Pre-simulation
IMRT Breast/Chest wall cases are most commonly seen with a prescription of 50 Gy in 25 fx, followed by a boost of 10 Gy in 5 fx.
You will not need to import any ancillary images for this plan, though, on rare occasions, a PET scan may be requested by the physician.
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).
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)
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.
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 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
Refer to the link below for a broad setup overview:
For a chest wall case, be sure to confirm with the physician whether he/she would like to crop the “PTV Chest wall” inside the body contour with a 0 mm additional margin and use a bolus, or crop the “PTV Chest wall” inside the body contour with a 3 mm additional margin and use no bolus. The latter is typically seen with patients with an expander put in place. If a bolus is to be used, insert a bolus which covers the entirety of the superficial treatment area with a slight extra margin to take setup error into account.
- IMRT breast/chest wall plans are generally treated using 7 to 9 static field.
- These static fields will be treated beginning in the counter clockwise direction. Thus, as you are determining the gantry rotation of your fields, it is recommended to do so starting at the most counter clockwise angle. For a left breast/chest wall, gantry angles of your fields can start around 285°-305° and end at around 140°-160°. For a right breast/chest wall, gantry angles start at around 230°-250° and end at around 60-80°.
- Fields should be placed by at least 10° apart, but not typically more than 30°-40°. Fields placed too nearby one another run the risk of redundancy or lack of diversification. Fields placed too far apart may result in missing optimal regions to deliver dose through.
- Round the gantry angles of your fields in increments of 5° or 10° unless you can justify the benefits of using a specific angle (e.g. G303 vs. G305).
- As you decide upon which gantry angle to use for your field, be aware of the OARs which may be in the direct path of the beam. Using the BEV and showing outlines will demonstrate how much overlap the field size will have with nearby OARs. To meet the OAR constraints may result in you eliminating gantry angles such as 90° for a left breast/chest wall patient as the exit dose of the field will pass directly through the contralateral breast.
- Properly name the fields based on the beam number and gantry angle (e.g. 02 G300, 07 G140). 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).
- Rotate the collimator angles of your fields to best fit the PTV and exclude OARs. 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” and minimize involvement of OARs. Sometimes, setting an en face field to cover just the superior lying nodes can help improve the overall distribution of your plan.
Using fluence editing after a plan has finished calculating can help decrease maximum hotspots which are of a very tiny volume.
After you are satisfied that is appropriate for treatment, the next step will be to add a flash that will account for breast swelling. By pressing play in the BEV, you will see that the field size and the MLC leaves close tightly onto the breast.
- Right click on first field’s fluence and select the “Skin Flash Tool”.
- Adjust the brush size to a size of about 1.5-2 CM.
- In the BEV of this field, turn on the contour of the breast and the body and paint fluence where air is present directly anterior to the breast to provide room for swelling and displacement.
- Certain fields will require no modifications such as an en face beam which treats the superiorly lying nodal volumes.
- After adding the appropriate amount of skin flash/fluence, re-calculate the plan.
- In the BEV, verify that there is a skin flash by pressing play to see the MLC motion for each field.
Optimization Tips
- Enable jaw tracking if available on the LINAC.
- Prioritize the constraints of the heart and contralateral breast while trying your best not to compromise the coverage of the PTV volumes as well.
- 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.
- If the coverage and/or constraints are not meeting, exiting optimization and trying a different field combination may be the solution. Sometimes, it can be either a field or two that requires a change in gantry angle, or it can be that you may need an additional field altogether.
Refer to the link below for more information on optimization:
An IMRT breast/chest wall plan WITH nodal involvement must meet the following constraints:
- At least 95% of the prescription dose covers at least 95% of the PTV Breast/Chest wall.
- 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.
The boost portion of the tutorial can be found in the 3D Supine Breast Boost Tutorial.
Refer to the link below for a 3D supine boost planning tutorial:
