The Role Of Replanning During Fractionated Adaptive Intensity Modulated Radiotherapy For Locally Advanced Nasopharyngeal Carcinoma

OBJECTIVE: Significant anatomic changes may occur during intensity-modulatedradiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). The aim of this study wasto evaluate the volume and dosimetric variations during IMRT for locally advancedNPC and to identify the benefit of a two-phase adaptive IMRT method.METHODS: Twenty patients with locally advanced NPC treated with IMRTconcurrently combined with chemotherapy were included in this retrospective study.Each patient had both an initial planning CT (CT-1) and a repeated CT scan (CT-2)after treatment at a dose of40Gy. Target volumes and OARs were recontoured on theCT-2images, which were completed by the same attending physician to minimizevariability between observers in the delineation. Three IMRT planning scenarios werecompared:(1) the initial plan on the CT-1(plan-1);(2) the hybrid plan copied andrecalculated the initial plan on the CT-2(plan-2);(3) the replan generated on the CT-2and used to complete the course of IMRT (plan-3).RESULTS: The mean gross target volume and mean volumes of the positive necklymph nodes, high-risk clinical target volume, and the left and right parotid glandswere significantly decreased by30.2%,45.1%,21.1%,14.7%and18.2%, respectivelyon the CT-2. The percentage of the volume reduction of the target volume and parotidglands correlated significantly with its initial volume (P <0.05). There were nosignificant qualitative dosimetric differences among the different IMRT plans forGTVnx, GTVndor CTV1. The dose delivered to all target volumes was maintainedduring the course of IMRT, but the number of high dose regions was increased.Comparing plan-2with plan-1, the dose coverage of the targets was maintained,whereas the dose delivered to the parotid glands and spinal cord increasedsignificantly (P <0.05). Comparing plan-2with plan-1, the mean dose of left/right parotid gland was increased by3.4Gy (13.0%) and3.5Gy (11.78%), the differencesare statistical significant (P <0.05). The average V30of left/right gland increased by39.7%and44.04%(P <0.05). The D50of the right parotid gland increased by4.1Gy(15.95%)(P <0.05), while the D50of the left parotid gland increased by2.4Gy (11.0%), with no statistical significant difference (P>0.05). The Dmaxand Dmeanof thespinal cord increased by2.2Gy (5.37%) and5.9Gy (26.22%), the differences arestatistical significant (P <0.05). Replanning reduced the dose delivered to the parotidglands and spinal cord (P <0.05). Comparing plan-3with plan-2, the mean dose ofleft/right parotid gland was decreased by3.0Gy (10.17%) and4.1Gy (13.80%), thedifferences are statistical significant (P <0.05). The average V30of left/right glanddecreased by22.28%and28.57%(P <0.05), while the D50of the left/right parotidgland decreased by-1.5Gy（-6.20%）and1.8Gy (7.53%), with no statistical significantdifference (P>0.05). The Dmaxand Dmeanof the spinal cord also decreased by3.9Gy(9.51%) and4.7Gy (16.55%), the differences are statistical significant (P <0.05).CONCLUSIONS: Measurable anatomic changes may occur throughout the course ofIMRT for patients with locally advanced NPC. The dose delivered to all targetvolumes was maintained during the course of IMRT. Although the dose differences inthe brain stem, mandible and lens between the hybrid IMRT plan and initial plan wereof no significance, the actual dose delivered to the parotid glands and spinal cord wasincreased significantly. Repeat CT imaging and replanning at40Gy intervals duringthe course of IMRT for patients with NPC may be helpful in identifying dosimetricchanges and to ensure adequate dose delivery to the target volumes and safe doseexposure regarding critical OARs.These patients might benefit from replanningbecause of the sparing of the parotid glands and spinal cord.