Comparison of treatment planning for stereotactic radiosurgery and stereotactic body radiation therapy techniques with 2.5 mm and 5 mm multileaf collimator (MLC): A pilot study [version 1; peer review: awaiting peer review]

Background The Elekta Apex micro-multileaf collimator (mMLC) for planning stereotactic radiosurgery (SRS)/stereotactic body radiation therapy (SBRT) provides excellent dose distribution; however, it offers disadvantages such as prolonged treatment duration and technical errors in terms of mMLC and gantry calibration, which adds to the total treatment duration. Hence, we aimed to compare the treatment planning performed with the 2.5 mm Apex mMLC and 5 mm MLC (Elekta Agility) for brain and lung targets treated with SRS and SBRT in Elekta Versa high definition (HD). Methods The study included 10 patients, five each with brain and lung targets. Two treatment plans were performed for each case using Elekta's Monaco (5.11.03) treatment planning system (TPS) with 2.5 mm and 5 mm MLC. An X-ray photon beam of energy 6FFF was used for planning purposes with various gantry, couch, and collimator combinations. These two plans were compared using target coverage (TC), conformity index (CI), homogeneity index (HI), gradient index (GI), and organ at risk (OAR) doses. Results No significant differences were found in the target coverage, CI, HI, or OAR doses in either MLC design. Volumetric modulated arc therapy (VMAT) with a 5 mm MLC provided equivalent tumor coverage with an additional number of monitor units. OAR doses were comparable in both MLC widths for brain targets, whereas for lung targets, OAR doses were slightly lower with 2.5 mm mMLC. GI was superior in the 2.5 mm mMLC compared to the 5 mm MLC giving a steep falloff in the dose distributions (p = 0.158). Conclusions The TC, CI, HI, and OAR doses were similar in both 2.5 and 5 mm based VMAT plans. The gradient index was better in the 2.5 mm mMLC resulting in steep dose gradients, which further reduced the isodose volumes. Therefore, a 5 mm MLC (agility) can also be used for SRS/SBRT treatment planning, with a further reduction in the gradient index. However, the study must be extended further with more samples and multiple comparison parameters. Clinical Trials Registry - India, registration number CTRI/2021/11/037842, registration date. 8 th November, 2021.