SU‐F‐T‐134: Can We Use the Same Dose Constrains Learnt From Photon World to Plan Proton for Lung Cancer?

Abstract

Purpose:To evaluate if the same DVH constrains used in photon plans can be safely used to plan proton therapy for lung cancer. Since protons and photons have different dose deposition patterns, the hypothesis is following DVH constrains derived from photon world is not safe for proton.Methods:We retrospectively evaluated plans for 11 lung cancer patients. Each patient was planned with photon and proton following the same dose constrains. Dose statistics on PTV, normal lung, heart and esophagus were extracted for comparison. gEUD for normal lung was calculated and compared between proton and photon plans. We calculated series of gEUDs for each plan by varying the parameter “a” in gEUD formula from 0.1 to 3, covering the whole confidence interval.Results:For all patients, proton plans yield similar PTV coverage and lower dose to heart and esophagus than photon plans. Normal lung V5 was 32.3 % on average in proton plans than 55.4 % in photon. Normal lung gEUD monotonically increased with increasing “a” for all proton and photon plans. For a given patient, the gEUD_proton(a) had a steeper slope than gEUD_photon(a). The two curves crossed for 8 out of 11 patients when “a” = [0.1, 3]. a_crossing ranged from 0.8 to 2.44 with an average of 1.15. For a<a_crossing, gEUD_proton was less than gEUD_photon and vice versa.Conclusion:The current clinical guideline is the lower normal lung V5 would associated with less complications. However, proton plans with a lower normal lung V5 could yield a higher gEUD than photon if the real “a” is larger than a_crossing. Since a_crossing was within the possible range of real “a”, simply following the normal lung V5 guideline for proton plan would not be a good practice. More comprehensive methods should be developed to evaluate the proton plan.