| Radiotherapy is one of the most important treatment for tumor therapy.As reports,about 70%patients with malignant tumor need to undergo radiotherapy.Nasopharyngeal carcinoma(NPC)and cervical carcinoma are common cancer in China.Radiation therapy is the best choice of treatment for them.At present,intensity-modulated radiotherapy(IMRT)has been widely used in clinical,not only can improve the curative effect,but also reduce side effects of normal tissue.However,a series of small segments and small monitor units(MUs)generated in IMRT may have a direct effect on the precise implementation of clinical treatment plans,so it is very important to study their dosimetry characteristics.Objective:To investigate dosimetric influences in Step-and-Shoot IMRT plans using the Prowess5.1 planning system through rounding subfield monitor units(MUs)up to their nearest integers.Methods:The ssIMRT plans for Fifty-five cancer patients with definite pathological diagnosis were enrolled into this study,including Thirty-three nasopharyngeal carcinoma(NPC)and Twenty-two cervical carcinoma.And these IMRT plans were divided into two groups:one group is composed of all the original plans which had been optimized completely by the TPS itself,named Porig;the other is a new group of which all the subfield monitor unit were rounded up to their nearest integers based on the Porig,named Pnew,and the segments shape of the Pnew were all the same as those of the Porig.The dosimetric characteristics and MU differences were recorded and compared between the Porig and Pnew The SPPS20.0 was used to perform the paired-t test for the data of two groups.Results:1.For NPC:(1)The target’s CI and HI of two groups were similar,having no statistical significance;The target coverage,D98、D95、D2、Dmean of PGTVnd and D98、D95 of PTV2 in Pnew were lower than those in Porig(P<0.05),V100%of PGTVnd、PTVnx and PTV1 decreased by 1.807%、0.655%and 1.258%respectively.(2)For OARs,only Dmean of left temporal lobe in two groups had statistical differences(P<0.05).(3)In addition,for the P1,the MU of per fraction in Pnew had increased by 1.6 MU(Pnew,519.758±46.410 VS.Porig,518.158±46.693;P<0.05).2.For cervical carcinoma:(1)The target’s CI and HI of two groups were similar,only HI of PGTV had statistical significance(P=0.037);The target coverage V100%,D98、D2、Dmean and V110%of PTV in Pnew were lower than those in Porig(P<0.001).and V100%of PGTV、PTV decreased by 12.709%and 5.632%respectively.(2)For OARs,all parameters in two groups had statistical differences(P<0.05)and Pnew were lower than those in Porig,Dmean of rectum、bladder、femoral head L and femoral head R decreased by 91.713cGy(P<0.001)、83.681cGy(P<0.001)、58.436cGy(P<0.001)and 67.364cGy(P<0.001)respectively;Dice of rectum、bladder and small intestine decreased by 91.582cGy(P<0.001)、80.968cGy(P<0.001)and 105.673cGy(P=0.012)respectively;V50of rectum、bladder、small intestine、femoral head_L and femoral head_R decreased by 3.973%(P<0.001)、4.186%(P<0.001)、0.818%(P<0.001)、0.427%(P=0.007)and 0.214%(P=0.049)respectively.(3)The MUs of two groups had no statistical significance(P=0.678).Conclusions:Using Prowess5.1 planning system to design step-and-shoot IMRT plan,the small changes of subfield monitor units will cause the change of the dosimetry,which should cause attention in clinical practice. |
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