Potentials of robust intensity modulated scanning proton plans for locally advanced lung cancer in comparison to intensity modulated photon plans.

BACKGROUND AND
PURPOSE: The potentials of lung sparing, dose escalation, and the robustness of intensity modulated proton plans (IMPT(robust)), obtained by minimax optimization on multiple scenarios, were studied.
MATERIALS AND METHODS: IMPT(robust) optimization as described by Fredriksson et al. [23] was evaluated by means of comparative treatment planning using breath hold CT data from 6 non-small cell lung cancer (NSCLC) patients. IMPT(robust) and single field uniform dose (SFUD) proton plans were compared to Tomotherapy and 7-field intensity modulated photon therapy (IMXT). Plan robustness against set-up errors, range uncertainties, and between field motions were analyzed as well as lung exposure quantified by the mean lung dose (MLD) and the partial lung volumes receiving at least 20, 10, and 5 Gy(RBE) (V20, V10, V5). Robustness was analyzed with regard to stability of the effective uniform dose (EUD) and the dose level reached or exceeded in 95% of the CTV (D95).
RESULTS: MLD by IMPT(robust) was less than by SFUD, and Tomotherapy in each patient, on average by 14.8% and 28.5% (p<0.05, Friedman test). V20-V5 were higher with Tomotherapy compared to both proton therapy techniques, on average by a factor of >1.8. Robustness of IMPT(robust) was high. EUD and D95 values were maintained above 96% and 94% of the reference plan values for all tested scenarios. With dose escalation to 86 Gy(RBE) lung tissue tolerances were maintained.
CONCLUSIONS: IMPT(robust) proved advantageous in terms of lung exposure and possible dose escalation while being also markedly robust. However, motion during delivery of a field remains a major problem of IMPT(robust) to be mitigated by high scanning speed and variable spot size.