Note: An expansion of 1.5 CM anteriorly and laterally is used here for the chest wall, whereas 2-2.5 CM may be used for a breast.
Pre-simulation
Breast cases that will be treated using VMAT will have nodal involvement (Axillary, IMC, and SCLAV) and most likely have a prescription of 50 Gy in 25 fx, followed by a boost of 10 Gy in 5 fx.
You may need to import a PET/CT or previous CT images, and perform a fusion afterwards. The physician will decide if the patient is to be treated with DIBH (Deep Inspiratory Breath Hold) or FB (Free Breathing), and may be important to know prior to proceeding with using the VMAT technique.
Simulation
Patients are simulated in the supine position while lying on a breast board. The breast board holds the patient’s ipsilateral arm above the head, and the patient will turn his/her head away from the treated breast. For left-sided breast cases, the physician may opt for a deep inspiration breath hold (DIBH) simulation in which the patient will take a deep breath and hold it for the full duration of the CT scan. The lungs expand upon taking a breath and push the heart inward and away from the left breast. The lungs in an expanded state have a greater volume, and with the heart also now further away from the treatment area, the dose to both of these OARs will receive significantly less (compared to non-DIBH). VMAT arcs will take a longer time to treat compared to static beams in 3D so be sure that patients can hold sufficiently hold their breath.
The therapists will align the CT machine laser indicators with the patient’s body to place radiopaque BBs on the patient’s AP and ipsilateral lateral of the same Z plane. A radiopaque adhesive wire will also be placed over the lumpectomy scar for only the scan, which is typically performed by the physician. 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 (tattoos) 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:
- Two unique structures: (RT or LT, Breast or Chest wall)
- e.g. RT Chest wall or LT Breast
- If Chest wall, you may make it into a High Resolution Structure
- PTV (RT or LT) (Breast or Chest wall)
- e.g. PTV RT Breast
- If Chest wall, you may make it into a High Resolution Structure
- Axillary Node
- IMC Node
- S_CLAV Node
- Two unique structures: “Lung LT” and “Lung RT”
- Heart
- Esophagus
- Spinal Cord
- Liver
- LT or RT Humeral Head (Ipsilateral)
- Mandible
- Brachial Plexus (Optional)
- BBs
- Body
- S_CLAV Wire
- Scar Wire or Lumpectomy Wire (Depending on Mastectomy or Intact breast)
- CTV Lumpectomy (If breast is still intact)
- PTV Lumpectomy (If breast is still intact)
- PTV Total (Combination of all target volumes)
If there is a DIBH scan, keep in mind that the primary treatment will be planned on the DIBH scan and the boost/CD will be planned on the Free Breathing (FB) scan. Therefore, if there is a lumpectomy, the physician will need to contour on the FB scan. The physician will contour the target volumes for this case. Do NOT modify physician drawn contours.
Before contouring the BB and the scar wire structures, make sure to convert them to High Resolution Structures which will allow you to define the wire location more precisely. Using the adaptive brush in combination with adjusting the contrast level can help with contouring wire/BB structures. When these contours are complete, you may crop the scar wire and BBs outside of the body contour since these markings were only used for the CT scan and will not be present at the time of treatment. Furthermore, go into the properties of these structures and set the HU = 0. This is a second precautionary measure taken to tell the TPS that there should not be such a high density structure in that place at the time of treatment.
The physician will contour the “RT/LT (Ipsilateral) Breast”, “RT/LT (ipsilateral) Chest wall”, “CTV Lumpectomy”, and/or nodal structures depending on the patient. Do NOT modify physician drawn contours.
The physician drawn contours are used as a reference, and you will instead work with the “PTV Breast” or “PTV Chest wall” structure.
- For the “PTV Breast”: Crop the physician drawn “RT/LT Breast” that extends outside the body contour by an additional 5 mm. This will bring the PTV volume inside the body and remove the 5 mm of skin.
- For the “PTV Chest wall”: Crop the physician drawn “RT/LT Chest wall” that extends outside the body with no additional margin. A bolus will be used to deliver superficial dose. For patients with an expander, the physician may ask to crop the volume by an additional 3 mm which extends outside the body, and treat without a bolus.
- For the physician drawn “CTV Lumpectomy”, add a 7 mm outer expansion to create a “PTV Lumpectomy” volume.
- If the “PTV Lumpectomy” now extends outside the “PTV Breast”, crop the “PTV Lumpectomy” inside the “PTV Breast”.
- If the original “CTV Lumpectomy” is EXCLUDED from the newly cropped “PTV Lumpectomy” on any slice, boolean the “CTV Lumpectomy” into the “PTV Lumpectomy”. A bolus may be used in such a case as the “CTV Lumpectomy” is most likely superficial, and it is up to the physician’s discretion.
- To summarize, the “PTV Lumpectomy” starts as a 7 mm margin around “CTV Lumpectomy”, but it should not extend outside the “PTV Breast”. If the “PTV Lumpectomy” is missing a part of the original “CTV Lumpectomy”, then the “CTV Lumpectomy” will be booleaned back into “PTV Lumpectomy”. At this point, the “PTV Lumpectomy” will extend outside “PTV Breast” in order to include the original “CTV Lumpectomy”.
- If the scar wire is being treated for the boost/CD (Generally indicated by an absence of a lumpectomy volume), no additional contouring modifications are required.
If the patient has artifact caused from the expander, 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.
If the patient has a pacemaker, it should be contoured as well. Pacemakers can receive no more than a max point dose of 2 Gy.
Planning Setup
In order to create the flash for a VMAT plan as seen in 3D breast plans, the current CT scan will need to be duplicated.
- In External Beam Planning, right click on the CT to “copy & paste” the initial scan, and name the duplicated scan as “copy”.
- The “copy” scan will have a structure set associated with it and will contain the exact same structures as the initial structure set. In Contouring, create the following structures and/or changes for the “copy” scan and images:
- Create a structure called “CT HU = 0”, and convert this structure to a high resolution structure. Proceed to create an outer margin from the PTV Breast only in the anterior and lateral direction between 1.5 to 2.5 CM. Then crop “CT HU = 0” extending inside the current body structure. This will result in a strip of contour along the anterior and lateral side of the PTV Breast without it entering the body contour. In the properties of “CT HU = 0”, set the HU to be 0.
- Take the body contour, and use the Boolean tool to amalgamate (combine) the body contour and “CT HU = 0”. This body contour should now include the flash, and use Post Processing to fill all cavities. DO NOT use the Smoothing function.
- Create a structure called “OPT PTV Breast”, and use the Boolean tool to amalgamate “CT HU = 0” and the physician’s contour of the treated breast. Now crop “OPT PTV Breast” which extends outside the recently modified body contour by an additional 5 mm. DO NOT amalgamate with the “PTV Breast” as gaps in the contour will be formed. If the right breast is to be treated, amalgamate “RT Breast” and “CT HU = 0” as opposed to “PTV Breast” and “CT HU = 0”.
- In External Beam Planning, create a new plan from the “copy” scan, and proceed to perform all the planning setup and optimizations on this scan.
- 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. It is recommended to use at least two arcs of varying collimator angles which focus on the breast, lower axillary nodal region, and IMC nodes, and 1-2 arcs which focus on the superior axillary region and the SCLAV nodes. The superiorly lying nodes can benefit from the arcs extending more posteriorly, but little advantage is gained for just the breast, so separating the arcs to treat separate regions can be useful.
- 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 center of the total target volume and round off the coordinates to the nearest decimal place (e.g. +2.4 CM shift, not +2.48 CM shift).
- Determine the angles of your arcs. Partial arcs are used covering angles that extend slightly beyond a conventional 3D tangent field setup.
- 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 total” throughout the entire length of the rotation.
- After you are satisfied with a plan on the “copy” scan, “copy & paste” this plan with a unique name such as “RT BREAST FINAL”.
- Assign the original structure set to this plan “RT BREAST FINAL”, and ONLY press calculate. No optimization is to be done on this plan, but only on the plan with the “copy” scan and structure set.
- Verify that the flash is present by playing the MLC motion in the BEV of every arc.
Optimization Tips
- Enable jaw tracking if available on the LINAC.
- Prioritize the constraints of the heart and contralateral breast as low dose spread is the trade-off going from 3D to VMAT. However, try your best not to compromise the coverage of the PTV volumes as well.
- It is recommended that 95% of the flash region receives at least 90% of the prescription dose.
- Avoid hotspots and/or minimize the max point dose for the esophagus by the supraclavicular/axillary region.
- Try to keep the plan as conformal as possible meaning that you should avoid high dose in normal tissue particularly in the area surrounding the supraclavicular/axillary region.
- Creating an optimization structure near the medial contralateral breast by the sternum can help bring the max point dose to meet its constraint.
A supine breast plan WITH nodal involvement must meet the following constraints:
- At least 95% of the prescription dose covers at least 95% of the PTV.
- At least 95% of the prescription dose covers at least 90% of each nodal volume.
- The maximum hot spot or “3D Dose Max” generally cannot exceed 110% prescription dose, but hot spots in the nodal structures can be up to 115%.
- 108% of the prescription dose cannot be delivered to more than 5% of the irradiated breast volume.
- The volume of the ipsilateral lung that receives 20 Gy cannot exceed 32%.
- The volume of the contralateral lung that receives 5 Gy cannot exceed 15% (preferably 10%).
- The mean dose to the heart cannot exceed 4 Gy.
- The contralateral breast cannot exceed a max point dose of 3-4 Gy.
- The spinal cord max dose cannot exceed 45 Gy.
- The volume of the esophagus that receives 45 Gy cannot exceed 33%.
Additionally, the following constraints are recommended:
- 105% of the prescription dose should not be delivered to more than 15-20% of the irradiated PTV breast/chest wall volume.
- No hot spots in the inframammary fold or close to the skin.
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