And more Protons Vs. Photons

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Dose-volume comparison of proton therapy and intensity-modulated

radiotherapy for prostate cancer.

Vargas C, Fryer A, Mahajan C, Indelicato D, Horne D, Chellini A,

McKenzie C, Lawlor P, R, Li Z, Lin L, Olivier K, Keole S.

University of Florida Proton Therapy Institute, ville, FL.

PURPOSE: The contrast in dose distribution between proton

radiotherapy (RT) and intensity-modulated RT (IMRT) is unclear,

particularly in regard to critical structures such as the rectum and

bladder. METHODS AND MATERIALS: Between August and November 2006,

the first 10 consecutive patients treated in our Phase II low-risk

prostate proton protocol (University of Florida Proton Therapy

Institute protocol 0001) were reviewed. The double-scatter proton

beam plans used in treatment were analyzed for various dosimetric

endpoints. For all plans, each beam dose distribution, angle,

smearing, and aperture margin were optimized. IMRT plans were

created for all patients and simultaneously analyzed. The IMRT plans

were optimized through multiple volume objectives, beam weighting,

and individual leaf movement. The patients were treated to 78 Gray-

equivalents (GE) in 2-GE fractions with a biologically equivalent

dose of 1.1. RESULTS: All rectal and rectal wall volumes treated to

10-80 GE (percentage of volume receiving 10-80 GE [V(10)-V(80)])

were significantly lower with proton therapy (p < 0.05). The rectal V

(50) was reduced from 31.3% +/- 4.1% with IMRT to 14.6% +/- 3.0%

with proton therapy for a relative improvement of 53.4% and an

absolute benefit of 16.7% (p < 0.001). The mean rectal dose

decreased 59% with proton therapy (p < 0.001). For the bladder and

bladder wall, proton therapy produced significantly smaller volumes

treated to doses of 10-35 GE (p < 0.05) with a nonsignificant

advantage demonstrated for the volume receiving </=60 GE. The

bladder V(30) was reduced with proton therapy for a relative

improvement of 35.3% and an absolute benefit of 15.1% (p = 0.02).

The mean bladder dose decreased 35% with proton therapy (p = 0.002).

CONCLUSION: Compared with IMRT, proton therapy reduced the dose to

the dose-limiting normal structures while maintaining excellent

planning target volume coverage.