The effect of treatment positioning on normal tissue dose in patients with prostate cancer treated with three-dimensional conformal radiotherapy.
OBJECTIVE
To prospectively assess the effect of supine vs. prone treatment position on the dose to normal tissues in prostate cancer patients treated with the three-dimensional conformal technique.


METHODS
Twenty-six patients underwent three-dimensional treatment planning in both the supine and prone treatment positions. The planning target volume and normal tissue structures were outlined on each CAT scan slice, and treatment plans were compared to assess the effect of treatment position on the volume of rectum, bladder, and bowel exposed to the high dose of irradiation.


RESULTS
The average dose to the rectal wall and the V95 (volume of rectal wall receiving at least 95% of the prescription dose) for the prone position were 64 and 24% of the prescription dose, respectively, compared to 72 and 29%, respectively, for the supine position (p < 0.05). When the average rectal wall dose was used as an endpoint, 14 of the 26 patients (54%) had an advantage for the prone position compared to 1 (4%) who demonstrated an advantage for the supine position (p < 0.0002). Similarly, when V95 of the rectal wall was used as a measure of comparison, 15 patients (58%) had an advantage for the prone position compared to 1 (4%) who demonstrated an advantage for the supine position (p < 0.0002). In 13 patients (50%), a change from supine to the prone position was associated with reduction of the V95 to levels < 30% of the prescription dose compared to 3 patients (11%) in whom such an advantage resulted from change of the prone to the supine position (p < 0.005). The effect of treatment position on the rectal wall dose was most pronounced in the region of the seminal vesicles. An increased volume of bowel was also noted in the supine position. The treatment position, however, had no significant impact on the dose to the bladder wall.


CONCLUSIONS
Three-dimensional conformal radiotherapy for prostate cancer in the prone position is associated with significant reduction of the dose to the rectum and bowel resulting in an improvement in the therapeutic ratio.
Zelefsky
M J
MJ
Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Happersett
L
L
Leibel
S A
SA
Burman
C M
CM
Schwartz
L
L
Dicker
A P
AP
Kutcher
G J
GJ
Fuks
Z
Z
D006801:Humans; D007421:Intestine, Small; D008297:Male; D011187:Posture; D011446:Prospective Studies; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011835:Radiation Protection; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012007:Rectum; D001743:Urinary Bladder