Twenty-one patients with stage I non-small-cell lung cancer (stage IA, n = 15 patients; stage IB, n = 6 patients) were studied. All tumors were located in the peripheral lung, and total dose was 66 Gray equivalents (GyE) (6.6 GyE/fraction). For treatment planning, beam incidence for proton beam technique was restricted to two to three directions for PT, and seven or eight noncoplanar beams were manually selected for SBRT to achieve optimal planning target volume (PTV) coverage and minimal dose to organs at risk.
Regarding lung tissues, mean dose, V5, V10, V13, V15, and V20 values were 4.6 Gy, 13.2 % , 11.4 % , 10.6 % , 10.1 % , and 9.1 % , respectively, for PT, whereas those values were 7.8 Gy, 32.0 % , 21.8 % , 17.4 % , 15.3 % , and 11.4 % , respectively, for SBRT with a prescribed dose of 66 Gy. Pearson product moment correlation coefficients between PTV and dose--volume parameters of V5, V10, V15, and V20 were 0.45, 0.52, 0.58, and 0.63, respectively, for PT, compared to 0.52, 0.45, 0.71, and 0.74, respectively, for SBRT.
Correlations between dose--volume parameters of the lung and PTV were observed and may indicate that PT is more advantageous than SBRT when treating a tumor with a relatively large PTV or several tumors.