Comparison Of Intensity Modulated Proton Therapy (IMPT) To Passively Scattered Proton Therapy (PSPT) In The Treatment of Prostate Cancer
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: October 4, 2011
Presenting Author: S. Choi Presenter's Affiliation:M.D. Anderson Cancer Center, Houston, TX
Prostate cancer can be definitively treated with radiotherapy. Higher doses of radiotherapy have been shown to improve biochemical progression-free survival (bPFS) (Zeitman et al. JAMA 2005, Kuban et al. IJROBP 2011).
Proton therapy, with its characteristic Bragg peak, has a sharp fall-off distally and laterally, potentially lowering dose to organs-at-risk. In prostate cancer, this is most importantly the rectum, as well as the bladder.
Proton therapy has classically been delivered as passively scattered proton therapy (PSPT).
Scanning beam proton therapy delivers a series of "spots" of proton radiation. These spots can be utilized in intensity modulated proton therapy (IMPT). Theoretically, IMPT has a better ability than PSPT to conform to a complex target shape.
This study examines if normal tissue sparing can be significantly improved without sacrificing target coverage using IMPT compared to PSPT for prostate cancer patients.
Materials and Methods
15 patients who were treated with PSPT for prostate cancer were evaluated in this study.
The clinical tumor volume (CTV) consisted of the prostate and the proximal seminal vesicles contoured by a single physician.
The planning tumor volume (PTV) consisted of the CTV plus 12 mm margin laterally, 5 mm margin posteriorly, and 6 mm anteriorly, posteriorly, and inferiorly.
IMPT plans utilizing scanning beam proton therapy were generated utilizing multifield optimization (MFO) using a PTV and were compared to the PSPT plans.
The total dose prescribed was 75.6 Cobalt-60 Gray Equivalent (CGE) in 1.8 CGE fractions.
Dose volume histograms were generated for the rectum, anterior rectal wall, and the bladder.
The anterior rectal wall was defined as a 3 mm layer of rectal tissue encompassing the edge of the anterior half of the rectum.
The coverage of the CTV was identical in IMPT and PSPT.
V100-CTV (volume of the CTV receiving 100% of the prescription dose) was 100% for all IMPT and PSPT plans.
The coverage of the PTV was improved with IMPT.
Mean V100-PTV was 98.6 ± 0.4% for IMPT and 96.2 ± 3.2% for PSPT (p=0.014).
There were significant differences in dose given to organs-at-risk:
Table 1 shows significant improvements in rectum and anterior rectal wall dose with IMPT over PSPT at low- and high-dose levels.
The dose given to the bladder was improved with IMPT, but the difference was not as pronounced as with the rectum and the anterior rectal wall.
Mean V70 of the bladder for IMPT was 6.7 ± 3.0% compared to 8.2% ± 3.0% for PSPT.
This is the largest presented dosimetric comparison of IMPT versus PSPT for prostate. Large and statistically significant reductions to the rectum and the anterior rectal wall were identified without sacrificing target coverage.
Importantly, this improvement was seen at all dose levels analyzed.
The authors use rigorous planning methods including multifield optimization to demonstrate that IMPT has benefit over PSPT in regards to dose to the rectum and the anterior rectal wall.
IMPT has advantages over PSPT above and beyond the dosimetric advantages shown. For example, apertures and compensators are not needed and therefore treatments could be delivered faster and with less physical work by therapists. In addition, IMPT could permit differential dosing and potentially simultaneous-integrated-boost (SIB) to the lobe of the prostate with known gross disease.
It is notable that in this study, only theprostate and proximal seminal vesicles were treated. Chera et al. (IJROPB, 2010)have shown dosimetric feasibility using proton therapy to treat the pelvic lymph nodes for high-risk patients. Additional studies will have to clarify the best treatment technique for these high-risk patients.
The authors do not state how they account for motion which is an important concern in both PSPT and IMPT.
Helical tomotherapy (HT) (Schwarz et. al Radiother Oncol 2011) has been shown to produce similar dosimetric outcomes as IMPT when designed with a SIB with hypofractionation. Further studies will need to be performed to elucidate the relative benefit of HT versus IMPT.
Prospective trials will be required to show a clinical benefit corresponding with the dosimetric benefit seen here.
Table 1: Comparison of Dosimetric Parameters Between IMPT and PSPT