CBCT-based Research On The Timing Of Replanning During Intensity Modulated Radiation Therapy For Locally Advanced Nasopharyngeal Carcinoma

Objective: To analysis the volumetric and dosimetric changes of target volumes and organs at risk(OARs)during intensity modulated radiation therapy(IMRT)for locally advanced nasopharyngeal carcinoma(NPC)patients based on weekly cone beam CT(CBCT)and the timing of replanning.Methods:18 newly diagnosed patients with locally advanced NPC were included and all cases received radical dose for IMRT.Patients repeated CBCT weekly for setup and acquired CBCT images,all cases were immobilized consistent with the planned CT.For each patient,each weekly CBCT image was registration to the planning CT individually.The target volumes and OARs were outlined in weekly CT-CBCT image fusions by the same oncologist who was responsible for delineating the planning CT image.Subsequently,the original IMRT plan was transferred onto each weekly scan.So that the actual dose delivered to the target volumes and OARs could be generated based on the hybrid IMRT plan,the dose distribution and dose volume histograms(DVHs)were generated.A pragmatic schedule for replanning was ultimately established by accounting for significant change appearing in all parameters.Results: The GTVnx,GTVnd,I-parotid and C-parotid mean shrinkage after 25 fractions were 4.72%,44.04%,31.34% and 28.21%(all p?0.001).Compare weekly hybrid plan with the Plan0,the Dmean and D95 of GTVnx,the D95 of GTVnd,the Dmean and D95 of PTV1 and the Dmean?D95?V95 of PTV2 were gradually improved during the course of radiation.The value of HI in PTV1 and PTV2 were decreased which means the homogeneity were improved.The V110 and V95 of GTVnx and GTVnd,the V95 of PTV1 and the Dmean of GTVnd remained the same with Plan0.But the CI of PTV1 and PTV2 were decreased significantly.Comparing with the original plan,there was no significant difference in dosimetric index of brain stem and spinal cord.The Dmean and V30 of I-parotid and C-parotid improved significantly.The Dmean of I-parotid improved significantly in week 2(p=0.001)while the V30 in week 2 and 4(p?0.001,=0.034).The Dmean and V30 of C-parotid both improved significantly in week 2-4(Dmean p=0.003,0.019,0.024;V30 p?0.001,=0.044,0.027).The Dmean,V30 of I-parotid and the V30 of C-parotid both improved significantly most in week 2 which increased 2.09 Gy,5.46% and 3.09%.The Dmean of C-parotid improved significantly most in week 1 which increased 0.91 Gy.Comparing with Plan0,the Dmean and V30 of I-parotid increased 3.72 Gy and 10.2% while of C-parotid increased 3.47 Gy and 9.08% after 25 fractions.Conclusions: The volume of targets and bilateral parotid decreased significantly during intensity modulated radiotherapy for locally advanced NPC.To modify plan during radiotherapy can ensure the conformal of PTV1/2 while protecting the parotid gland.The best timing for replanning were 10?20 fractions during radiation.