Commissioning and implementation of a stereotactic conformal radiotherapy technique using a general‐purpose planning system

M. Amin Mosleh‐Shirazi,Frank H. Saran,Vibeke N. Hansen,Peter J. Childs,Alan P. Warrington

doi:10.1120/jacmp.v5i3.1948

M. Amin Mosleh‐Shirazi, Frank H. Saran + Show 3 more

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https://doi.org/10.1120/jacmp.v5i3.1948

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Abstract

The purpose of this paper is to report on commissioning and clinical implementation of a customized system for pediatric stereotactic conformal radiotherapy (SCRT). The system is based on the Pinnacle treatment‐planning system and its interfaces with other equipment: (1) Beam models were optimized for our compact blocking system and a new LINAC. (2) Three CT‐to‐density conversion tables were evaluated, one using tabulated data for a commercial phantom, the second including additional points from the manufacturer's data for the inserts in an in‐house phantom, and the third using measured densities for the in‐house phantom materials combined with tabulated data for the commercial phantom. (3) Blocks were transferred to a computerized block cutter using in‐house software that extracted the block shape from the export file and custom‐fitted the additional necessary shapes. (4) In the absence of a DICOM RT Image link, a method based on screen data capture was used to export digitally reconstructed radiographs (DRRs) to two portal imaging systems for treatment verification. Lens shielding by multileaf collimation in the anterior‐posterior isocenter verification field was investigated. (1) Computed dose distributions using the beam models agreed with measurements well within published acceptability criteria. A difference of up to 1.0 mm was measured between the beam's eye views of aperture blocks and computed 50% isodose contours for a 2×2×2 mm dose calculation grid. (2) The third table, which included measured densities, improved the accuracy of the calculated isocenter dose by up to 0.5% in typical patient SCRT treatments and up to 1.0% in a phantom with 5‐cm diameter inhomogeneity inserts. (3) The block export and customization process was shown to introduce no additional uncertainty. A 1‐mm block production uncertainty was measured using film dosimetry on six blocks. (4) The DRR transfer method did not introduce uncertainty into the process. Verification field shielding reduced lens dose by 12 to 15 times. In conclusion, this customized system for planning and verification of pediatric SCRT provides a high level of precision as well as reasonable practical efficiency.PACS numbers: 87.53.Kn, 87.53.Ly, 87.53.Oq, 87.53.Tf, 87.53.Uv

Highlights

Small-to-medium sized benign or low-grade childhood brain tumors are routinely treated at The Royal Marsden Hospital with fractionated stereotactic conformal radiotherapy (SCRT) using static noncoplanar conformal fields
Beam modeling Machine data incorporating small-field beam models, manually optimized for our SCRT blocks, had been created on Pinnacle for the 6-MV beam of an Elekta SL15 LINAC (Elekta AB, Stockholm, Sweden).(2) Instead of using the published or interpolated spectra available on the system, a Monte Carlo–generated photon energy spectrum had been used in that model.[2]. The commissioning process was repeated for the 6-MV beam of an Elekta Precise LINAC
As with the model for the Elekta SL15 LINAC, computed dose distributions with SCRT blocks for the Elekta Precise LINAC agreed with measurements well within published acceptability criteria.[3,4,5] Concentrating on a limited set of treatment parameters has helped produce a model that is clinically acceptable for precise SCRT treatments

Summary

Introduction

Small-to-medium sized benign or low-grade childhood brain tumors are routinely treated at The Royal Marsden Hospital with fractionated stereotactic conformal radiotherapy (SCRT) using static noncoplanar conformal fields. The patient is immobilized in either an in-house relocatable stereotactic frame designed for children or a high-precision thermoplastic cast system.[1] Gross tumor volume delineation is performed based on coregistered CT-MR images. Customized blocks offer the highest conformity to targets. We use an in-house, compact blocking system for SCRT. Its compactness offers a greater beam access compared to tray-mounted blocks or tertiary multileaf collimators (MLCs), thereby reducing the overlap of noncoplanar beam paths. The positional reproducibility of the SCRT blocks is better than 0.5 mm,(1) and the beam passes through the block aperture unattenuated since no accessory tray is used

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