Determining appropriate timing of adaptive radiation therapy for nasopharyngeal carcinoma during intensity-modulated radiation therapy

Abstract

BackgroundTo determine appropriate timing of an adaptive radiation therapy (ART) replan by evaluating anatomic and dosimetric changes of target volumes and organs at risk (OARs) during intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC).MethodsNineteen NPC patients were recruited. Each patient had repeat computed tomography (CT) scans after each five fractions and at treatment completion. Automatic re-contouring the targets and OARs by using deformable registration algorithm was conducted through CT-CT fusion. Anatomic changes were assessed by comparing the initial CT and repeated CT. Hybrid plans with re-contouring were generated and the dose-volume histograms (DVH) of the hybrid plan and the original plan were compared.ResultsProgressive volume reductions in gross target volume for primary disease (GTVnx), gross target volume for involved lymph nodes (GTVnd), and parotids were observed over time. Comparing with the original plan, each hybrid plan had no significant difference in homogeneity index (HI) for all the targets. Some parameters for planning target volumes for primary disease and high-risk clinical target volume (PTVnx and PTV1, respectively) improved significantly, notably starting from the 10th fraction. These parameters included mean dose (Dmean), dose to 95 % of the volume (D95), percentage of the volume receiving 95 % of the prescription dose (V95), and conformity index (CI) for PTVnx, and Dmean, D95, and CI for PTV1. The dosimetric parameters for PTVnd remained the same in general except for D95 and V95 which had significant improvement at specific time points; whereas for PTV2, similar trend of dosimetric changes was also observed. Dose to some OARs increased significantly at some time points.ConclusionsThere were significant anatomic and dosimetric changes in the targets and OARs. The target dose coverage in the hybrid plans did not get worse, but overdose occurred in some critical structures. Significant dosimetric changes should be considered as a trigger point at which ART replanning is indicated. D95/V95/CI for PTV2, Dmax for the brain stem, spinal cord, right eyeball and left lens, and Dmean/V30 for the parotids and glottis were taken into account for predicting the need for ART. Two replans at the 5th and 15th fractions were suggested.

Optimal adaptive IMRT strategy to spare the parotid glands in oropharyngeal cancer

“…While different numbers of replannings aimed at sparing the PG have been investigated in the literature, the optimal timing of these replannings has not been thoroughly assessed [8,13,15,16,[25][26][27][28][29][30][31][32][33][34]. The endpoint of these studies was the quantification of the dosimetric benefit of sparing the PG, while keeping or even increasing the minimum dose in the PTV, using different CT-based IMRT replanning scenarios.…”

Potential proton and photon dose degradation in advanced head and neck cancer patients by intratherapy changes

“…Treatment plan adaptation can be used to prevent severe dose degradation throughout the fractionated treatment course11 and is related with lower normal tissue complication probabilities 12. For IMRT treatment, up to two adaptation steps was reported to be sufficient and is logistically feasible 13, 14, 15, 16. Adaptive IMRT has been shown to be associated with improved locoregional control,17 especially for advanced tumor stages 18…”

Three-Phase Adaptive Radiation Therapy for Patients with Nasopharyngeal Carcinoma Undergoing Intensity-Modulated Radiation Therapy: Dosimetric Analysis

“…2 Hansen et al 7 found increased maximum doses to the brains stem occurred in 85% of patients, and all patients had increased maximum doses to the spinal cord, if adaptive replanning was not considered. Another study by Fung et al showed that the actual delivered doses to all listed OARs were higher than expected.…”

“…2 Briefly, GTVnx and GTVnd included the primary gross volume and the involved cervical lymphadenopathy, respectively, determined by the imaging, clinical, and endoscopic findings. The high-risk clinical target volume (CTV1) was defined as GTVnx plus 5-mm margin and entire nasopharyngeal mucosa plus 5-mm submucosal volume.…”

“…There are, indeed, large anatomic changes both in the targets and in OARs, which subsequently result in underdose to the targets and/or overdose to the surrounding critical structures according to studies from other investigators and ours. [2][3][4][5] Thus, the initial planning based on pretreatment condition may not truly reflect the dosimetric variations during the course of IMRT, and an intervention with adaptive radiation therapy (ART) is indicated.…”