To compare the dosimetric characteristics and treatment delivery efficiency of trigeminal neuralgia (TN) stereotactic radiosurgery (SRS) patients previously treated with a 6 MV-FFF (flattening filter-free; radiation beam obtained by removing the flattening filter) beam versus those re-planned with a 10 MV-FFF beam using a conical collimator on the TrueBeam Novalis STx linear accelerator.

Eleven patients with TN previously treated with a 6 MV-FFF beam following the SRS protocol of 90 Gy in a single fraction were selected. Plans were recalculated using 10 MV-FFF beam, maintaining the same dose prescription and beam angle configuration used with 6 MV-FFF beam. The dose gradient, volumes receiving 20 and 10 Gy, maximum dose and dose to 10% of the brainstem were recorded for both the energies. Efficiency was assessed by the average monitor unit (MU) and time per arc. The 10 MV-FFF machine was configured in the treatment planning system (TPS) to measure the tissue phantom ratio (TPR), dose profiles and scatter factors using RAZOR, PTW-60012 diodes and EBT3 radiochromic films.

Compared to the 6 MV-FFF, the 10 MV-FFF plans exhibit average increments in dose gradient, volume of 20 Gy and volume of 10 Gy of 3.8, 17.1 and 17.8%, respectively. Average increases of 6.5 and 18.1% were obtained for maximum dose and dose to 10% of the brainstem, respectively. An average increase of 31 MU/arc was observed for the 10 MV-FFF plans, with a 40% reduction in treatment time per arc. The TPR for the 10 MV-FFF beams increased by 10%, and a penumbra width of 0.3 mm was observed. Scatter factor increments of 15, 13.5, 12.7 and 10.3% were observed for the 6 MV-FFF over the 10 MV-FFF for cones of 4, 5, 6 and 7 mm, respectively.

In TN SRS, the utilisation of 10 MV-FFF beams reduces treatment duration but results in an increased brainstem radiation dose. To mitigate this increase in brainstem dose, it is necessary to adjust the isocentre position.

In modulated radiotherapy treatments with the jaw tracking technique (JTT), the collimator jaws can dynamically follow the multileaf collimator apertures and reduce radiation leakage. This reduction protects normal tissue from unwanted doses. Previous research has highlighted the importance of defining which patients will benefit most from JTT. Besides, some authors have expressed their concerns about possible increases in monitor units (MUs). Treatments of patients with peripheral targets and isocentre located in the patient’s midline are of particular interest. The current work assessed the effect of JTT on these cases.

JTT plans for thirty-two patients were compared to plans with the static jaws technique. The volumes of normal tissue receiving 5 Gy (V5), 10 Gy (V10) and 20 Gy (V20), mean dose (Dmean), target coverage parameters D95, D2% and Paddick’s conformity index (PCI) were compared. MUs were also registered for comparisons. The decrease in the jaws opening with JTT was correlated to the decrease in dose values in normal tissue.

Small decreases were observed in D95 and in D2% values, without statistical significance. A 5% average decrease in PCI values was noticed as well as significant decreases in V5, V10 and Dmean values, 9% on average. A 3% decrease in V20 was also observed. The number of MUs decreased by 2%. A significant correlation was found between the reduction of the secondary collimation opening areas and the dose delivered to normal tissue.

JTT technique improved normal tissue protection in volumetric modulated arc therapy treatments for the patients included in the present study.

To analyse the impact of multileaf collimator (MLC) leaf width in multiple metastases radiosurgery (SRS) considering the target distance to isocenter and rotational displacements.

Ten plans were optimised. The plans were created with Elements Multiple Mets SRS v2·0 (Brainlab AG, Munchen, Germany). The mean number of metastases per plan was 5 ± 2 [min 3, max 9], and the mean volume of gross tumour volume (GTV) was 1·1 ± 1·3 cc [min 0·02, max 5·1]. Planning target volume margin criterion was based on GTV-isocenter distance and target dimensions. Plans were performed using 6 MV with high-definition MLC (HDMLC) and reoptimised using 5-mm MLC (MLC-5). Plans were compared using Paddick conformity index (PCI), gradient index, monitor units , volume receiving half of prescription isodose (PIV50), maximum dose to brainstem, optic chiasm and optic nerves, and V12Gy, V10Gy and V5Gy for healthy brain were analysed. The maximum displacement due to rotational combinations was optimised by a genetic algorithm for both plans. Plans were reoptimised and compared using optimised margin.

HDMLC plans had better conformity and higher dose falloff than MLC-5 plans. Dosimetric differences were statistically significant (p < 0·05). The smaller the lesion volume, the higher the dosimetric differences between both plans. The effect of rotational displacements produced for each target in SRS was not dependent on the MLC (p > 0·05).

The finer HDMLC offers dosimetric advantages compared with the MLC-5 in terms of target conformity and dose to the surrounding organs at risk. However, only dose falloff differences due to rotations depend on MLC.