Pre-simulation

Lung cases that are treated with the conventional fractionation are commonly seen with a prescription of 66 Gy in 33 fx. Lung SBRT cases are commonly seen treated in 4 to 5 fx to a dose of 40 Gy or more.

You may need to import a PET/CT or previous CT images, and perform a fusion afterwards.

Simulation

The patient will be simulated in the supine position. The patient will lie on a lung board to which an inflated vac lock bag is attached. There are pegs at the head of the lung board 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. They will proceed to align the patient and use the lasers of the CT machine to place radiopaque BBs on the AP and laterals of the patient of the same Z plane. After the simulation, the BBs will be removed and dots will be tattooed in their places. Markings will also be made on the vac lock at the Z plane of the CT lasers to properly align both the vac lock and the patient during setup.

As the lungs are constantly in motion by breathing, respiratory gating is used to track a patient’s breathing pattern to deliver radiation within the selected range of a patient’s breathing pattern. A plastic cube with reflectors are placed by the xiphoid process of the patient, and a fixed camera by the feet of the table monitors the vertical motion of the reflectors. The full up-and-down motion of the cube will represent the patient’s breathing cycle and will allow for the acquisition of a “4D-CT”. 10 CT sets or phases are acquired with each set equivalent to a distinct breath hold position within the breathing cycle. From the phases, the AVG CT can be created which will define the tumor averaged with time and will be used as the planning CT. The MIP scan is also created by picking out the maximum value of the each of the phases’ corresponding voxel and in reconstructing the scan, the MIP scan can estimate for the ITV (Possible because of contrast between air and tissue). Push the phases, the AVG, and the MIP scans to the TPS.

Contouring

Create a structure set that includes the following structures:

  1. Body
  2. Heart
  3. Esophagus
  4. Carina
  5. Liver
  6. Brachial Plexus
  7. Three unique structures: “Lung LT”, “Lung RT”, and “Whole Lung – GTV”
  8. Three unique structures: “GTVp” (Primary), “GTVn” (Nodal), and “PTV PET” (PET avid volume)
  9. Two unique structures: “CTVp” (Primary) and “CTVn” (Nodal)
  10. PTV
  11. Two unique structures: “Spinal Cord”
    • From this structure, create the following unique structure using the margin function: “Spinal Cord + 5mm”.

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

  1. Aorta
  2. Bronchus
  3. Chestwall
  4. Pulmonary Trunk
  5. Ribs
  6. Skin
  7. Trachea
  8. Vena Cava
  9. ITV (Internal Target Volume)

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

Planning Setup

Refer to the link below for a broad setup overview:

  1. 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. Using 2-3 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.
  2. 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.
  3. Align the fields to the center of the target volume and round off the coordinates to the nearest decimal place (e.g. +2.4 CM shift, not +2.48 CM shift). For SBRT cases, it may be warranted to avoid rounding off the isocenter shifts. Avoid using shifts if there is no significant dosimetric advantage.
  4. Determine the angles of your arcs. Partial arcs are generally used in VMAT lung cases if the target volume is predominantly unilateral to spare as much healthy lung tissue as possible, and sometimes to completely avoid dose to the lens. However, in cases with a bilateral target volume, full arcs can be used.
  5. 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.
  6. Adjust the field size in the BEV to fit the tightly to the PTV throughout the entire length of the rotation.
  7. For SBRT cases, use the field specifications of 6X-FFF, 1400 MU/min, and a tolerance of SRS_SRT_SBRT.

Optimization Tips

  1. Enable jaw tracking if available on the LINAC.
  2. 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.
  3. Create an optimization structure that encompasses only the PTV which extends into the both lung structures. It is usually more difficult to control the dose deposition in air, and in optimizing to this structure may aid in obtaining adequate coverage.
  4. Following the intermediate dose calculation, pause the optimization before it exits the screen and enters the calculation. If your PTV overlaps noticeably with lung, pausing the optimization will allow for the dose to “come back”.
  5. Be aware that the PTV may have direct overlap with the brachial plexus and the physician may prefer to sacrifice PTV coverage to meet its constraint.
  6. 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 dose to the lungs as much as possible while maintaining adequate coverage.
    • Further minimize the max point dose to any other OAR, particularly that of the spinal cord, esophagus, and heart, 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 lung plan must meet the following constraints:

  1. At least 95% of the prescription dose covers at least 95% of the PTV.
  2. At least 99% of the GTV should receive at least 100% of the prescription dose.
  3. The maximum hot spot or “3D Dose Max” cannot exceed 110-113% prescription dose.
  4. The Whole Lung – GTV has four constraints:
    • The volume of the Whole Lung – GTV that receives 20 Gy should not exceed 30-35%.
    • The volume of the Whole Lung – GTV that receives 10 Gy should not exceed 45%.
    • The volume of the Whole Lung – GTV that receives 5 Gy should not exceed 65%.
    • The mean dose to the Whole Lung – GTV should not exceed 20-23 Gy.
  5. The max point dose to the spinal cord should not exceed 45 Gy.
  6. The max point dose to the spinal cord + 5mm should not exceed 50 Gy.
  7. The heart has three constraints:
    • The volume of the heart that receives 40 Gy should not exceed 100%.
    • The volume of the heart that receives 45 Gy should not exceed 67%.
    • The volume of the heart that receives 60 Gy should not exceed 33%.
  8. The esophagus has two constraints:
    • The max point dose to the esophagus should exceed 70 Gy, preferably under 63-66 Gy.
    • The mean dose to the esophagus should not exceed 34 Gy.
  9. The brachial plexus has two constraints:
    • The volume of the brachial pelxus that receives 54 Gy should not exceed 90%.
    • The max point dose to the brachial plexus should not exceed 66 Gy, preferable under 60-63 Gy.
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