A treatment planning comparison of intensity-modulated photon and proton therapy for paraspinal sarcomas

Abstract

Purpose/Objective: A comparative treatment planning study has been undertaken between intensity-modulated (IM) photon and IM proton therapy (IMPT) in paraspinal sarcomas, as to assess the potential benefits and limitations of these treatment modalities. Proton therapy planning was performed assuming two different sizes of the pencil beam: IMPT with a beam of full width at half-maximum (FWHM) of 20 mm, and IMPT-M with a “mini-beam” (FWHM=12mm). Finally, a 10% and 20% dose-escalation with IM protons was planned and the consequential organ at risk (OAR) irradiation evaluated. Materials/Methods: Plans for five patients were created for IM photons (5 and 7 coplanar fields) and protons (3 coplanar beams), using KonRad inverse treatment planning system (developed at the German Cancer Research Center). Prescribed dose was 77.4 Gy-CGE to the gross tumor volume (GTV). Surface and center spinal cord dose-constraint for all techniques was 64 and 53 Gy-CGE, respectively. Tumor and organs at risk (OAR) dose-volume histograms (DVH) were calculated. Results were analyzed using the DVH parameters, inhomogeneity coefficient (IC) and conformity index (CI). Results: GTV coverage was optimal and equally homogeneous with both IM photon and IM proton plans. Compared to the IM photon plans, the use of the IM proton beam lead to a substantial reduction of the OAR total integral dose in the low- to mid-dose level. Heart, lung, kidney and stomach mean dose and median tumor dose at the 50% volume level were consistently reduced by a factor of 1.8–40. Tumor dose homogeneity in IMPT-M plans was always better than in IMPT plans (median IC, 0.19 vs. 0.25). IMPT dose escalation (to 85.1 and 92.9 Gy-CGE in GTV) was possible in all patients. Conclusions: These results suggest that the use of IM proton therapy, when compared to IM photons, can result in similar levels of tumor conformation. IM proton therapy, however, reduces the OAR integral dose substantially, compared to IM photon radiation therapy. As a result, tumor dose escalation was always possible with IM proton planning, within the specified OAR dose-constraints. In IM proton planning, reducing the size of the proton pencil beam (using the “mini-beam”) improved the dose homogeneity, but did not have a significant effect on the dose conformity.