Implementation of full/half bowtie filter models in a commercial treatment planning system for kilovoltage cone-beam CT dose estimations

Abstract

The purpose of this study was to implement full/half bowtie filter models in a com-mercial treatment planning system (TPS) to calculate kilovoltage (kV) cone-beam CT (CBCT) doses of Varian On-Board Imager (OBI) kV X-ray imaging system. The full/half bowtie filter models were created as compensators in Pinnacle TPS using MATLAB software. The physical profiles of both bowtie filters were imported and hard-coded in the MATLAB system. Pinnacle scripts were written to import bowtie filter models into Pinnacle treatment plans. Bowtie filter-free kV X-ray beam models were commissioned and the bowtie filter models were validated by analyzing the lateral and percent-depth-dose (PDD) profiles of anterior/posterior X-ray beams in water phantoms. A CT dose index (CTDI) phantom was employed to calculate CTDI and weighted CTDI values for pelvis and pelvis-spotlight CBCT protocols. A five-year-old pediatric anthropomorphic phantom was utilized to evaluate absorbed and effective doses (ED) for standard and low-dose head CBCT protocols. The CBCT dose calculation results were compared to ion chamber (IC) and Monte Carlo (MC) data for the CTDI phantom and MOSFET and MC results for the pediatric phantom, respectively. The differences of lateral and PDD profiles between TPS calculations and IC measurements were within 6%. The CTDI and weighted CTDI values of the TPS were respectively within 0.25 cGy and 0.08 cGy compared to IC measurements. The absorbed doses ranged from 0 to 7.22 cGy for the standard dose CBCT and 0 to 1.56 cGy for the low-dose CBCT. The ED values were found to be 36-38 mSv and 7-8 mSv for the standard and low-dose CBCT protocols, respectively. This study demonstrated that the established full/half bowtie filter beam models can produce reasonable dose calculation results. Further study is to be performed to evaluate the models in clinical situations.

Figure 1

Experimental setup of IC measurements in a CTDI body phantom. Note that an IC is separately placed in the middle of five locations (center and four peripheries) per each CBCT irradiation.

Figure 2

Pinnacle dose distributions of a kV static X‐ray beam with (a) full‐bowtie filter geometry and (b) half‐bowtie filter geometry in the cuboid water phantom.

Figure 3

Comparison of lateral and PDD profiles among Pinnacle dose calculations, IC measurements, and MC simulations: (a) lateral profiles of full‐bowtie filter model, (b) lateral profiles of half‐bowtie filter model, (c) PDD profiles of full‐bowtie filter model, and (d) PDD profiles of half‐bowtie filter model. The x‐axis is in cm.

Figure 4

Dose distributions of kV CBCT imaging protocols in the CTDI body phantom: pelvis spotlight CBCT protocol with full‐bowtie filter in (a) Pinnacle TPS and (b) MC simulations, and pelvis CBCT protocol with half‐bowtie filter in (c) Pinnacle TPS and (d) MC simulations. *MC data is presented with permission of published journal. ⁽¹³⁾ Note that the color scale in the MC simulations is in mGy.

Figure 5

Pinnacle dose distributions of a kV standard dose CBCT imaging protocol in five‐year‐old pediatric anthropomorphic phantom. Note that the CBCT imaging is calculated for the abdominal irradiation.

Figure 6

Comparison of absorbed dose distributions among MOSFET measurements, MC data, and Pinnacle calculations from (a) standard dose head CBCT imaging protocol and (b) low‐dose head CBCT imaging protocol. Note that the absorbed dose is assumed to be equivalent with the measured/calculated point dose. *MOSFET and MC data are presented with permission of published journal. ⁽¹⁵⁾ $\text{BM}=\text{bone marrow}$; $\text{CCC}=\text{Pinnacle collapsed cone convolution}$.