Pre-simulation
Prone Breast cases are most commonly seen with a prescription of 50 Gy in 25 fx (conventional) or 42.56 Gy in 16 fx (hypofractionation), followed by a boost of 10 Gy in 5 fx.
You will not need to import any ancillary images for this plan.
Simulation
For patients who have large or pendulous breasts, they are simulated and treated in the prone position. The prone position allows for a reproducible set-up while also significantly minimizing dose to the organs at risk (OARs). If the patient were to lay supine, the breast could rest in a unique position on different treatment days, and hotspots may also form in the infra-mammary fold. Instead, by using the prone position, the patient utilizes a prone breast board which is designed with an opening for the treated breast to fall into. A typical prone breast treatment will remove nearly all OARs from the treatment field, resulting in minimal dose to the lungs, heart, and contralateral breast (relative to the supine position). Since the dose to the heart is of particular concern for left breast cancer patients, the prone position is especially desired for those cases.
The setup will begin with the patient lying prone on the breast board. The patient’s head will face away from the treated breast and he/she grasp pegs located above the head. The therapists will align the CT machine laser indicators with the patient’s body to place radiopaque BBs on the patient’s PA and laterals 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)
- PTV (RT or LT) (Breast)
- e.g. PTV RT Breast
- Two unique structures: “Lung LT” and “Lung RT”
- Heart
- Esophagus
- Spinal Cord
- Liver
- LT or RT Humeral Head (Ipsilateral)
- Mandible
- BBs
- Body
- Lumpectomy Wire
- CTV Lumpectomy (If breast is still intact)
- PTV Lumpectomy (If breast is still intact)
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” and “CTV Lumpectomy”. Do NOT modify physician drawn contours.
The physician drawn contours are used as a reference, and you will instead work with the “PTV Breast” 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 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 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:
Prone breasts are generally treated using two opposing tangential fields (medial & lateral) at a fixed SSD of 100 CM. The two fields will be un-grouped and each set to a unique isocenter. The placement of the fields should meet the following criteria:
- No Z shift from the user origin.
- The isocenter for each field will have identical Y shifts (ex: Both at +27.00 CM) based on approximating the half way point between the anterior to posterior breast length. This is done so that the therapist can visually verify the optical distance indicator (ODI), which projects from the LINAC onto the treatment area.
- The medial and lateral fields will have distinct X shifts.
- The gantry angles chosen should attempt to avoid all OARs while also matching the posterior borders of the medial and lateral fields. Matching the posterior field borders will prevent the overlap of beam divergence in the body, thereby minimizing the spread and buildup of low dose beyond the field edge.
- Consider a slight collimation to have the posterior field border become near parallel to the chest wall viewable on the DRR. Doing so will allow the MLCs to conform better to the PTV Breast and also keep OARs out of the field. Sometimes, a 90 degree collimation may be required for large breasts as the MLCs which move in the Y direction can traverse greater distances.
- For the ease of setup, ensure all coordinates are rounded to the nearest decimal (ex: +2.4 CM shift, not +2.48 CM). Sometimes, this changes the SSD to a value other than 100 CM. Re-type 100 CM SSD and the rounded X/Y values until they are of acceptable values.
You will only use one setup field: “PA SETUP” with a gantry rotation 0° (Keeping in mind that the patient was simulated in the prone position). The central axis of the setup field will pass through the BB on the posterior side of the body, which means that there will be a X shift of 0 CM if the User Origin is set to the BBs. In summary, your three fields (medial, lateral, & PA Setup) will have the same Y and Z coordinates with differing X coordinates.
In the event of a superficial lumpectomy volume, the physician will typically approve of a 0.5 CM thick bolus over the superficial treatment area. Insert a bolus which covers the entirety of the superficial treatment area with a slight extra margin to take setup error into account. Be sure to consider the practicality of placing a bolus as well.
Most prone breast tangents are treated using an energy of 6X, but large breasts in particular may be treated using an energy of 15X if it is dosimetrically advantageous (with the physician’s approval), or even mixed energies. This may occur in patients with a large breast separation (>23 CM breast separation).
3D Breast/Chest Wall Planning
Modify the gantry angles of both fields to deliver adequate dose to the “PTV Breast” while minimizing OAR overlap and also matching the posterior borders of the medial and lateral fields. Matching the posterior field borders will prevent the overlap of beam divergence in the body, thereby minimizing the spread and buildup of low dose beyond the field edge.
Once the fields are setup, starting with one field, create MLC leaves and use the ‘fit to structure’ function. Input the following parameters and press ok:
- Select the target structure to be the PTV.
- Select elliptical margin, collimator coordinate system, leaf edge-contour meet point = outside, closed leaf meeting position = center, and only check “optimize collimator jaws.
- Give a 3 CM anterior margin and a 0 CM margin elsewhere. This 3 CM margin will be the “flash” that accounts for breast swelling.
After using this tool, you should see that the field size, on the axial view and the BEV, have been adjusted to include the entirety of the PTV. Verify that the contours for the contralateral breast, heart, and ipsilateral lung have minimal overlap as possible with this field in the BEV. If there is an unsatisfactory overlap:
- Rotate the gantry and re-fit to structure until an acceptable angle is achieved.
- Possibly rotate the collimator by 5° to 15° such that the posterior field edge becomes near parallel to the chest wall seen through the DRR to keep an OAR out of the field.
- Include more of the ipsilateral lung and/or the heart (if the previously mentioned suggestions did not work). Sometimes, simply the positioning of the patient from simulation or the patient anatomy may not be ideal to minimally involve all the OARs, and so including more of the ipsilateral lung and/or the heart in the field in this scenario would be acceptable. It is best to consult with the physician on his/her preference.
Remember that in making these field adjustments, the opposing field may no longer be tangential and will require further adjustment of the gantry angle afterwards.
When you are satisfied with the field, manually create a 1.5 CM superior (as there is no nodal involvement) and inferior margin on the PTV by adding 1.5 to the appropriate X/Y field size (depending on collimator rotation). Furthermore, the MLC leaves need to be manually adjusted as well:
- All the MLC leaves which enter anteriorly will need to be dragged out manually to match the new anterior field border. The MLC leaves will only be kept in the field if it is to block part of the raised ipsilateral arm.
- The posterior MLC leaves will generally match the outline of the PTV contour (as there is no nodal involvement). These leaves are adjusted in rare cases such as to give more coverage to posterior regions of the breast.
You can then repeat the method above to create a flash and add margins on the second opposing tangent field.
Place your reference point near the breast-lung interface and calculate.
Refer to the link below for more information on 3D breast field-in-field planning:
Refer to the link below for more information on breast electronic (irregular surface) compensator planning:
In general, a prone breast plan WITHOUT nodal involvement 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 (108% for hypofractionated cases).
- 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 20% (preferably 15%).
- The mean dose to the heart cannot exceed 4 Gy.
- The contralateral breast cannot exceed a max point dose of 3-4 Gy.
- Additionally, the following constraints are recommended:
- 105% of the prescription dose should not be delivered to more than 15-20% of the irradiated breast volume.
- No hot spots close to the skin.
Planning for the Lumpectomy Boost
The isocenter of the fields for the boost plan do not need to change unless doing so provides a significant dosimetric advantage. Remember to name/number the fields appropriately.
A lumpectomy that is significantly one sided (medial or lateral) can utilize mixed energies with 15X for the contralateral field and 6X for the ipsilateral field.
Start with one field and use the fit to structure function to set the MLC leaves. Input the following parameters and press ok:
- Select the target structure to be the “PTV Lumpectomy”.
- Select circular margin, collimator coordinate system, leaf edge-contour meet point = outside, closed leaf meeting position = center, and only check “optimize collimator jaws”.
- Give a 0.7 CM margin around the “PTV Lumpectomy”.
After using this tool, you should see that the field size, on the axial view and the BEV, have been adjusted to include the entirety of the “PTV Lumpectomy”. Repeat the method above on the second opposing field. The opposing field may no longer remain tangential after fitting the MLC leaves to the “PTV Lumpectomy” and will require a gantry rotation.
Place your reference point in the “PTV Lumpectomy” and calculate. Change the weighting of the fields to reduce the hotspot and/or improve the dose distribution.
In general, a Boost plan must meet the following constraints:
- At least 95% of the prescription dose covers at least 95% of the PTV Lumpectomy.
- The maximum hot spot or “3D Dose Max” cannot exceed 110% prescription dose.
- No OAR should receive significant dose in the CD plan.
- 100% of the prescription dose from the Lumpectomy CD plan should not irradiate more than 33% of the original “PTV Breast”.
