VMAT Head and Neck Tutorial

5 Minute Tutorials: Head and Neck Planning

Pre-simulation

Head and neck cases will commonly treat the primary tumor and also the nodal chains which drain from it. Thus, it is normal to see multiple dose levels and various prescriptions depending on where the cancer arose from.

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

Simulation

Patients are simulated in the supine position. The patient will lie on the table, and either grasp the pegs by the sides of the table or rest their hands on their abdomen.

The therapists will align the patient and place a plastic headrest underneath the head/neck for support. A thermoplastic head and neck mask is placed over the head and shoulders and attaches firmly onto the table. This mask is used to ensure treatment reproducibility and also allows for shorter setup times during treatment. The properties of the mask are as such that it becomes elastic as it warms and hardens as it cools. The therapists will warm the mask in an oven and stretch it over the patient. It is particularly important that the mask properly secures the chin as it will prevent neck flexion. They will gently press on the mask with their fingers to conform the mask securely to the shape of the head and shoulders. Sometimes, the therapists may create a hole in the mask by the mouth and insert a block for the patient to bite on to enhance reproducibility.

Once the mask hardens, the therapists will proceed to align the patient and use the lasers of the CT machine to place radiopaque BBs on the mask at the AP and the laterals of the same Z plane in the center of the brain. 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 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:

  1. Body
  2. Three unique structures: “GTVp”, “GTVn”, and “GTV PET”
  3. Three unique structures: “CTV54”, “CTV60”, and “CTV70”
  4. Three unique structures: “PTV54”, “PTV60”, and “PTV70”
  5. Brain
  6. Brainstem
  7. Two unique structures: “Cochlea LT” and “Cochlea RT”
  8. Two unique structures: “Eye LT” and “Eye RT”
  9. Two unique structures: “Lens LT” and “Lens RT”
    • Set each to a high Resolution Structure
  10. Three unique structures: “Optic Nerve LT”, “Optic Nerve RT”, and “Optic Chiasm”.
    • From these structures, create the following unique structures using the margin function: “Optic Nerve LT +3mm”, “Optic Nerve Rt +3mm”, and “Optic Chiasm +3mm”.
  11. Pituitary Gland
  12. Brachial Plexus
  13. Esophagus
  14. Mandible
  15. Oral Cavity
  16. Pharyngeal Constrictors (Optional)
  17. Two unique structures: “Lung LT” and “Lung RT”
  18. Two unique structures: “Spinal Cord”
    • From this structure, create the following unique structure using the margin function: “Spinal Cord + 5mm”.

The body contour in the area of the nose may need to be fixed as Eclipse may either leave parts of the nose missing or leave holes in the nasal cavity. The physician will contour the target volumes for this case. The physician may re-name these and/or contour additional structures. If the CTV is contoured, but the PTV is not, ask the physician for instructions to create the PTV. For example, the physician may request for an outer 0.5 CM margin around the CTV to create the PTV. Do NOT modify physician drawn contours.

If the patient has artifact caused from dentures, 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.

Planning Setup

Refer to Checklist Guide 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. A H&N case with lymph nodal involvement will typically abut multiple OARs and so significant modulation may be required. Thus, using 3-4 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 largest target volume and round off the coordinates to the nearest decimal place (e.g. +2.4 CM shift, not +2.48 CM shift).
    4. Determine the angles of your arcs. Partial arcs may be used if only the unilateral H&N is treated. However, in bilateral H&N cases, using full arcs is recommended. The inferior nodal levels are located anteriorly, and creating an optimization structure to decrease dose deposition from the posterior body can be helpful.
    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.
  2. Adjust the field size in the BEV to fit the tightly to the PTV throughout the entire length of the rotation.

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. To keep the nodal volumes from becoming too hot, assigning an upper objective to the PTV 54 will minimize the high dose from the PTV 60. For example, the upper objective on the PTV 54 can be: 5% of the volume receives no more than 57-58 Gy (assuming the PTVs are cropped from one another). The volume is not set to 0% as doing so may reduce coverage to the PTV 50 as the PTV contours abut one another. An alternative would be to create an optimization structure in which the PTV 54 is cropped 0.3 CM from the PTV 60, and assigning an upper objective 0% of the volume receives no more than 57-58 Gy. The greater the difference in dose levels, the more the optimization structure will need to be cropped.
      • For OPT PTV54, crop PTV54 that extends inside PTV60 with a margin of 3 mm and PTV70 with a margin of more than 3 mm.
      • For OPT PTV60, crop the PTV60 volume that extends inside PTV70 with a margin of more than 4-5 mm.
      • There will not be an OPT PTV70 because there is not a higher dose level to crop from.
      • To prevent the dose levels from hotspots, use upper objectives during optimization on: OPT PTV54, OPT PTV60, and PTV70. The PTV54 and PTV60 can be used for DVH analysis.
  2. The parotid will commonly overlap with the nodal volumes contoured by the physician. If there is significant overlap, pushing on the parotid to meet the constraint will result in under covering the PTV. It is recommended to ask the physician on what his preference is to dealing with this particular case.
  3. 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 any other OAR without significantly altering coverage.

In general, a H&N 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 110-115% prescription dose.
  4. The max point dose to the spinal cord should not exceed 45 Gy.
  5. The max point dose to the spinal cord + 5mm should not exceed 48 Gy.
  6. The brain has two constraints:
    • The max point dose to the brainstem should not exceed 72 Gy.
    • The volume of the brain that receives 60 Gy should not exceed 33%.
  7. The brainstem has two constraints:
    • The max point dose to the brainstem should not exceed 59 Gy.
    • The volume of the brainstem that receives 54 Gy should not exceed 100%.
  8. The cochleas have two constraints:
    • The max point dose to the cochleas should not exceed 45 Gy.
    • The mean dose to an individual cochlea should not exceed 35-45 Gy (Depending on chemotherapy).
  9. The eyes have three constraints:
    • The max point dose to an eye should not exceed 45 Gy.
    • The volume of an eye that receives 30 Gy should not exceed 40%.
    • The mean dose to an eye should not exceed 35 Gy.
  10. The lenses have two constraints:
    • The max point dose to a lens should not exceed 8 Gy.
    • The volume of a lens that receives 6 Gy should not exceed 50%.
  11. The optic nerves and optic chiasm have two constraints:
    • The max point dose to the optic nerve and chiasm should not exceed 54 Gy.
    • The volume of an optic nerve or the optic chiasm that receives 50 Gy should not exceed 100%.
  12. The pituitary gland has two constraints:
    • The max point dose to the pituitary gland should not exceed 50 Gy.
    • The volume of the pituitary that receives 45 Gy cannot exceed 100%.
  13. The larynx has three constraints:
    • The max point dose to the larynx should not exceed 66 Gy.
    • The volume of the larynx that receives 45 Gy should not exceed 50%.
    • The mean dose to the larynx should not exceed 20 Gy.
  14. The mandible has two constraints:
    • The max point dose to the mandible should not exceed 66 Gy, and cannot not exceed 70 Gy.
    • The volume of the mandible that receives 60 Gy should not exceed 50%.
  15. The oral cavity has three constraints:
    • The max point dose to the oral cavity should not exceed 60 Gy unless physician approves due to PTV overlap.
    • The volume of the oral cavity that receives 40 Gy should not exceed 50%.
    • The mean dose to the oral cavity should not exceed 30 Gy.
  16. The parotids have two constraints:
    • The mean dose to an individual parotid should not exceed 20 Gy.
    • The volume of 20 cc of combined parotid should not exceed 20 Gy.
    • If the above is not achievable, the mean dose to both parotids combined should not exceed 25 Gy.
  17. The brachial plexus has two constraints:
    • The max point dose to the brachial plexus should not exceed 60 Gy.
    • The volume of the brachial plexus that receives 54 Gy should not exceed 50%.
  18. The esophagus has four constraints:
    • The max point dose to the esophagus should not exceed 63-66 Gy, and cannot exceed 70 Gy.
    • The volume of the esophagus that receives 55 Gy should not exceed 67%.
    • The volume of the esophagus that receives 65 Gy should not exceed 33%.
    • The mean dose to the esophagus should not exceed 34 Gy and cannot exceed 30 Gy.