2 Minute Tutorials: VMAT Prostate Planning

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2 Minute Tutorials: VMAT Prostate & Lymph Nodes Planning

Pre-simulation

Prostate cases are commonly seen with a prescription of 50 Gy to the prostate and seminal vesicles, and 45 Gy to the nodes if nodes are involved, in 25 fx. A sequential boost follows commonly treating just the prostate with 28 Gy in 14 fx, totaling 78 Gy. Certain patients may qualify as candidates for prostate SBRT in which 36.25 Gy is treated in 5 fx to only the prostate.

You will need to import an MRI scan for all prostate plans unless otherwise specified by the physician. It is required to delineate the target volume and will help identify organs at risk such as the penile bulb and the neurovascular bundle.

Simulation

About two weeks prior to the CT simulation, patients may be required for a SpaceOAR and/or fiducial insertion. SpaceOAR is a polyethylene hydrogel solution that is injected between the prostate and the rectum, and will create “space” between the prostate-rectal interface to minimize potential side effects to the rectum. Fiducials are tiny gold seeds that are inserted into the prostate to track intra-fractional motion of the prostate during the course of treatment. If the seeds are moved out of range from their point of reference, the beam will shut off in the midst of treatment.

Patients are simulated in the supine position. The patient will lie on the table to which an inflated vac lock bag is attached. There are pegs at the head of the table for the patient to grab onto. Once the patient is appropriately positioned, the therapists will remove air from the vac lock bag, and doing so will create a mold/outline of the patient’s body contour for treatment reproducibility.

The physician will also request that the patient be simulated with a comfortably full bladder as a full bladder will push the bowel superiorly out of the treatment area and also decrease the volume of the total bladder that receives dose. The therapists will proceed to align the patient and use the lasers of the CT machine to place radiopaque BBs on the laterals and AP of the patient on the same Z plane. 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).

For SBRT patients, a MRI will take place almost immediately following the CT simulation. This is done so that the MRI and CT can be fused as accurately as possible.

Contouring

Create a structure set that includes the following structures:

Body
Bladder
Bladder CF
Two unique structures: “Femoral Head LT” and “Femoral Head RT”
Penile Bulb
Rectum
Small Bowel
Large Bowel
Bowel
SpaceOAR (If present)

For a Prostate, Seminal Vesicles, and Lymph Node plan, include the following structures:

CTV_SV – Clinical Target Volume (Seminal Vesicles)
CTV_LNs – Clinical Target Volume (Lymph Nodes)
CTV_Prostate – Clinical Target Volume (Prostate)
CTV_Prostate_SV – Clinical Target Volume (Prostate & Seminal Vesicles)
PTV_28 – Planning Target Volume for the Boost
PTV_45 – Planning Target Volume for the Primary Plan (Lymph Nodes)
PTV_50 – Planning Target Volume for the Primary Plan (Prostate & Seminal Vesicles)

For a Prostate SBRT case, include the following structures and exclude the target volumes mentioned directly above:

GTV_Prostate
PTV_36 Gy
Fiducials (Will not be defined by a contour, but as a Marker)
Bladder Wall
Neurovascular Bundle
Skin
Urethra

The physician will contour the target volumes for this case. Do NOT modify physician drawn contours.

If the patient has artifact caused from a prosthesis, you should contour out the artifact region in a separate structure. Then assign it a HU of 0 to have this region become equivalent to tissue as the artifact will not be present during the time of treatment. Be sure not to include the metallic or artifact causing structure itself within this structure. It is recommended to avoid depositing dose through this artifact region of the body.

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. Prostate cases will require a sharp dose fall-off by the prostate-rectal interface and/or the involvement of lymph nodes may make for a large PTV volume. Thus, using 3-4 arcs with varying collimator angles may be warranted. 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 PTV 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. If there is lymph node involvement, remember to “Boolean” all the PTV volumes into a PTV TOTAL structure prior to making the alignment.
Determine the angles of your arcs. Full arcs are generally used in VMAT prostate cases. However, in cases without lymph node involvement (e.g. prostate SBRT or prostate IMRT boost), planners may opt to exclude arc rotation along the posterior aspects of the rectum.
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.

Adjust the field size in the BEV to fit the tightly to the PTV throughout the entire length of the rotation.
For Prostate SBRT cases, use the field specifications of 6X-FFF, 1400 MU/min, and a tolerance of SRS_SRT_SBRT.

Optimization Tips

Enable jaw tracking if available on the LINAC.
Prioritize coverage of the target volume above all else since at least 95% of the target volume needs to receive 100% of the prescription dose.
Scale down constraints if a boost will follow the primary treatment. Though constraints may meet easily for only the primary treatment, in the Plan Sum with the boost plan, constraints may not meet anymore. Thus, it is important to factor this in as you set objectives for nearby OARs.
Minimize dose to the rectum and bladder, which will result in greater amounts of dose that will penetrate through the femoral heads. This is acceptable so long as the femoral head constraints meet.
If a lymph node target volume is present (PTV 45 in IMRT plans), creating an optimization structure between the right and left lymph nodes will avert dose in that normal tissue region. There is no reason to deliver dose to the region between the lymph nodes, so dose to it should be minimized.
To keep the nodal volumes from becoming too hot, assigning an upper objective to the PTV 45 will minimize the high dose from the PTV 50. For example, the upper objective on the PTV 45 can be: 5% of the volume receives no more than 48-49 Gy. The volume is not set to 0% as doing so may reduce coverage to the PTV 50 as the PTV contours connect. An alternative would be to create an optimization structure in which the PTV 45 is cropped 0.3 CM from the PTV 50, and assigning an upper objective 0% of the volume receives no more than 48-49 Gy.
If all objectives and constraints are met and there is opportunity to further preserve organs at risk, it may be recommended 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 dose to the rectum, bladder, and the inter-lymph node region in their respective order as much as possible. If the current PTV coverage exceeds the required PTV coverage, it is acceptable to sacrifice some coverage to further reduce dose to the rectum.

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

At least 95% of the prescription dose covers at least 95% of the PTV.
At least 99% of the GTV should receive at least 100% of the prescription dose.
The maximum hot spot or “3D Dose Max” cannot exceed 110% prescription dose.
The rectum has four constraints:
- The volume of the rectum that receives 60 Gy should be below 50%.
- The volume of the rectum that receives 65 Gy should be below 35%.
- The volume of the rectum that receives 70 Gy should be below 10%.
- The volume of the rectum that receives 70 Gy should be below 10cc.
The bladder has four constraints:
- The volume of the rectum that receives 65 Gy should be below 50%.
- The volume of the rectum that receives 70 Gy should be below 35%.
- The volume of the rectum that receives 75 Gy should be below 25%.
- The volume of the rectum that receives 80 Gy should be below 15%.
The small bowel has four constraints:
- The volume of the small bowel that receives 35 Gy should be below 180 cc.
- The volume of the small bowel that receives 40 Gy should be below 100 cc.
- The volume of the small bowel that receives 45 Gy should be below 65 cc, no more than 150 cc.
- The max point dose of the small bowel should be no greater than 50 Gy (0.035 cc point dose).
The volume of each femoral head that receives 50 Gy cannot exceed 10%.
The mean dose to the penile bulb cannot exceed 52.5 Gy.

The following constraints must be met for a Prostate SBRT plan:

The target volume has four requirements:
- Max effective point dose to 0.03 cc shouldn’t exceed 107% prescription dose, absolutely not above 110%.
- At least 95% of the PTV should receive at least 100% of the prescription dose.
- The minimum point dose (0.03 cc) to the PTV should be greater than 34.4 Gy.
- At least 99% of the GTV should receive at least 100% of the prescription dose.
The dominant nodules have two requirements:
- The maximum effective point dose to 0.03 cc should be no more than 42.8 Gy.
- At least 95% of the nodules receive at least 100% of the prescribed dose.
The rectum has four constraints:
- The volume of the rectum that receives 36.25 Gy should be below 1cc.
- The volume of the rectum that receives 32.625 Gy should be below 10%.
- The volume of the rectum that receives 29 Gy should be below 20%.
- The volume of the rectum that receives 18.125 Gy should be below 50%.
The bladder has three constraints:
- The volume of the bladder that receives 38 Gy should be below 1 cc.
- The volume of the bladder that receives 32.625 Gy should be below 10%.
- The volume of the bladder that receives 18.125 Gy should be below 40%.
The bladder wall has one constraint:
- The volume of the bladder wall that receives 18.3 Gy should be below 18cc.
The small and large bowel have two constraints:
- The volume of the small and large bowel that receives 30 Gy should be below 1 cc.
- The volume of the small and large bowel that receives 18 Gy should be below 5 cc.
The femoral heads have three constraints:
- The maximum point dose (0.03cc) cannot exceed 30 Gy.
- The volume of each femoral head that receives 20 Gy cannot exceed 10 cc.
- The volume of each femoral head that receives 14.5 Gy cannot exceed 5%.
The volume of the neurovascular bundle that receives 38 Gy cannot exceed 50%.
The maximum point dose (0.03 cc) to the urethra cannot exceed 105% of the prescription dose.
The penile bulb has two constraints:
The maximum point dose (0.03 cc) to the skin cannot exceed 30 Gy.