The effect of extremely narrow MLC leaf width on the plan quality of VMAT for prostate cancer
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RESEARCH
Open Access
The effect of extremely narrow MLC leaf width on the plan quality of VMAT for prostate cancer Jong Min Park1,2,3,4, So-Yeon Park1,2,3,4, Jin Ho Kim1,2,3, Joel Carlson5 and Jung-in Kim1,2,3,4*
Abstract Background: To investigate the effect of multi-leaf collimators (MLCs) with leaf width of 1.25 mm on the plan quality of volumetric modulated arc therapy (VMAT) for prostate cancer. Methods: A total of 20 patients with prostate cancer were retrospectively selected. Using a high definition MLC (HD MLC), primary and boost VMAT plans with two full arcs were generated for each patient (original plan). After that, by shifting the isocenter position of the 2nd arc by 1.25 mm in the cranio-caudal direction, we simulated fluences made with MLCs with leaf width of 1.25 mm. After shifting, primary and boost plans were generated for each patient (shifted plan). A sum plan was generated by summation of the primary and boost plan for each patient. Dose-volumetric parameters were calculated and compared. Results: Both the homogeneity index (HI) and conformity index (CI) of the shifted plans were better than those of the original plans in primary plans (HI = 0.065 vs. 0.059 with p < 0.001 and CI = 1.056 vs. 1.044 with p = 0.006). Similarly, the shifted plans for the boost target volume showed better homogeneity and conformity than did the original plans (HI = 0.060 vs. 0.053 with p < 0.001 and CI = 1.015 vs. 1.009 with p < 0.001). The target mean dose of the original plans was closer to the prescription dose than that of the shifted plans in the case of sum plans (81.45 Gy vs. 81.12 Gy with p = 0.001). Conclusions: Use of extremely narrow MLCs could increase dose homogeneity and conformity of the target volume for prostate VMAT. Keywords: Multi-leaf collimator, Leaf width, Volumetric modulated arc therapy, Prostate cancer
Background Volumetric modulated arc therapy (VMAT) has been widely adopted in the clinic due to its superior ability to generate optimal dose distributions that deliver prescription doses to target volumes while reducing dose to radiosensitive organs [1–3]. Moreover, VMAT is more efficient in delivery than is intensity modulated radiation therapy (IMRT), spending fewer monitor units (MUs) as well as taking less time to deliver a dose distribution to the patient [4–6]. To acquire the optimal dose distribution, VMAT modulates photon beam intensities by * Correspondence: [email protected] 1 Department of Radiation Oncology, Seoul National University Hospital, Seoul, South Korea 2 Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, South Korea Full list of author information is available at the end of the article
varying multi-leaf collimator (MLC) positions, gantry rotation speed and dose rate, simultaneously, while rotating the gantry around the patient [1, 7–9]. For VMAT, the resolution of the modulated photon beam intensities, i.e. the resolution of fluence, is determined by the MLC leaf width, just as in IMRT [8, 9]. The resolution of the fluence ca
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