normalize the prescription to your desired coverage with the “Brain” as the target volume, and the TPS will achieve the set normalization. Then, by moving the reference to the 100% isodose line, and changing the normalization back to “100% at reference point”, you can find the exact location to achieve the coverage you were looking for. For many simple 3D plans, it is unconventional to normalize the prescription to a target volume, even though it is very commonly seen in VMAT planning.
Refer to the link below for more information on field-in-field:
WBRT cases are treated to 20-30 Gy and thus, nearly all the OARs are well within tolerance. In general, a WBRT plan must meet the following constraints:
- The brain volume must receive adequate coverage.
- The maximum hot spot or “3D Dose Max” cannot exceed 110% prescription dose
- The each lens should not exceed a max point dose of 8-10 Gy (depends on physician preference).
- Some physicians prefer that the lens volume which receives 6 Gy does not exceed 50%.
