Equivalent‐quality unflattened photon beam modeling, planning, and delivery

Yunfei Huang,Ryan T Flynn,R Alfredo,C Siochi,John E Bayouth

doi:10.1120/jacmp.v14i4.4211

Yunfei Huang, Ryan T Flynn + Show 3 more

Open Access

https://doi.org/10.1120/jacmp.v14i4.4211

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Abstract

The clinical application of the flattening filter‐free photon beam (FFF) has enjoyed greater use due to its advantage of reduced treatment time because of the increased dose rate. Its unique beam characteristics, along with the very high‐dose rate, require a thorough knowledge of the capability and accuracy in FFF beam modeling, planning, and delivery. This work verifies the feasibility of modeling an equivalent quality unflattened photon beam (eqUF), and the dosimetric accuracy in eqUF beam planning and delivery. An eqUF beam with a beam quality equivalent to a conventional 6 MV photon beam with the filter in place (WF) was modeled for the Pinnacle3 TPS and the beam model quality was evaluated by gamma index test. Results showed that the eqUF beam modeling was similar to that of the WF beam, as the overall passing rate of the 2%/2mm gamma index test was 99.5% in the eqUF beam model and 96% in the WF beam model. Hypofractionated IMRT plans were then generated with the same constraints using both WF and eqUF beams, and the similarity was evaluated by DVH comparison and generalized 3D gamma index test. The WF and eqUF plans showed no clinically significant differences in DVH comparison and, on average >98% voxels passed the 3%/3mm 3D gamma index test. Dosimetric accuracy in gated phantom delivery was verified by ion chamber and film measurements. All ion chamber measurements at the isocenter were within 1% of calculated values and film measurements passed the 3mm/3% gamma index test with an overall passing rate >95% in the high‐dose and low‐gradient region in both WF and eqUF cases. Treatment plan quality assurance (QA), using either measurement‐based or independent calculation‐based methods of ten clinically treated eqUF IMRT plans were analyzed. In both methods, the point dose differences were all within 2% difference. In the relative 2D dose distribution comparison, >95% points were within 3% dose difference or 3 mm DTA.PACS number: 87.55.kh

Highlights

We provided a comprehensive 3D dose distribution comparison to quantitatively evaluate the similarity between the WF and equivalent quality unflattened photon beam (eqUF) plans
While the photon fluence in the WF model was roughly constant across the lateral regions, it is the greatest at the central axis (CAX) and decreases rapidly away from the CAX in the eqUF beam model
This effect results in a difference in the off-axis softening factor, which is much smaller in the eqUF beam model (0.9 in the eqUF model vs. 9.8 in the WF model), a fact observed by other groups, as well.[28]. Differences in other modeling parameters were observed, such as a 43% lower electron contamination dose at the surface and a 60% smaller Gaussian height in the eqUF beam model

Summary

Introduction

109 Huang et al.: eqUF beam modeling/planning generated by removal of the flattening filter.[1,2,3,4,5,6] One of the implementation challenges of the UF photon beam is that its characteristics differ significantly from that of the conventional flat photon beam with the flattening filter in place (WF), as documented in previous studies, including one by the author.[7,8,9,10,11,12,13,14,15,16,17]Some groups have reported UF beam modeling and planning results using various dose calculation algorithms.[3,6,18,19,20,21,22] Hrbacek et al[22] described in detail the UF beam modeling process for a TPS using the anisotropic analytical algorithm, and Stathakis et al[23] commissioned an UF beam for the TPS using the superposition/convolution algorithm. Previous work showed that the integral dose to normal tissue increased in UF beam planning due to the softer energy spectrum.[5] In our work, the energy of the UF beam was tuned to match the original flat beam (eqUF) to avoid the increase in skin dose, as well as to obtain greater increase in dose rate. The increase in dose rate is even greater (four- to five-fold increase) for energy modified eqUF beam[16,24] compared to a two-fold increase in the energy unaltered mode. This requires the verification of the UF beam delivery accuracy, especially for gated treatments where the beams are turned on and off frequently. Sites affected by respiratory motion (such as liver and lung cancer) that need gated technique have yet to be investigated

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