Strategy to implement RapidArc® using HD120™MLC for irradiation of whole pelvis and lower para-aortic lymph nodes in cervical carcinoma

Purpose: To compare the utility of RapidArc® technique with mono or dual isocentre using high-definition 120 multileaf collimator (HD120™MLC) for irradiation of whole pelvis and lower para-aortic lymph node (PALN) in cervical carcinoma.Methods: Ten patients having more than 22-cm treatment target length were delivered 45 Gy in 25 fractions by RapidArc® technique using mono or dual isocenter. Dose distribution characteristics were monitored using various indices. Doses to critical organs were also compared for the sparing effects.Results: Conformity and dose-gradient indices for mono or dual isocenter plans were comparable. However, mono-isocentric plan decreased the heterogeneity index (0.07±0.01) as compared to dual-isocentric plan (0.08±0.01; p<0.05). Mean and maximal doses to various critical organs remained comparable. The mean dose to bowel bag was significantly reduced (p<0.05) in mono-isocentric plan (18.78±5.18 Gy) as compared to dual-isocentric plan (19.20±5.27 Gy). Mono-isocentric plan required 1.3 times higher exposure time than the dual-isocentric plans but reduced the overall treatment time by eliminating intra-fraction patient shift that is inevitable in dual isocenter plan.Discussion: Feasibility of mono-isocentric RapidArc® plan using HD120™MLC for irradiation of whole pelvis and PALN relied on ‘Maximum Leaf Span’, which restricted the ‘leading-leaf’ to surpass the ‘trailing-leaf’ beyond 15-cm. This difficulty was overcome by truncating the field size and fixing the jaws at dedicated positions in mono-isocentric plan.Conclusion: The strategy used in mono-isocentric plan enhanced the utility of RapidArc® using HD120™MLC and also minimized the intra-fraction set-up variations and overall treatment time.