Optimization of the oblique angles in the treatment of prostate cancer during six-field conformal radiotherapy.
Historically, four perpendicular treatment fields or bilateral arcs have been used in the treatment of prostate cancer. As new techniques challenge the four-field box technique for their superiority of tumor coverage and adjacent critical structure sparing, oblique beam angles (in addition to right and left laterals) have been introduced as an alternative to anterior (AP) and posterior (PA) beams. Among the most popular of these alternative approaches is a six-field technique. Traditionally 45 degrees angles have been used with this technique. In this study, opposed coplanar oblique beams angled 20, 25, 30, 35, 40, and 45 degrees off the lateral beam position, were compared for their ability to minimize adjacent critical structure doses, while maintaining maximum clinical target volume (CTV) coverage. This analysis compared rectum, bladder, and femoral head dose volume histograms (DVH) for each of these varying oblique gantry angles. As the angle of the posterior oblique beams became more horizontal, it is more difficult to encompass the apex of the prostate in the 95% isodose value. On inferior CT slices near the apex of the prostate, the density of the pelvic bones in the path of the posterior oblique fields causes the beam to be slightly attenuated, thereby underdosing the CTV. The oblique angles most affected by this bone heterogeneity are beams angled from 20 to 30 degrees off the lateral beam position. As the angles approach the vertical direction, rectal and bladder doses increase, while femoral head doses decrease. Oblique gantry angles approaching the horizontal direction result in a decrease in rectal and bladder doses, while femoral head doses increase. Of the oblique angles studied, 35 degrees off the lateral position provides lower rectum and bladder doses than 30, 40, and 45 degrees; lower femoral head doses than 20, 25, and 30 degrees, and the maximum CTV coverage on all CT slices studied.
Pickett
B
B
University of California, San Francisco 94143.
Roach
M
M
Horine
P
P
Verhey
L
L
Phillips
T L
TL
D000230:Adenocarcinoma / Q000532:radiotherapy*; D005270:Femur Head / Q000528:radiation effects; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011835:Radiation Protection; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000379:methods*; D012007:Rectum / Q000528:radiation effects; D014057:Tomography, X-Ray Computed / Q000379:methods; D001743:Urinary Bladder / Q000528:radiation effects