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Open Access Research

Optimal set of grid size and angular increment for practical dose calculation using the dynamic conformal arc technique: a systematic evaluation of the dosimetric effects in lung stereotactic body radiation therapy

Ji-Yeon Park12, Siyong Kim3, Hae-Jin Park124, Jeong-Woo Lee5, Yeon-Sil Kim6 and Tae-Suk Suh12*

Author Affiliations

1 Department of Biomedical Engineering, The Catholic University of Korea Songeui Campus, Banpo4-dong, Seocho-gu, Seoul 137-701, Korea

2 Research Institute of Biomedical Engineering, The Catholic University of Korea Songeui Campus, Banpo4-dong, Seocho-gu, Seoul 137-701, Korea

3 Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA 23298, USA

4 Department Radiation Oncology, Ajou University School of Medicine, Suwon 443-721, Korea

5 Department of Radiation Oncology, Konkuk University Medical Center, Seoul 143-729, Korea

6 Department of Radiation Oncology, Seoul St. Mary’s Hospital, Seoul 137-701, Korea

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Radiation Oncology 2014, 9:5  doi:10.1186/1748-717X-9-5

Published: 4 January 2014

Abstract

Purpose

To recommend the optimal plan parameter set of grid size and angular increment for dose calculations in treatment planning for lung stereotactic body radiation therapy (SBRT) using dynamic conformal arc therapy (DCAT) considering both accuracy and computational efficiency.

Materials and methods

Dose variations with varying grid sizes (2, 3, and 4 mm) and angular increments (2°, 4°, 6°, and 10°) were analyzed in a thorax phantom for 3 spherical target volumes and in 9 patient cases. A 2-mm grid size and 2° angular increment are assumed sufficient to serve as reference values. The dosimetric effect was evaluated using dose–volume histograms, monitor units (MUs), and dose to organs at risk (OARs) for a definite volume corresponding to the dose–volume constraint in lung SBRT. The times required for dose calculations using each parameter set were compared for clinical practicality.

Results

Larger grid sizes caused a dose increase to the structures and required higher MUs to achieve the target coverage. The discrete beam arrangements at each angular increment led to over- and under-estimated OARs doses due to the undulating dose distribution. When a 2° angular increment was used in both studies, a 4-mm grid size changed the dose variation by up to 3–4% (50 cGy) for the heart and the spinal cord, while a 3-mm grid size produced a dose difference of <1% (12 cGy) in all tested OARs. When a 3-mm grid size was employed, angular increments of 6° and 10° caused maximum dose variations of 3% (23 cGy) and 10% (61 cGy) in the spinal cord, respectively, while a 4° increment resulted in a dose difference of <1% (8 cGy) in all cases except for that of one patient. The 3-mm grid size and 4° angular increment enabled a 78% savings in computation time without making any critical sacrifices to dose accuracy.

Conclusions

A parameter set with a 3-mm grid size and a 4° angular increment is found to be appropriate for predicting patient dose distributions with a dose difference below 1% while reducing the computation time by more than half for lung SBRT using DCAT.

Keywords:
Dynamic conformal arc therapy; Grid size; Angular increment; Lung; Stereotactic body radiation therapy