Abstract
BackgroundTo investigate the feasibility and benefits of a modified adaptive radiotherapy (ART) by replanning in the initial CT (iCT) with new contours from a repeat CT (rCT) based on CT-CT image fusion for nasopharyngeal cancer (NPC) patients underwent volumetric modulated arc radiotherapy (VMAT).Materials and methodsNine NPC patients underwent VMAT treatment with a rCT at 23rd fraction were enrolled in this study. Dosimetric differences for replanning VMAT plans in the iCT and in the rCT were compared. Volumetric and dosimetric changes of gross tumor volume (GTV) and organs at risk (OARs) of this modified ART were also investigated.ResultsNo dosimetric differences between replanning in the iCT and in the rCT were observed. The average volume of GTV decreased from 78.83 ± 38.42 cm3 in the iCT to 71.44 ± 37.46 cm3 in the rCT, but with no significant difference (p = 0.42).The average volume of the left and right parotid decreased from 19.91 ± 4.89 cm3 and 21.58 ± 6.16 cm3 in the iCT to 11.80 ± 2.79 cm3 and 13.29 ± 4.17 cm3 in the rCT (both p < 0.01), respectively. The volume of other OARs did not shrink very much. No significant differences on PTVGTV and PTVCTV coverage were observed for replanning with this modified ART. Compared to the initial plans, the average mean dose of the left and right parotid after re-optimization were decreased by 62.5 cGy (p = 0.05) and 67.3 cGy (p = 0.02), respectively, and the V5 (the volume receiving 5 Gy) of the left and right parotids were decreased by 7.8% (p = 0.01) and 11.2% (p = 0.001), respectively. There was no significant difference on the dose delivered to other OARs.ConclusionPatients with NPC undergoing VMAT have significant anatomic and dosimetric changes to parotids. Repeat CT as an anatomic changes reference and re-optimization in the iCT based on CT-CT image fusion was accurate enough to identify the volume changes and to ensure safe dose to parotids.
Highlights
Due to its dose painting capability and sharp dose gradient, intensity modulated radiotherapy (IMRT), and recently developed new IMRT delivery method: volumetric modulated arc therapy (VMAT), have been accepted as the primary treatment modalities for nasopharyngeal cancer (NPC) patients [1,2]
The average volume of gross tumor volume (GTV) decreased from 78.83 ± 38.42 cm3 in the initial computed tomography (CT) (iCT) to 71.44 ± 37.46 cm3 in the repeat CT (rCT), but with no significant difference (p = 0.42).The average volume of the left and right parotid decreased from 19.91 ± 4.89 cm3 and 21.58 ± 6.16 cm3 in the iCT to 11.80 ± 2.79 cm3 and 13.29 ± 4.17 cm3 in the rCT, respectively
No significant differences on planning target volume of GTV (PTVGTV) and PTVCTV coverage were observed for replanning with this modified adaptive radiotherapy (ART)
Summary
Due to its dose painting capability and sharp dose gradient, intensity modulated radiotherapy (IMRT), and recently developed new IMRT delivery method: volumetric modulated arc therapy (VMAT), have been accepted as the primary treatment modalities for nasopharyngeal cancer (NPC) patients [1,2]. One retrospective study demonstrated that repeat CT (rCT) imaging and IMRT replanning helped to ensure adequate doses to target volumes and safe doses to normal structures for patients who had clinically identified anatomic changes during the course of IMRT [6]. It is hard for physicians to delineate the same superior and inferior boundary of targets in two different CT sets It might introduce additional errors during the transmission of the beam configuration due to the beam isocenter displacement resulted from volume changes. To investigate the feasibility and benefits of a modified adaptive radiotherapy (ART) by replanning in the initial CT (iCT) with new contours from a repeat CT (rCT) based on CT-CT image fusion for nasopharyngeal cancer (NPC) patients underwent volumetric modulated arc radiotherapy (VMAT)
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