Utility of Smart Arc CDR for intensity-modulated radiation therapy for prostate cancer

摘要

A LightSpeed RT16 (GE) was used for the planning CT. Body-fix (Medical Intelligence) and supine set-ups were used to keep the patients stationary. CT images were acquired in 2.5-mm slice thicknesses. The procedure to acquire the CT images is described below. The patients were classified into the following four groups according to the National Comprehensive Cancer Network (NCCN) [9] criteria: ‘low risk' (T1–T2a, Gleason score 2–6 and PSA 10 ng/ml), ‘intermediate risk' (T2b–T2c or Gleason score 7 or PSA 10–20 ng/ml), ‘high risk' (T3a or Gleason score 8–10 or PSA 20 ng/ml), and ‘very high risk' (T3b–T4). The target volume was determined based on the risk group. There were 3 low risk patients, 16 intermediate risk patients, 9 high risk patients, and no very high risk patients in this study. The planning target volume (PTV) was defined by the CTV + 10-mm margin (rectum side: 6 mm).The rectum (from the ischial tuberosities to the rectosigmoid flexure), the whole bladder, the small intestine surrounding 1.5 cm of the PTV, the sigmoid colon surrounding 1.5 cm of the PTV, and the femoral heads and body were defined as the organs at risk (OARs). Table 1 shows the characteristics and planning conditions of f-IMRT, CDR and VMAT. f-IMRT was planned using seven beams; VMAT and CDR were planned using a single arc. The prescribed dose to 95% (D95) of the PTV was 74 Gy in 37 fractions. The dose calculation grid size was set to 2 mm for all examples in this study. The isocenter was set to the PTV center. Adaptive Convolve [10] with heterogeneous correction was used for the dose calculation algorithm. Figures 1–3 display the average DVH for 28 patients of the PTV, rectum and bladder. Table 3 lists the dose indexes of the PTV, rectum and bladder, which are the average of all cases. The DVHs of the PTV for all of the methods were in good agreement, and D2, D50 and D95 were within 0.5 Gy. D2 of the rectum and bladder for all methods were within 0.5 Gy, and the volume receiving 50–70 Gy (V50–V70) were within 3%. The V20–V40 using the CDR approach was ～5% higher than with the f-IMRT and VMAT methods. The achievement level of the dose limit is evident for the f-IMRT, VMAT and CDR techniques. There were no ‘rejects'; 25 (89%), 26 (93%) and 21 (75%) patients were classified as ‘optimal' and the remaining patients were classified as ‘suboptimal' (f-IMRT, VMAT and CDR, respectively). The optimal CDR achievement rate was slightly lower than those of the other two methods. Tables 4 and 5 reveal the correlation between the achievement level of the dose limit and the risk group or organ volume. Our results indicate a significant reduction in the dose limit achievement rate for the CDR method when the bladder volume was 100 cm3. Figures 4–6 display the total number of MUs, the optimization time, and the irradiation time, respectively. For the f-IMRT, VMAT and CDR methods, the total numbers of MUs (average ± 1σ) were 469 ± 53, 357 ± 35 and 365 ± 33, respectively; the optimization times were ～50 min, 2 h and 2 h 40 min, respectively; and the irradiation times were ～280 s, 60 s and 110 s, respectively. For f-IMRT, the beam loading time for each field and the time between each segment could not be calculated accurately because this method used a step-and-shoot approach. Therefore, the irradiation times for f-IMRT were acquired using a stopwatch.