Gi Rectal Field

General Comments

  • Belly board could reduce the irradiated small-bowel by as much as 70%, regardless of patient’s weight, age, or sex.
  • In the Stockholm trials, mortality rates werehigher among patients treated with AP/PA vs 3 or 4-field (15% vs 3% p<0.001).
  • 3-field technique is in general. Anterior field (AP/PA or 4-field) is used when disease extends anteriorly to urogential region.
  • Always Cover Sacrum
  • Inguinal and External iliac does not need to be in the field in general, except when inguinals are clinically involved or T4 tumor.
  • Anal canal involvement in rectal cancer still confers < 5% inguinal node involvement.

Post-operative Field

  • Post-op Dose is up to 50-55 Gy but limited by small bowel tolerance

Pre-operative Field

  • Pre-op Dose is 45 Gy
  • 5.4 boost for “large” tumors (> 4 cm? T3/T4).

Radiation Fields and Doses

Initial Whole Pelvic Field

  • Minimum boundaries of whole pelvic fields:
  • Standard opposed anterior-posterior portals:
    • Inferior - The minimum would be at least a 5 cm margin from the inferior extent of the cancer or the anal verge for the distal cancers as identified by a marker on simulation.
    • Lateral - 2 cm lateral to the bony pelvis taken at its widest point.
    • Superior – L5-S1 junction.
  • Standard opposed lateral portals:
    • Superior - To correspond to A/P fields.
    • Inferior - To correspond to A/P fields.
    • Anterior - This will cover the lower common and external iliac to 1 cm anterior to the symphysis pubis for anterior wall lesions and be at the mid symphysis for posterior lesions.
    • Posterior - This must include the entire sacrum with a 1 cm margin for T4 lesions or 2 cm posterior to the presacrum for T3 lesions.

Boost Field

  • The boost field shall have a 3 cm margin around the tumor but must include the whole of the sacral hollow.

Dose

  • Original Pelvic Treatment Volume: The total dose to the prescription point shall be 45 Gy in 25 (1.8 Gy/fx) fractions. For CT planning, the PTV1 will receive 45 Gy in 25 fractions.
  • Boost Volume: The dose for the boost volume or PTV2 is 5.4 Gy in 3 (1.8 Gy/fx) fractions for a cumulative dose within the tumor volume to the prescription point (or GTV) of 50.4 Gy.
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