The Estimated Difference in Biologically Effective Dose Between Two-Opposing-Fields-Per-Day and...

This study aims to determine whether two-opposing-fields-per-day (2-fields-per-day) carbon ion radiation therapy for prostate cancer provides a significant advantage in terms of biologically effective dose (BED) for normal tissue compared to the one-alternating-field-per-day (1-field-per-day) approach.

The prescribed dose for carbon ion radiation therapy is 54 Gy (relative biological effectiveness [RBE]) in 12 fractions, and 10 patients were randomly selected for analysis. BED was calculated using 3 methods: the linear-quadratic model, modified microdosimetric kinetic model (mMKM) with a fixed clinical RBE of 2.41 (adopted in most treatment planning systems), and the mMKM without rescaling, using domain radius ( $r_d$ ) values of 0.28, 0.38, and 0.45 µm with $\alpha /\beta$ ratios of 3, 5, and 10 Gy. Organs at risk (OARs) were contoured following standard guidelines. The dosimetric and dose-volume histogram differences, based on BEDs estimated using various methods, were compared between the 2-fields-per-day and 1-field-per-day approaches. Statistical analyses were performed using the Wilcoxon signed rank test with P < .05 considered statistically significant.

The 2-fields-per-day approach resulted in significantly lower BED for noncentrally located OARs, particularly the femoral head, skin, and bone marrow. Among these, the femoral head showed the largest relative mean BED difference, with approximately 17% using the linear-quadratic model ( $\alpha /\beta$ = 3 Gy), 7% with the mMKM model using a fixed clinical RBE, and 9% with the mMKM model ( $r_d$ = 0.38 µm, $\alpha /\beta$ = 3 Gy), with all P values <.01. In contrast, the differences observed for the obturator muscles and other centrally located OARs, including the urethra, bladder, and rectum, were minimal, with relative BED differences remaining below 3%.

Compared to the 1-field-per-day approach, the 2-fields-per-day approach results in lower BED for noncentrally located OARs, though significant variation is observed between the different methods used to estimate BED. The reduction in BED requires further investigation to evaluate its clinical significance and impact on toxicity.