Effective spatially fractionated GRID radiation treatment planning for a passive grid block

Abstract

Objective: To commission a grid block for spatially fractionated grid radiation therapy (SFGRT) treatments and describe its clinical implementation and verification through the record and verify (R&V) system. Methods: SFGRT was developed as a treatment modality for bulky tumours that cannot be easily controlled with conventionally fractionated radiation. Treatment is delivered in the form of open-closed areas. Currently, SFGRT is performed by either using a commercially available grid block or a multileaf collimator (MLC) of a linear accelerator. In this work, 6-MV photon beam was used to study dosimetric characteristics of the grid block. We inserted the grid block into a commercially available treatment planning system (TPS), and the feasibility of delivering such treatment plans on a linear accelerator using a R&V system was verified. Dose measurements were performed using a miniature PinPoint TM ion chamber (PTW, Freiburg, Germany) in a water phantom and radiochromic film within solid water slabs. PinPoint ion chamber was used to measure the output factors, percentage depth dose (PDD) curves and beam profiles at two depths, depth of maximum dose (z max ) and 10 cm.Film sheets were used to measure dose profiles at z max and 10-cm depth.Results: The largest observed percentage difference between output factors for the grid block technique calculated by the TPS and measured with the PinPoint ion chamber was 3.6% for the 5 3 5-cm 2 field size. Relatively significant discrepancies between measured and calculated PDD values appear only in the build-up region, which was found to amount to ,4%, while a good agreement (differences ,2%) at depths beyond z max was observed. Dose verification comparisons performed between calculated and measured dose distributions were in clinically acceptable agreements. When comparing the MLC-based with the grid block technique, the advantage of treating large tumours with a single field reduces treatment time by at least 3-5 times, having significant impact on patient throughput. Conclusion:The proposed method supports and helps to standardize the clinical implementation of the grid block in a safer and more accurate way. Advances in knowledge: This work describes the method to implement treatment planning for the grid block technique in radiotherapy departments.

Technical Note: Dosimetric feasibility of lattice radiotherapy for breast cancer using GammaPod

“…25 Dose delivery using GRID typically employs a single field that deposits the maximum dose close to the skin surface and drops off along the depth resulting in non-uniform distribution of dose within the target volume. 26 In contrast, with GammaPod LRT, each shot location receives the same dose. The simulated and delivered plans illustrated that the tighter the spacing among the shots, the lower the peak-to-valley difference.…”

Spatially fractionated (GRID) radiation therapy using proton pencil beam scanning (PBS): Feasibility study and clinical implementation

“…10,12 The jaw travel limitation in some Linacs may also limit the maximal tumor size that can be treated with MLCs or require complex field matching. 11 In addition, due to MLC leakage, MLC-based GRID treatment results in 2-3 times higher surface doses 12 and smeared spatial fractionation that produces higher valley-to-peak ratios, 10 which may weaken the benefits of GRID therapy to some extent. In contrast, the PBS approach facilitates larger achievable field sizes and easily changeable spot separation.…”

“…The standard delivery method for GRID therapy involves using collimating blocks with a distinctive transmission pattern mounted on a linear accelerator (Linac) . The blocks have equally spaced holes that allow full photon intensity transmission.…”

“…[7][8][9] The standard delivery method for GRID therapy involves using collimating blocks with a distinctive transmission pattern mounted on a linear accelerator (Linac). 10,11 The blocks have equally spaced holes that allow full photon intensity transmission. The diameter of the holes and the spacing between the holes have been optimized to yield the best combination of tumor response and skin sparing.…”

Grid block design based on monte carlo simulated dosimetry, the linear quadratic and Hug–Kellerer radiobiological models

“…However, it has found from clinical experience, that the main drawback of the MLC-based approach is the relatively long delivery time in comparison with the Grid block approach. [ 12 ]…”