How is conventional external beam radiotherapy (EBRT) delivered for the treatment of prostate cancer?

Updated: Nov 29, 2018
  • Author: Isamettin Andrew Aral, MD, MS; Chief Editor: Edward David Kim, MD, FACS  more...
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Answer

Answer

Conventional EBRT is typically delivered by means of a 4-field technique (see the image below). The 4 fields (AP, PA, left lateral, and right lateral) are designed to include the prostate, the seminal vesicles, and the regional lymphatic vessels. A cumulative dose of 45-50 Gy is delivered over a period of 5-5.5 weeks. An additional dose of approximately 20 Gy to a smaller field (ie, a boost) is delivered to the prostate and the periprostatic tissues.

Unit used to deliver conventional external beam ra Unit used to deliver conventional external beam radiotherapy.

Total doses of 66.6-70 Gy were once typically used; however, these doses are too low to provide the same rates of local and regional control achieved with currently used higher doses (72-80 Gy). [14, 15] The boost field is designed to limit treatment to the target volume (prostate, seminal vesicles, and 1- to 2-cm margin) and to offer additional shielding to the posterior wall of the rectum, the urethra, and the small bowel. The reduced volume of normal tissue included within the radiation field is associated with a reduction in morbidity.

Whole-pelvis radiotherapy (ie, superior border at L5-S1 junction) is rarely used, because of the increased bowel toxicity and lack of clear outcome improvement. Whole-pelvis radiotherapy is offered to some patients when extensive regional disease is either present or expected. RTOG trial 94-13 suggests that large-field radiation treatment may be beneficial in patients with higher Gleason scores and in those who receive adjuvant hormonal blockade. [13]

When regional lymph nodes are to be treated, the superior border of the pelvic field is at the level of the midsacroiliac joints, and the inferior border is usually 1-1.5 cm inferior to the junction of the membranous and prostatic urethra, as demonstrated on urethrography (ie, pencil point). The lateral borders on the AP and PA fields are 1.5-2 cm lateral to the pelvic brim.

The superior and inferior borders remain unchanged on the lateral fields. The posterior border of the lateral field is commonly placed at the S2-S3 interspace. The anterior border is established to include the anterior portion of the symphysis pubis. The field edges for cone-down, or boost-field treatments, share the same inferior border as the primary field.

Superiorly, the fields extend to the top of the acetabulum and laterally to include two thirds of the obturator foramen. Dose distributions for conventional treatment are typically generated in a single plane, and the dose is prescribed at the isocenter and normalized at the 100%-isodose line.


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