Physical and dosimetric characteristic of high‐definition multileaf collimator (HDMLC) for SRS and IMRT

Abstract

Physical and dosimetric characteristics of HDMLC were studied for SRS6, 6, and 10 MV X‐rays from Novalis Tx. This in‐built tertiary collimator consists of 60 pairs (32×0.25 cm; 26×0.5 cm and 2×0.7 cm) of leaves. Properties of HDMLC studied included alignment, readout and radiation field congruence, radiation penumbra, accuracy and reproducibility of leaf position and gap width, static and dynamic leaf shift, tongue‐and‐groove effect, leaf transmission and leakage, leaf travel speed, and delivery of dynamic conformal arc and IMRT. All tests were performed using a calibrated ionization chamber, film dosimetry and DynaLog file analysis. Alignment of leaves with isocenter plane was better than 0.03 cm at all gantry and collimator positions. The congruence of HDMLC readout and radiation field agreed to within ± 0.03cm for filed sizes ranging from 1×1 to 20×20 cm2. Mean 80% to 20% penumbra width parallel (perpendicular) to leaf motion was 0.24±0.05(0.21±0.02) cm, 0.37±0.12(0.29±0.07) cm, and 0.51±0.13(0.43±0.07) cm for SRS6, 6, and 10 MV X‐rays, respectively. Circular field penumbra was comparable to corresponding square field. Average penumbra of 1×20 cm2 field was effectively constant over off‐axis positions of up to 12 cm with mean value of 0.16 (± 0.01)cm at 1.5 cm depth and 0.38 (± 0.04)cm at 10 cm depth. Minimum and maximum effective penumbra along the straight diagonal edge of irregular fields increased from 0.3 and 0.32 cm at 70° steep angle to 0.35 and 0.56 cm at 20° steep angle. Modified Picket Fence test showed average FWHM of 0.18 cm and peak‐to‐peak distance of 1.99 cm for 0.1 cm band and 2 cm interband separation. Dynamic multileaf collimation (DMLC) output factor remained within ± 1% for 6 MV and ± 0.5% for 10 MV X‐rays at all gantry positions, and was reproducible within ± 0.5% over a period of 14 months. The static leaf shift was 0.03 cm for all energies, while dynamic leaf shift was 0.044 cm for 10 MV and 0.039 cm for both SRS6 and 6 MV X‐rays. The dose depression and corresponding tongue‐and‐groove size were 24% and 0.17 cm for 6 MV and 19% and 0.20 cm for 10 MV X‐rays. Average transmission through HDMLC was 1.09%, 1.14% and 1.34% for SRS6, 6 and 10 MV X‐rays. Analysis of DynaLog files for leaf speed test in arc dynamic mode, delivery test of dynamic conformal arc, and step‐and‐shoot and sliding window IMRT showed at least 95% or more of the error counts had misplacements < 0.2cm, with maximum root mean square (RMS) error value calculated at 0.13 cm. Accurate and reproducible leaf position and gap width, and less leakage and small consistent penumbra over the fields demonstrate HDMLC suitable for high‐dose resolution SRS and IMRT.PACS number: 87.56.N‐, 87.55.Qr, 87.50.cm, 87.55.de, 87.53.Ly

Characteristics and performance of the first commercial multileaf collimator for a robotic radiosurgery system

“…Other MLCs have been reported to achieve comparable accuracy. 17,19 Second, AQA and robot pointing tests with MLC did not show any deterioration over the test cycle and yielded similar numerical data as for fixed cone assemblies. 23,24 This demonstrates that the mechanical stability of the robotic arm is adequate to handle the higher weight of the MLC (48 kg compared to 30 kg for fixed cones/35 kg for Iris) in routine use.…”

“…However, its specific design and leaf tilt caused slight differences between Y + and Y − penumbrae as well as a dependence on off-axis position, which were not seen with other MLCs. 19,21 Transmission and leakage of the InCise™ MLC did not exceed 0.5% for any assessed Linac head orientation. This result compares favorably to several other current MLCs with small leaf width, which yielded larger values of around 1% or higher.…”

“…Fischer et al 17 and García-Garduño et al 18 determined penumbrae of around 2.5 mm, in a slightly different measurement setup. Sharma et al 19 found a range of 2-4 mm for 6 MV delivery modes and different field sizes, and Solberg et al 20 reported around 4.5 mm for a 10×10 cm 2 field on a gimbaled Linac. It is important to keep in mind that different exposure conditions chosen by various authors have an impact on numerical results and impede direct comparability.…”

“…This result compares favorably to several other current MLCs with small leaf width, which yielded larger values of around 1% or higher. [16][17][18][19]21,22 Since the current finite sized pencil beam algorithm of Multiplan™ assumes zero transmission, low values for leakage and transmission are required to minimize dose discrepancies.…”

Critical neurological structure sparing radiosurgery of vestibular schwannoma: Dosimetric comparison of different techniques and dose prescription methods

“…The detailed physical and dosimetric characteristics of HDMLC especially for radiosurgery and IMRT were reported elsewhere. [22] In comparison to other mMLC/MLC, HDMLC has thinner leaf width, less effective penumbra and leakage. A margin dose of 12-13 Gy (mean 12.3 Gy) was prescribed at 80% isodose while maintaining the maximum dose to brainstem at 12 Gy and minimizing the dose to cochlea as low as possible.…”