Abstract
The purpose of this study was to compare anatomical and dosimetric variations in first 15 fractions, and between fractions 16 and 25, during intensity‐modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). Twenty‐three NPC patients who received IMRT in 33 fractions were enrolled. Each patient had two repeat computed tomography (CT) scans before the 16th and 25th fraction. Hybrid IMRT plans were generated to evaluate the dosimetric changes. There was a significant decrease of the transverse diameter of nasopharyngeal and neck as well as gross tumor volume (GTV) in the primary nasopharyngeal carcinoma (GTVnx) and involved lymph nodes (GTVnd) during the first 15 fractions, and between fraction 16 and 25 (p<0.05). Consequently, there was a significant reduction of the percentage of the volume receiving the prescribed dose (V100) of CTV1 and GTVnd, which was more prominent after the first 15 fractions treatment compared to that between fraction 16 and 25 (p<0.05). Additionally, there was a significant increase in the mean dose (Dmean) and percentage of volume receiving ≥30Gy(V30) to the bilateral parotid in the first 15 fractions (p<0.05), but not between fraction 16 and 25. While the maximum dose to the spinal cord was significantly increased both in the first 15 fractions, and between fraction 16 and 25 (p<0.05), the increase of the percent of spinal cord volume receiving ≥40Gy(V40) was significantly higher in the first 15 fractions compared to that between fraction 16and25(p<0.05). Based on the dose constraint criterion in the RTOG0225 protocol, a total 39.1%(9/23) of phantom plan 1 (generated by applying the beam configurations of the original IMRT treatment plan to the anatomy of the second CT scan) and 17.4%(4/23) of phantom 2 (generated by applying the beam configurations of the replan 1 to the anatomy of the third CT scan) were out of limit for the dose to the normal critical structures. In conclusion, our data indicated that anatomic changes resulted in more predominant dosimetric effects in the first 15 fractions, and between fractions 16 and 25, of IMRT.PACS number: 87.53.Bn, 87.55.de, 87.55.Qr
Highlights
Nasopharyngeal carcinoma (NPC) is common among Asians, especially the Southern Chinese.[1]. Radiation therapy with or without chemotherapy is the definitive treatment for NPC.[2,3,4] In external beam radiotherapy, treatment has always aimed at administering an adequate dose coverage to the entire tumor volume while protecting the surrounding normal tissues
There is a lack of studies that compare the effects of different time periods of repeat computed tomography (CT) scans and replans on conformality and dose distributions during intensity-modulated radiotherapy (IMRT) treatment, which may be helpful to decide the optimal timing of replans during IMRT
Our results were consistent with the previous report that the doses to 95% of the planning target volumes of the gross tumor volume and the clinical target volume were reduced during the course of IMRT for patients with head and neck cancer.[17] it was reported in another study that the anatomical changes had no effect on tumor dose coverage in patients with head and neck cancer.[26]
Summary
Nasopharyngeal carcinoma (NPC) is common among Asians, especially the Southern Chinese.[1]. Some patients receiving radiation therapy (RT) to the head and neck will have significant anatomic changes during their treatment course, including shrinking primary tumors or nodal masses, resolving postoperative changes/edema, and changes in overall body habitus/weight loss.[5,6,7,8,9,10,11,12] These variables could theoretically cause deviations in radiation dose delivery from the initial treatment plan, especially the highly conformal treatment approaches,(13-15) such as intensity-modulated radiotherapy (IMRT).(16-21) It has been reported that replanning by using the second CT scan with an average interval of 19 ± 6 fractions during the course of IMRT for head and neck cancer patients significantly reduced the normal organ dose and increased the target dose coverage, compared with using the original plan on the new anatomy.[17] Our previous studies implicated that 50% of IMRT plans may need replanning before the 25th fraction because of the overdose to the normal sensitive structures.[20,22] Recently, Zhao et al[23] conducted a retrospective study to demonstrate that the IMRT replan improved the three-year local progression-free survival for patients who had American Joint Committee on Cancer (AJCC) stage higher than T3 (T3, 4Nx) and eased the after effects for patients who had large lymph nodes (AJCC stage TxN2,3). We conducted the present study to compare anatomical and dosimetric variations in the first fractions, and between fraction and 25, during the course of IMRT for NPC patients
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