The aim of this study is to evaluate the influence of flattened and flattening filter-free (FFF) beam 6 MV photon beam for liver stereotactic body radiation therapy by using volumetric modulated arc therapy (VMAT) technique in deep inspiration breath hold (DIBH) and free breathing condition.

Eight liver metastasis patients (one to three metastasis lesions) were simulated in breath hold and free breathing condition. VMAT-based treatment plans were created for a prescription dose of 50 Gy in 10 fractions, using a 230° coplaner arc and 60° non-coplanar arc for both DIBH and free breathing study set. Treatment plans were evaluated for planning target volume (PTV) dose coverage, conformity and hot spots. Parallel and serial organs at risk were compared for average and maximum dose, respectively. Dose spillages were evaluated for different isodose volumes from 5 to 80%.

Mean D98% (dose received by 98% target volume) for FFF in DIBH, flattened beam in DIBH, FFF in free breathing and flatten beam in free breathing dataset were 48·9, 47·81, 48·5 and 48·3 Gy, respectively. D98% was not statistically different between FFF and flatten beam (p = 0·34 and 0·69 for DIBH and free breathing condition). PTV V105% (volume receiving 105% dose) for the same set were 3·76, 0·25, 1·2 and 0·4%, respectively. Mean heterogeneity index for all study sets and beam models varies between 1·05 and 1·07. Paddik conformity index using unflattened and flattened beam in DIBH at 98% prescription dose were 0·91 and 0·79, respectively. Maximum variation of isodose volume was observed for I-5%, which was ranging between 2288·8 and 2427·2 cm3. Increase in isodose value shows a diminishing difference in isodose volumes between different techniques. DIBH yields a significant reduction in the chest wall dose compared with free breathing condition. Average monitor units for FFF beam in DIBH, flattened beam in DIBH, FFF beam in free breathing CT dataset and flattened beam in free breathing CT dataset were 1318·6 ± 265·1, 1940·3 ± 287·6, 1343·3 ± 238·1 and 2192·5 ± 252·6 MU.

DIBH and FFF is a good combination to reduce the treatment time and to achieve better tumour conformity. No other dosimetric gain was observed for FFF in either DIBH or free breathing condition.