Optimum Parameters for Imaging and Dose Calculation in Lung SBRT

Abstract

<h3>Purpose/Objective(s)</h3> Efficiency, set up accuracy, patient comfort and accurate dose calculations are critical requirements in radiation therapy especially in stereotactic body radiotherapy (SBRT). There are several CT imaging parameters such as kV, mAs, image reconstruction algorithm filters, field of view and pitch that affect image quality used for volume delineation and dose calculation. Modifying the imaging parameters may lead to variation in Hounsfield Units (HU) that may affect the dose calculations. We investigated the relationship between the image reconstruction algorithms (Body (B) and Lung filter (L)), HU and dose distribution. We hypothesize that variation in the imaging parameters may lead to better image quality with clinically insignificant dose to target and organs at risk (OARs). <h3>Materials/Methods</h3> Ten lung SBRT patients were imaged on a 16 slice CT scanner. The standard clinical scan (C) is a B- reconstruction filter-based scan with low pitch. For selected patients, in additional to standard scan, sharp scans were acquired using greater pitch and L-filter algorithm to see the effect of density and dose calculation. Clinical workflow includes fusing the images acquired with B and L filter. Clinically B filter images are used for planning and treatment. For this study purposes, an additional plan was generated on L filtered images using appropriate HU to density curve. CT density phantom was scanned on a 16 slice CT scanner with standard Body (B) filter as well as standard images of CT density phantom were reconstructed with lung (L) filter algorithm to generate HU to physical density table for a treatment planning system and used for dose calculation. Image qualities for the two filter were evaluated visually by an attending radiation oncologist. The dosimetric differences in the target coverages were evaluated using the HU curve generated for B and L filter. <h3>Results</h3> For clinical cases, using lung SBRT protocols, planning target volume (PTV) coverage V100% were recorded. In addition to the target coverage mean physical density was recorded for the B and L filter and C-clinical scans. Average difference in the PTV physical density was 2.4%, 0.3% and 2.1% for B-C, L-C and B-L, where B stands for – B Filter density curve on the sharp scan, L – L Filter density curve on the sharp scan, C – Clinical scan. No significant differences were noted in PTV coverages for B-C 1.7% (p < 0.0004), L-C 2.03% (p < 0.0007) and B-L -0.29% (p < 0.0005) based plans for V100%. <h3>Conclusion</h3> The L filter high pitch combination improved image contrast and thus helped in contouring. No significant difference in the PTV coverage was observed when B filter HU curve was applied to sharp quality scan used for dose calculation. Thus, images acquired with L filter can be used clinically with existing B filter HU curve. Overall Differences in dosimetric parameters are insignificant compared to the small uncertainties in HU numbers.