A novel arc geometry setting for pelvic radiotherapy with extensive nodal involvement

The aim of this study was to find optimal planning approach for large planning targets with complicated geometry requiring wide field openings. The study presents a novel approach for arc geometry design for pelvic targets with extensive nodal involvement. A total of 15 patients with anorectal carcinoma or vulvar cancer were selected retrospectively. For each patient, one seven‐field IMRT plan and three VMAT plans were calculated: one with two 360° arcs with no limitations for the field size (VMATw); one with two asymmetrically field‐size‐restricted 360° arcs (VMATr); and the proposed novel approach which consisted of one 360° arc with the field size restricted to the central PTV, and another arc divided into two 180° arcs, restricting the field sizes with the focus on the lymph nodes. The techniques were compared in terms of PTV coverage [Formula: see text] , dose maximum (D(max)), dose conformity index (CI), homogeneity index (HI), and organs at risk doses. The proposed novel approach with one full and two half arcs tended to have better PTV coverage ([Formula: see text] , compared to [Formula: see text] , and [Formula: see text] in VMATw, VMATr, and 7f‐IMRT, respectively) and lower maxima ([Formula: see text] , compared to [Formula: see text] , and [Formula: see text] in VMATw, VMATr, and 7f‐IMRT, respectively); and lower or equal organs at risk doses. The superiority of the proposed technique [Formula: see text] was more pronounced compared with the VMATw plans ([Formula: see text] , paired t‐test [Formula: see text]), but the proposed technique was slightly better also in comparison with the VMATr plans [Formula: see text] and 7f‐IMRT plans [Formula: see text]. Radiotherapy treatment planning for large and complicated treatment volumes benefits not only from restricting the field size but also from careful field design that considers PTV geometry. This optimizes multileaf collimator movements, leading to better dose conformity and homogeneity. PACS number(s): 87.53.Jw, 87.55.D, 87.56.jk: