The prescription varies based on the pathology/extent of disease. Examples of common prescriptions include 54 Gy in 27 fx, and 46 Gy in 23 fx with a sequential boost of 14 Gy in 7 fx.
You may need to import an MRI or previous CT images, and perform a fusion afterwards. The “T2 Flair” and “T1 Post Contrast” MRI will help the physician delineate the target volume, and a previous CT may show changes in the target volume over time (e.g. preoperative vs. postoperative).
Patients are simulated in the supine position. The patient will lie 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 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 for IMRT or 0.625 mm for SBRT, 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.
Create a structure set that includes the following structures:
- GTV – Gross Tumor Volume
- CTV – Clinical Target Volume
- PTV – Planning Target Volume
- 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”.
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 target volumes for this case. The physician may re-name these and/or contour additional structures. If the CTV is contoured, but the PTV is not, ask the physician for instructions to create the PTV. For example, the physician may request for an outer 0.5 CM margin around the CTV to create the PTV. Do NOT modify physician drawn contours.
Refer to the link below for a broad setup overview:
- 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). For SRS cases, it may be warranted to avoid rounding off the isocenter shifts. Avoid using shifts if there is no significant dosimetric advantage.
- Determine the angles of your arcs. Partial arcs are generally used in VMAT brain cases to spare as much healthy brain tissue as possible, and sometimes to completely avoid dose to the lens. However, in cases where the volume is centrally located, full arcs can be used.
- Rotate the collimator angles of your arcs to best fit the PTV. MLC leaf specification vary for every LINAC, but for SBUH’s machines, 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 if the target volume is longer than 15 CM.
- Adjust the field size in the BEV to fit the tightly to the PTV throughout the entire length of the rotation.
- Enable jaw tracking if available on the LINAC.
- Prioritize coverage of the target volume above all else since this plan requires at least 95% of the prescription dose to cover at least 95% of each individual volume.
- Scale down constraints if a boost will follow the primary treatment. For example:
- For a prescription of 46 Gy with a 14 Gy boost, the optic nerve constraint of a 54 Gy max point dose will easily meet on the primary plan. However, if the dose to be deposited from the boost plan is unaccounted for, the optic nerve may receive over 54 Gy in the plan sum. By taking the percentage of the dose from the primary plan relative to the total dose, multiplying it to the constraint will give an adjusted dose constraint to meet for the primary plan (e.g. 46/60 = 75%*54 = 40.5 Gy). In certain cases, the primary volume will be close to an OAR, but the boost volume will be far away from the OAR so scaling the constraints may be unnecessary.
- Utilize an arc with a couch rotation if it provides significant dosimetric advantage. For example, rotating the couch by 90° is commonly seen for target volumes that reside in between the orbits which help spare the dose to surrounding OARs. Additionally, utilizing a couch rotation may allow you spare more healthy brain tissue as well. However, be aware that introducing couch rotations will spread low dose more superiorly and inferiorly, and will also lengthen treatment time.
- If all objectives and constraints are met and there is opportunity to further preserve organs at risk, you may desire to proceed with the following:
- Optimize to the conformity index to be as close to 1.00 as possible. This is achieved by manipulating the 100% isodose line to be as tight to the target volume as possible.
- Further minimize the max point dose delivered to the lens without altering coverage.
- Further minimize the max point dose to any other OAR without significantly altering coverage.
- Further minimize the volume outside the PTV that receives 50% or greater dose.
Refer to the link below for more information on optimization:
In general, a brain plan must meet the following constraints:
- At least 95% of the prescription dose covers at least 95% of the PTV.
- The maximum hot spot or “3D Dose Max” cannot exceed 110% prescription dose.
- The max point dose to the spinal cord should not exceed 45 Gy.
- The max point dose to the spinal cord + 5mm should not exceed 48 Gy.
- The brain has two constraints:
- The max point dose to the brainstem should not exceed 72 Gy.
- The volume of the brain that receives 60 Gy should not exceed 33%.
- The brainstem has two constraints:
- The max point dose to the brainstem should not exceed 59 Gy.
- The volume of the brainstem that receives 54 Gy should not exceed 100%.
- The cochleas have two constraints:
- The max point dose to the cochleas should not exceed 45 Gy.
- The mean dose to an individual cochlea should not exceed 35-45 Gy (Depending on chemotherapy).
- The eyes have three constraints:
- The max point dose to an eye should not exceed 45 Gy.
- The volume of an eye that receives 30 Gy should not exceed 40%.
- The mean dose to an eye should not exceed 35 Gy.
- The lenses have two constraints:
- The max point dose to a lens should not exceed 8 Gy.
- The volume of a lens that receives 6 Gy should not exceed 50%.
- The optic nerves and optic chiasm have two constraints:
- The max point dose to the optic nerve and chiasm should not exceed 54 Gy.
- The volume of an optic nerve or the optic chiasm that receives 50 Gy should not exceed 100%.
- The pituitary gland has two constraints:
- The max point dose to the pituitary gland should not exceed 50 Gy.
- The volume of the pituitary that receives 45 Gy cannot exceed 100%.