Clinical Study of the Necessity of Replanning Before the 25th Fraction During the Course of Intensity-modulated Radiotherapy for Patients with Nasopharyngeal Carcinoma

Purpose: To quantify the target and normal structures on dose distributing variations during intensity-modulated radiotherapy (IMRT) and to assess the value of replanning for nasopharyngeal carcinoma (NPC) patients.

Methods And Materials: Twenty-eight NPC patients treated with IMRT were recruited. The IMRT was delivered in 33 fractions, to 70 to 76Gy, to the gross tumor volume (GTV). Before the 25th fraction of IMRT, a new simulation computed tomography (CT) scan was acquired for all patients. According to the dose constraint criterion in the Radiation Therapy Oncology Group (RTOG) 0225 protocol, the replanning was generated on the new simulation CT. With the Quality Assessment Center of a CORVUS 6.3 treatment planning system, a phantom plan was generated for each patient by applying the beam configurations of the initial plan to the anatomy of the new simulation CT. The dose-volume histograms of the phantom plan were compared with the replanning.

Results: The percentage of prescription dose delivered to the clinical target volume (CTV1) was significantly increased by 4.91% +/- 10.89%, whereas the maximum dose to the spinal cord, mean dose to the left parotid, and V30 to the right parotid were significantly decreased by 5.00 +/- 9.23Gy, 4.23 +/- 10.03Gy, and 11.47% +/- 18.89% respectively in the replanning, compared with the phantom plan (p < 0.05). Based on the dose constraint criterion in the RTOG0225 protocol, 50% of phantom plans (14/28) were out of limit for the dose to the normal critical structures, whereas no plan was out of limit in replanning (p < 0.001).

Conclusion: Replanning for patients with NPC before the 25th fraction during IMRT helps to ensure adequate dose to the target volumes and safe doses to critical normal structures.