March 1, 2013

Today, we will be discussing breast cancer and its associated external beam therapy fields.

A note:  Patients who require only breast or chest wall irradiation are treated with tangential (glancing) fields, which maximize coverage to the tissues at risk and minimize dose to underlying structures (primarily heart and lung).

• Considerations for field margins of tangential fields:  Superior

1.      First intercostals space.

2.      As far cephalad as possible without including the arm.

3.      Superior extent of the palpable breast tissue.

4.      Cephalad (>2cm) to original location of mass.

·        Considerations for field margins of tangential fields:  Inferior

1.      Caudad (1-2cm) to the inframammary fold.

2.      In a postmastectomy patient, this can be extrapolated from inframammary fold of the other breast.

• Considerations for field margins of tangential fields:  Medial

1.      At midline, as determined by palpation of suprasternal notch and xiphoid process.

2.      Exceptions include patients whose mass or incision extends to or beyond midline and patients who will receive (IM) Internal Mammary lymphatic radiation.

• Considerations for field margins of tangential fields:  Lateral

1.      Corresponding to midaxillary line.

2.      Including drain sites or incisions considered at risk, original tumor bed, and appropriate amount of lung margin.

• Tangential fields:  isocenter

1.      Placed at some depth in breast tissue or chest wall.

2.      Sometimes halfway between ribs and skin.

3.      Sometimes midway between medial and lateral beam entrance points.

4.      Sometimes at the deep edge of the tangential field (split beam in half and block deep half of beam).

• Tangential fields

1.      An important feature of the tangential field arrangement is the coplanar nature of the deep (posterior) margin of the ports.

2.      Deep border of the medial and lateral tangents form a single plane.

3.      Important for homogeneous dose.

4.      Achieved with a split-beam technique or an unblocked technique.

5.      Multileaf collimator (MLC) provides the best result with a split-beam technique.

6.      Unblocked technique does best at limiting dose to opposite breast.

7.      Planning tangential fields can be a relatively complex process.

8.      Several commercial vendors market systems that include immobilization devices and programmable calculators to help (other techniques use factors that are derived empirically).

9.      Computerized treatment planning of the tangential field pair is necessary to visualize the dose distribution throughout the treatment volume and surrounding normal tissues (i.e. lung and heart).

10.  Wedges or a compensator may be needed to improve homogeneity.

11.  Skin doses are often adequate, so bolus is not usually required.

• Supraclavicular field

1.      When used, it is planned before tangential fields.

2.      Superior border is about 5cm above the suprasternal notch and avoiding flash over the skin of the supraclavicular area.

3.      Medial border is at the midpoint of the suprasternal notch.

4.      Lateral border is 2-3cm off the humeral head.

5.      Inferior border is at one of the following

a.       Approximately at the angle of Louis.

b.      Just above the superior extent of the palpable breast tissue.

c.       A point >2cm cephalad to the original location of the mass.

6.      Central rays is placed at the inferior margin of the treatment volume and the inferior half of the beam is blocked , creating a vertical straight edge at the inferior border of the field.

7.      Gantry is angled 10-15º mediolaterally to avoid exiting through the spinal cord.

• Posterior axillary boost field (PAB)

1.      Sometimes used to increase the midaxillary dose.

2.      Field should be parallel opposed to the supraclavicular field and uses the identical inferior margin, preserving the vertical straight edge.

3.      Should be treated with the same fractionation scheme as the supraclavicular field.

4.      Considerations for field margins:

a.       Superior – Mid to upper clavicle.

b.      Medial – A strip of lung approximately 1cm wide.

c.       Lateral – Approximately 1-2cm of the humeral head, which is subsequently blocked.

d.      Inferior – Corresponding to the inferior border of the supraclavicular field.

• Tangential breast or chest wall field

1.      Planned in the same way as the tangential fields described above, with several notable additions necessary for an appropriate junction with the supraclavicular-axillary fields.

2.      To establish a vertical straight edge at the superior border of the tangential fields, correction must be made.

3.      Couch assembly is rotated to put the patient’s feet directly away from the collimator and a block is placed at the superior edges of the tangential fields.

4.      This forces correspondence to the vertical straight edge created by the inferior border of the supraclavicular-axillary field pair.

5.      Single isocenter setup is another way to treat both supraclavicular field and breast if the field length is less than 20cm, half of the field length of the machine limit.

6.      Advantage:  simple setup in daily treatment with no need of blocking and couch rotation.

• Internal mammary lymph nodes

1.      A small number of patients may be at risk for IM lymph involvement, and whether to irradiate remains controversial.

• Breast boost

1.      Patients with conservatively managed care usually receive additional radiation treatment to the tumor bed immediately after completion of tangential irradiation.

2.      Must ensure the treatment volume adequately encompasses the tumor bed; the assumption that the location and length of scar accurately reflect the position and size of the tumor bed may be inaccurate.

Reflections:

When treating the breast, I will be using 3D static fields, meaning that I will be irradiating the cancer and breast tissue with radiation for long periods of time without moving the gantry until the field receives the expected monitor units.  It will be important to make sure she is in the same position everyday so that the cancer is destroyed and not the rest of the breast tissue.  Each field is also parallel opposed.  Parallel opposition means that a beam of radiation will pass through the patient one direction and then the next beam opposes it at 180º, thus increasing the skin sparing effect.  Parallel opposition took me a long time to understand.  Thus, the PAB and supraclav. should be parallel opposed and the tangential fields as well.  At Nevada Cancer Center, the wedges are all built into the machine, which makes things easy on the therapist.  At 21^st century, the wedges must manually be inserted, which is not difficult to do right.  If you do it wrong on the newer machines, you are informed by the status light and treatment setup on the monitor in the room.  Something else that is important to me is that a mastectomy deforms the breast more than a lumpectomy because the mastectomy takes more tissue where the lumpectomy takes the cancer with some healthy tissue.

I decided to look up "posterior axillary boost" on google and found a student doctor forum.  http://forums.studentdoctor.net/showthread.php?t=952352 There were a bunch of acronyms they used that I didn't understand.  Though I did find out that not all breast cancer patients need a posterior axillary boost if they haven't had a sufficient lymph node dissection or there N staging is less than or equal to N2.  The student doctors discuss that N2 stages need to be looked at on a case by case basis rather than all N2s receive PABs or not.  It looks like there is a trial going on right now in Canada that is looking at having nodal irradiation to females with node-positive breast cancer.  Thus, changing PAB from sometimes to everytime; the study is not yet done though.