(P066) Dosimetric Analysis of Proton Therapy for Prostate Cancer: A Single Institutional Experience

Jing Zeng, Kent McCune, Matthias Cook, Grayden MacLennan, Malin Mao, George E. Laramore, Kenneth Russell, Jay Liao; University of Washington

BACKGROUND: With the increasing number of proton therapy centers in the United States, more patients with prostate cancer have access to proton radiation as a treatment option. While dose constraints for organs at risk (OARs) are well defined for photon treatment, less is known for proton radiation, and photon constraints are often used after correcting for relative biological effectiveness (RBE = 1.1). However, it is often possible to achieve lower doses to OARs with proton therapy. Therefore, we investigated dose to OARs in a series of patients treated at our institution to consider lowering dose constraints for the proton population.

METHODS: Consecutive patients with prostate cancer treated at our center from April–September 2014 were analyzed. All patients were treated with fiducial markers, daily kV image-guided radiation therapy (IGRT), full bladder, and rectal balloon. Clinical characteristics and OAR doses were extracted. Low-risk patients were treated to prostate plus 3–5-mm planning target volume (PTV) margin. Intermediate- and high-risk patients were treated to prostate plus proximal seminal vesicles, with optional pelvic nodal irradiation for high-risk patients. Treatment groups were compared with t-tests.

RESULTS: A total of 40 patients with prostate cancer were treated, with a median age of 68 years (range: 51–79 yr); 10 patients had low-risk disease, 21 had intermediate-risk disease, and 9 had high-risk disease (5 received whole-pelvis radiation). A total of 31 patients were treated with pencil beam scanning, and 9 patients were treated with uniform scanning. Median prescription was 79.2 Cobalt Gray Equivalent (CGE).

For the bladder: median V80 CGE = 4.9 cc (range: 0–13.8 cc) with constraint < 8 cc; median V70 CGE = 6.6% (range: 3.6%–22.1%) with constraint < 10%; and median V50 CGE = 11.5% (range: 6.1%–32.8%) with constraint < 35%. For the bladder wall (3-mm internal rind): median V80 CGE = 3.5% (range: 0%–14.5%) with constraint < 15%; median V70 CGE = 13% (range: 8%–31%) with constraint < 35%, and median V55 CGE = 16.5% (range: 10.8%–36.5%) with constraint < 50%.

For the rectum: median V81 CGE = 0 cc (range: 0–6.61cc) with constraint < 1 cc; median V70 CGE = 10.8% (range: 3.0%–19.3%) with constraint < 25%; and median V50 CGE = 19.6% (range: 10.9%–33.8%) with constraint < 35%. For the rectal wall (3–mm internal rind): median V81 CGE = 0 cc (range: 0–5.36 cc) with constraint < 1 cc; median V70 CGE = 22.6% (range: 5.7%–34.3%) with constraint < 25%; and median V50 CGE = 32.5% (range: 15.8%–43%) with constraint < 50%.

Comparing the low-, intermediate-, and high-risk disease groups, the only significant dose difference was in rectal wall V50 CGE (low = 28.4%, intermediate = 32.1%, high = 35.0%).

CONCLUSIONS: Proton therapy for prostate cancer typically generates treatment plans that are below dose constraints defined in the photon literature, with the exception of a small subset of patients. In order to promote the best possible proton treatment plans, lower dose constraints for OARs should be considered. 

Proceedings of the 97th Annual Meeting of the American Radium Society- americanradiumsociety.org