5 Minute Tutorials: Hippocampal Sparing Planning

Pre-simulation

The physician may opt for hippocampal sparing as opposed to 3D whole brain radiation therapy (WBRT) for the purpose of sparing neurocognitive function. Instead of using two lateral fields, hippocampal sparing will utilize VMAT to treat the whole brain while limiting dose to the hippocampus. Hippocampal sparing cases have a prescription of 30 Gy in 10 fx.

You may need to import an MRI for the physician to delineate the hippocampus.

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. A thermoplastic mask is placed over the head and attaches firmly onto the table. This mask is used to ensure treatment reproducibility and also allows for shorter setup times during treatment. The properties of the mask are as such that it becomes elastic as it warms and hardens as it cools. The therapists will warm the mask in an oven and stretch it over the head of the patient. It is particularly important that the mask properly secures the chin as it will prevent neck flexion. They will gently press on the mask with their fingers to conform the mask securely to the shape of the head.

Once the mask hardens, the therapists will place BBs on the mask at the AP and the laterals of the same Z plane in the center of the brain. 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.

Once the mask hardens, the therapists will proceed to align the patient and use the lasers of the CT machine to place radiopaque BBs on the mask at the AP and the laterals of the same Z plane in the center of the brain. The CT scan will then take place using a slice thickness of 2.5 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:

PTV – Planning Target Volume
Body
Brain
Brainstem
Two unique structures: “Cochlea LT” and “Cochlea RT”
Two unique structures: “Eye LT” and “Eye RT”
Two unique structures: “Lens LT” and “Lens RT”
- Set each to a high Resolution Structure
Three unique structures: “Optic Nerve LT”, “Optic Nerve RT”, and “Optic Chiasm”.
- From these structures, create the following unique structures using the margin function: “Optic Nerve LT +3mm”, “Optic Nerve Rt +3mm”, and “Optic Chiasm +3mm”.
Pituitary Gland
Two unique structures: “Spinal Cord”
- From this structure, create the following unique structure using the margin function: “Spinal Cord + 5mm”.
Two unique structures: “Hippocampus”, “Hippocampus + 5mm”

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 contour the hippocampus, and you will need to create a 0.5 CM margin around the hippocampus to create the “Hippocampus + 5mm”. The PTV structure will crop the “Hippocampus + 5mm” out of the brain volume for optimization and DVH analysis purposes. Do NOT modify physician drawn contours.

Planning Setup

Create at least two rotational therapy fields (arcs). For any VMAT case, the number of arcs to use will depend on how much dose modulation you expect will be needed. Your brain case may have a target volume which abuts or is nearby a radiosensitive structure, and may require significant modulation. Thus, using 2-4 arcs with varying collimator angles may be recommended. Keep in mind that any additional arcs you create should provide a significant dosimetric advantage as the patient will be kept on the treatment table for a greater duration.
Properly name the fields based on the beam number and gantry direction (e.g. 02 RA CCW, 07 RA CW). Remember to check for previous treatments in the process of naming beams.
Align the fields to the largest target volume and round off the coordinates to the nearest decimal place (e.g. +2.4 CM shift, not +2.48 CM shift). Avoid using shifts if there is no significant dosimetric advantage.
Unlike non-hippocampal Brain VMAT cases where planners look to use partial arcs when applicable to spare as much healthy brain tissue and avoid OAR as possible, full arcs are used in hippocampal sparing cases to encompass the entirety of the brain.
Rotate the collimator angles of the arcs to best fit the PTV and common complementary angles include 85° and 95°. Collimator angles of 85° and 95° are used as opposed to say 5° and 355° because the MLC leaves can travel a lesser distance to block dose to the hippocampus which is discussed in greater depth in the following step. Nonetheless, in general, it is recommended to weigh the following factors:
- The thickness of the MLC leaves. The central leaves are thinner and can therefore modulate the dose more intensely and precisely. Thus, the central leaves should be placed in a location which requires the most dose modulation.
- The distance the MLC leaves of the X-jaw travel. MLC leaves of the X-jaw traverse a lesser distance (15 CM) relative to MLC leaves of the Y-jaw, and so it may be dosimetrically advantageous to limit the distance traveled by these leaves.
Adjust the field size in the BEV to fit tightly to the PTV throughout the entire length of the rotation. Having done so, beginning with one arc, close the inferior border of the field size to the inferior outline of the hippocampus contour. For the second arc, close the superior border of the field size to the superior outline of the hippocampus contour. Repeat this step for any additional arcs by which the MLC leaves travel in the superior-inferior direction. Now increase the field size on the field edge which was just closed such that all field sizes overlap enough to prevent cold spots. As opposed to a field size which envelops the entire brain, using this half field size technique allows for the MLC leaves to traverse a shorter length to block the hippocampus.

Optimization Tips

Enable jaw tracking if available on the LINAC.
To meet the constraints of the hippocampus, you may need to assign it and its PRV higher priorities relative to your other objectives.
It is common to notice significant holes in your dose distribution which span numerous slices and is due to heavy modulation at the hippocampus for which the target volume surrounds it. Thus, it can be difficult to achieve adequate PTV coverage, and you may desire to use optimization structures. Contouring out regions where dose is lacking, and assigning lower objectives onto these structures may help with improving coverage to these regions.
Keep hotspots off of the normal structures within the PTV if possible. Though there are not specific dose constraints for the lens, you may desire to avert dose to these structures.

In general, a brain plan must meet the following constraints:

At least 100% of the prescription dose covers at least 90% of the PTV.
There is no maximum point hot spot or “3D Dose Max” point constraint, but try to limit it to around 120%.
The maximum dose to 2% of the PTV (D2%) is 37.5 Gy.
The minimum dose to 98% of the PTV (D98%) is 25 Gy.
The hippocampus has two constraints:
- The maximum point dose to the hippocampus must not exceed 16 Gy.
- The dose to 100% (D100%) of the hippocampus must be less than or equal to 9 Gy.
The maximum dose to the optic nerves and chiasm must not exceed 37.5 Gy.