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

A comparative treatment planning study has been undertaken between intensity modulated (IM) photon therapy and IM proton therapy (IMPT) in paraspinal sarcomas, so as to assess the potential benefits and limitations of these treatment modalities. In the case of IM proton therapy, plans were compared also for two different sizes of the pencil beam. Finally, a 10% and 20% dose escalation with IM protons was planned, and the consequential organ at risk (OAR) irradiation was evaluated.

Plans for 5 patients were computed for IM photons (7 coplanar fields) and protons (3 coplanar beams), using the KonRad inverse treatment planning system (developed at the German Cancer Research Center). IMPT planning was performed assuming 2 different sizes of the pencil beam: IMPT with a beam of full width at half-maximum of 20 mm, and IMPT with a "mini-beam" (IMPT(M), full width at half-maximum = 12 mm). Prescribed dose was 77.4 Gy or cobalt Gray equivalent (CGE) for protons 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 OAR dose-volume histograms were calculated. Results were analyzed using dose-volume histogram parameters, inhomogeneity coefficient, and conformity index.

Gross tumor volume coverage was optimal and equally homogeneous with both IM photon and IM proton plans. Compared to the IM photon plans, the use of IM proton beam therapy leads to a substantial reduction of the OAR total integral dose in the low-level to mid-dose level. Median heart, lung, kidney, stomach, and liver mean dose and dose at the 50% volume level were consistently reduced by a factor of 1.3 to 25. Tumor dose homogeneity in IMPT(M) plans was always better than with IMPT planning (median inhomogeneity coefficient, 0.19 vs. 0.25). IMPT dose escalation (to 92.9 CGE to the GTV) was possible in all patients without exceeding the normal-tissue dose limits.

These results suggest that the use of IM photon therapy, when compared to IM protons, 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 maximal OAR dose constraints. In IM proton planning, reducing the size of the proton pencil beam (using the "mini-beam") improved the dose homogeneity, but it did not have a significant effect on the dose conformity.