Investigation of fusion dose distribution for locally advanced cervical cancer under different bladder statuses for intensity‐modulated radiotherapy combined with intracavitary brachytherapy

Objective Intensity‐modulated radiotherapy (IMRT) combined with intracavitary brachytherapy (ICBT) is a standard radiotherapy technology for locally advanced cervical cancer (LACC), and bladder status is a common factor that affects dose distribution of the target and organs at risk (OARs). Under different bladder statuses, fusion dose distribution of IMRT combined with ICBT is unclear. The aim of the present study was to analyze the fusion dose distribution of targets and OARs for IMRT combined with ICBT in LACC under different bladder statuses. Methods A total of 20 patients with LACC who were treated in our department from 1 January 2015 to 31 July 2015 underwent magnetic resonance imaging and simulation computed tomography (Sim‐CT) scans under filled and empty bladder status. The magnetic resonance imaging and Sim‐CT scans were transmitted by Sim‐CT to the Oncentra treatment planning system and fused. The gross tumor volume (GTV) was delineated in the magnetic resonance imaging, and the clinical target volume (CTV), planning target volume (PTV) and OARs (intestine, bladder, rectum, left, and right femoral head) were delineated in Sim‐CT. The IMRT plan was designed with seven fields and 3‐D ICBT in the treatment planning system, and the radiation sources were X‐ray (6 MV) and 192Ir. The doses of the targets (D95%, D90%, D85%, D80%) and OARs (D1 cc and 2cc for intestine, D5%, 10%, and 30% for bladder, D1cc, 2cc, and 5cc for rectum, D1% for femoral head) were planned separately with IMRT and ICBT, and the geometric sum was used as the geometric dose. The treatment planning system plan used the superposition function to superimpose the IMRT and ICBT plans as a fusion plan, and the doses of the targets and OARs were calculated as a fusion dose. The relationship between the geometric and fusion doses of the targets and OARs was analyzed under different bladder statuses, and the dose contribution rates to the targets and OARs were calculated from ICBT. Results For the empty bladder: D95% (uGTV = 3.92, tCTV = 11.28, tPTV = 10.79), D90% (uGTV, CTV = 3.92, uPTV = 3.25), and D85% (u = 3.92), D80% (u = 3.92). The geometric doses of the targets were lower than the fusion doses. For the full bladder: D95% (uGTV, PTV = 3.92, tCTV = 15.96), D90% (uGTV = 3.81, uCTV, PTV = 3.92), D85% (u = 3.92), and D80% (uGTV = 4.70, uCTV, PTV = 3.92). The geometric doses of the targets (D95%, D90%, D85%, D80%) were lower than the fusion doses at P  < 0.001. The dose difference rate of GTV under the filled bladder condition was lower than that of the empty bladder (0.17–0.93% and 0.32–1.07%, respectively), whereas these values were similar in the empty bladder condition for CTV and PTV (1.10–2.75% and 1.22–3.40%, and 0.98–2.29% and 0.94–3.17%, respectively). For the empty bladder, the geometric doses of OARs (uintestine = 3.92; tintestine = 11.59; ubladder=3.92, 3.92, 3.36; urectum = 3.92; tfemoral head = 4.77 and 6.06) were higher than the fusion doses. For the full bladder, the geometric doses of OARs (tintestine = 10.27 and 8.84; tbladder = 10.69, 11.77, and 4.91; urectum = 3.36, 3.21, and 3.25) were higher than the fusion doses at P  < 0.005. The average geometric dose differences of D30% for the bladder and D1cc, 2cc, and 5cc for the rectum were higher than those of the fusion dose (1.90 Gy, 1.01 Gy, 0.87 Gy, 0.86 Gy and 1.86 Gy, 0.95 Gy, 0.79 Gy, 0.59 Gy). The D1% values for the right and left femoral head were 0.76 Gy, 0.41 Gy, 0.26 Gy, and 0.73 Gy. For the empty bladder: D95% (uGTV = 3.92, tCTV = 11.40, tPTV = 10.84), D90% (uGTV = 3.92, uCTV = 3.29, tPTV = 6.00), D85% (uGTV = 3.92, tCTV = 17.29, tPTV = 13.87), and D80% (uGTV = 3.92, tCTV = 16.60, tPTV = 15.41). The geometric dose contribution rate of ICBT to the targets was lower than that of the fusion dose; for the full bladder: D95% (uGTV = 9.87, uCTV = 15.78, uPTV = 10.65), D90% (uGTV = 3.81, tCTV = 20.70, tPTV = 17.64), D85% (tGTV = 8.31, tCTV = 23.27, tPTV = 19.78), D80% (tGTV = 4.68, uCTV = 3.92, tPTV = 19.90). The geometric dose contribution rate to the targets was lower than that of the fusion dose at P  < 0.005. The highest dose contribution rate of ICBT was to GTV. The geometric and fusion contribution rates were 51.12–63.89% and 48.10–60.80%, and 49.52–63.35% and 46.74–60.52% under the empty and filled bladder conditions, respectively. These values were <10.00% for CTV and PTV. For the empty bladder, the geometric dose contribution rate of ICBT to OARs (uintestine = 3.92; ubladder = 3.92, 3.92, and 3.36; urectum = 3.92; tfemoral head = 4.67 and 6.16) was higher than that of fusion. For the filled bladder, the geometric dose contribution rate to OARs (tintestine = 10.14 and 8.77; tbladder = 10.74, 11.82, and 4.93; urectum = 3.25, 3.21, and 3.21) was higher than that of fusion at P  < 0.005. Comparing the empty bladder with the filled bladder case, the dose contribution rates of ICBT to the rectum were 47.77–59.45% and 40.87–52.40%, and 47.82–58.78% and 41.61–52.00%, respectively, and the dose contribution rates to the bladder were 27.60–45.17% and 26.04–41.80%, and 23.36–43.67% and 21.89–40.22%, respectively. The dose contribution rates to the intestine were 30.90–36.90% and 28.85–34.79%, and 20.68–25.13% and 18.69–22.88%, respectively, with <10% to the femoral head.