| Purpose:1. To investigate the design of intensity-modulated whole pelvic radiation therapy (IMRT).2. To evaluate the dose distribution difference in organ at risk (OAR) in IMRT delivered by different fields.3. To evaluate the dose distribution difference in OAR and PTV in IMRT and conventional radiotherapy (CRT).Method:1. A patient with cervical cancer, immobilized in vacuum pad, was given a CT simulation scan in supine position. The CTV, and OAR (small bowel, rectum, bladder) were delineated. IMRT plan was generated using PrecisePLAN system inversely under the constraints. Through comparing the isodose curve and DVH in IMRT with CRT, dose distribution characteristics of pelvic external intensity-modulated radiation therapy were gained.2. 6 patients were selected. After fixed and simulated following above way, 4 plans were generated for each patient: a 2-field conventional radiotherapy plan, and 3 IMRT plans delivered by 7-,8-,9-field respectively. Then dose in OAR were calculated in all three IMRT plans. The one in which dose OAR received was the lowest was chosen to have further comparison with CRT , mainly in PTV Dmax, Dmin, HI (Heterogeneity Index), and CI (Conformal Index). The percent volumes of small bowel, rectum, and bladder were also analyzed at 5 dose levels (10Gy, 20Gy, 30Gy, 40Gy, 45Gy).Results:1. The PTV dose distribution parameters in IMRT and CRT planning were : Dmin were 41.4Gy and 43.2Gy, Dmax 50.5Gy and 48.1Gy, HI: 1.10 and 1.06, CI: 0.91 and 0.32 ; The volume received more than 45Gy in the plan of IMRT and CRT were as follows: small bowel: 15% and 85%, rectum: 31% and 90%, bladder: 7% and 88% .2. No obviously difference could be noted in the 3 IMRT plans. But numerically, Volume receiving more than 30Gy of small bowel were smallest in 8-field, followed by 9-field, then 7-field; and of rectum and bladder, the sequence was 9-field<8-field< 7-field.3. The PTV dose distribution parameters in IMRT and CRT planning were: Dmin were (41.9±0.3)Gy and (42.5±0.2)Gy, Dmax (50.1±0.9)Gy and (47.9±0.3)Gy, HI: 1.10±0.01 and 1.06±0.01, CI: 0.87±0.02 and 0.31±0.03 respectively. All the above parameters had statistical significance (P<0.05).4. The OAR dose distribution parameters in IMRT and CRT planning were: (1) At low radiation level, a larger volume of small bowel was irradiated in the IMRT than CRT, V10 was 77.3%±6.9% and 71.5%±5.1% respectively (P=0.03). However, IMRT plan resulted in significant reduction in SBR volume when receiving more than 30Gy; the volume of which was 1/4 of that of CRT plan at the prescribed dose.(2) IMRT plan resulted in significant reduction in rectum volume when receiving more than 30Gy; the volume of which were 23.1%±20.1% and 87.6±13.7 respectively (P<0.001). (3) IMRT plan resulted in significant reduction in bladder volume when receiving more than 40Gy; the volumes of which were 6.9%±3.7%,93.7%±5.9% respectively (P<0.001). Conclusion:1. IMRT was better when delivered in 8-field technique in pelvic external irradiation in cervical cancer.2. Compared with CRT, IMRT could improve CI, and reduce the dose of OAR received , but these benefits were achieved at a cost of higher dose inhomogeniety in target volume.3. Compared with CRT, IMRT could significantly reduce the volume of small bowel , rectum ,and bladder that receiving high dose .
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