Comparison of stereotactic plans for brain tumors with two different multileaf collimating systems

Comparison of treatment planning for stereotactic radiosurgery and stereotactic body radiation therapy techniques with 2.5mm and 5mm multileaf collimator (MLC): A pilot study

Background

The Elekta micro-multileaf collimator (mMLC) known as apex for planning stereotactic radiosurgery (SRS)/stereotactic radiotherapy (SRT)/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 2.5mm mMLC and 5mm MLC known as 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 2.5mm and 5mm 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 2.5mm mMLC. GI was superior in the 2.5mm mMLC compared to the 5mm MLC giving a steep falloff in the dose distributions (p = 0.158).

Conclusions

The TC, CI, HI, and OAR doses were similar in both 2.5mm and 5mm based VMAT plans. The gradient index was better in the 2.5mm mMLC resulting in steep dose gradients, which further reduced the isodose volumes. Therefore, a 5mm 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.

Sparing the Hippocampus in Prophylactic Cranial Irradiation Using Three Different Linear Accelerators: A Comparative Study

Hippocampus protection, as an organ at risk in brain radiotherapy, might protect patients' quality of life. Prophylactic cranial irradiation (PCI) has been used traditionally in small cell lung cancer (SCLC) patients as it increases survival. This study aimed to discover the contributing parameters for a successful PCI with simultaneous protection of the hippocampus by using three different treatment machines. For this purpose, treatment plans were generated for 45 SCLC patients using three half-arcs in three linear accelerators (LINACs; Elekta Infinity, Synergy, and Axesse; Elekta Ltd, Stockholm, Sweden) with different radiation field sizes and multileaf collimator (MLC) leaf thickness characteristics. The prescribed dose was 25 Gy in 10 fractions. Thresholds for the hippocampus were calculated based on the Radiation Therapy Oncology Group 0933 dose constraints. The planning and treatment system templates were common to all three LINACs. Plan evaluation was based on the dosimetric target coverage by the 95% isodose, the maximum dose of the plan, the conformity index (CI), the degree of plan modulation (MOD), and the patient-specific quality assurance (QA) pass rate. The mean target coverage was highest for Infinity (97.3%), followed by Axesse (96.6%) and Synergy (95.5%). The mean maximum dose was higher for Synergy (27.5 Gy), followed by Infinity (27.0 Gy) and Axesse (26.9 Gy). Axesse plans had the highest CI (0.93), followed by Infinity (0.91) and Synergy (0.88). Plan MOD was lower for Synergy (2.88) compared with Infinity (3.07) and Axesse (3.69). Finally, patient-specific QA was successful in all Infinity plans, in all but one Synergy plan, and in 17/45 Axesse plans, as was expected from the field size in that treatment unit. Based on overall performance, the most favorable combination of target coverage, hippocampus sparing, and plan deliverability was obtained with the LINAC, which has the largest field opening and thinnest MLC leaves.

Rotational effect and dosimetric impact: HDMLC vs 5-mm MLC leaf width in single isocenter multiple metastases radiosurgery with Brainlab Elements™

Purpose:

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

Methods:

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.

Results:

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).

Conclusions:

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.

Plan quality comparison for cervical carcinoma treated with Halcyon and Trilogy intensity-modulated radiotherapy

Purpose: Varian Halcyon is a novel machine with dual-layer leaves, single flattening filter free (FFF) energy and an enclosed bore. The purpose of this study was to compare the differences in dosimetry and plan parameters of intensity-modulated radiation therapy (IMRT) plans between the Halcyon and Trilogy accelerators. Methods and Materials: A total of 30 IMRT plans from cervical carcinoma patients were retrospectively analyzed on the Trilogy and Eclipse v13.5 treatment planning systems (TPSs). For each patient, a new plan based on Halcyon was created with the same planning parameters and optimization constraints using the Eclipse Version 15.1 TPS. To compare plan qualities, dosimetry parameters regarding planning target volume (PTV), organs at risk (OARs), monitor unit (MU) efficiency, segment size and treatment time were evaluated. Evaluation of the helical diode array system was performed with gamma-index analysis. Results: The dose distribution of the target volume of the Halcyon and Trilogy plans showed no significant difference (p > 0.05). The mean doses of rectum and both femoral heads for Halcyon plans were significantly reduced compared to those for Trilogy plans (p < 0.05). Compared to Trilogy, Halcyon increased the number of MUs from 1542.9±248.3 MU to 2514.9±328.2 MU (p = 0.00) and decreased the delivery time from 11.28±1.36 min to 3.26±0.26 min (p = 0.00). The average segment areas of Halcyon plans for proximal and distal multileaf collimators (MLCs) were 42.1 ± 31.2 cm2 and 28.4 ± 23.7 cm2, respectively, and that of Trilogy plans was 27.3 ± 16.9 cm2. The mean gamma index (3 mm/3%) results for the Halcyon and Trilogy plans were 99.41±0.26 and 99.76±0.32 (p > 0.05), respectively. Conclusions: All Halcyon treatment plans were recognized as clinically acceptable and had statistically better OAR sparing with higher delivery efficiency. The Halcyon system exhibited fast treatment delivery of IMRT with good dosimetric agreement using ArcCHECK.