Hypothalamic Contouring And Radiation Dose To The Hypothalamus For Patients Undergoing IMRT With Nasopharyngeal Carcinoma

Part one: Hypothalamic contouring and radiation dose to the hypothalamus for patients undergoing IMRT with nasopharyngeal carcinoma and its relationship to tumor and nodal stageObjective: To present a guide to contouring the hypothalamus on axial images as would be done in the standard process of planning conformal radiotherapy and determine tumor factors contributing to the dose received by the HT among patients undergoing IMRT for NPC.Methods: Ninety-seven patients with biopsy-proven NPC included in the study.The total prescribed dose to the planning target volume(PTV)was 70Gy(D95%)delivered in 2.12-Gy daily fractions using IMRT.The HT was delineated separately on axial imaging from the CT scan obtained at simulation for each patient.We explain how to contour the HT on axial 3D-FSPGR MRI with intravenous gadolinium.IMRT treatment plans were generated without dose-volume constraints to the HT.A range of dose-volume statistics was calculated.HT dose parameters were compared with tumor and nodal stage.Results: The hypothalamic contour was created using MRI-based anatomic landmarks representing surrogate boundaries for the hypothalamus itself.The mean HT volume was 0.97±0.11cm3.The mean HT maximum dose with T1/2 and T3/4 was 25.13±1.95 Gy and 45.46±16.34Gy(P<0.0001),and the HT mean dose was 13.46±8.64 Gy and 28.87±15.72Gy(P<0.0001)respectively.Conclusion: The article presents a guide to contouring the HT on axial MRI for radiation therapy treatment planning.Our dosimetric analysis suggests that for NPC patients treated with IMRT,the HT receives significantly high doses.The mean values for HT maximum and mean dose were higher in T3/4 group compared with T1/2 group.HT sparing IMRT for NPC is necessary.Part two: Evaluation of the dosimetric feasibility of hypothalamus sparing IMRT in patients with nasopharyngeal carcinomaObjective: To evaluate the dosimetric feasibility of using HT sparing IMRT in patients with NPC.Methods: Twenty cases of either T1-2 or T3-4 NPC were selected for this study.Standard IMRT treatment plans were constructed using the volume and dose constraints for the targets and organs at risk(OAR).Experimental plans were constructed using the same criteria,with the addition of the HT as OAR.The two dose-volume histograms for each case were compared for the targets and OARs.Results: All plans achieved the protocol dose criteria.The homogeneity index,conformity index,and coverage index for the planning target volumes(PTV)were not significantly compromised by the avoidance of the HT.The doses to all OARS,excluding the HT,were similar.Both the dose(Dmean,Dmax)and volume(V20,V30)parameters for the HT were significantly lower in the HT sparing plans(P<0.05).Conclusions: IMRT for patients with NPC exposes the HT to a significant radiation dose.HT sparing IMRT planning significantly decreases this dose,with minimal impact on the therapeutic targets and other OARs.Part three: Dosimetric change of hypothalamus during the treatment course of intensity-modulated radiation therapy for locally advanced nasopharyngeal carcinomaObjective: To evaluate the dosimetric change of HT in nasopharyngeal carcinoma patients during IMRT.Methods: Ten patients with locally advanced NPC treated with IMRT had repeated computed tomography after twenty fractions.A hybrid plan was made to the anatomy of the second computed tomography scan.The dose of the original plan,hybrid plan,and new plan were compared.Results: There was volume reduction of target volumes and parotid gland over the course of IMRT.Relative to the original plan,the hybrid plans demonstrated significantly higher maximal dose and mean dose to the HT,lower D95 to target volumes with greater dose inhomogeneity and lower conformity,higher maximum doses to the brainstem and spinal cord,and higher median doses to the parotid glands.A new plan reduced the dose of HT,brainstem,spinal cord,and parotids,with sufficient dose coverage of targets.Conclusions: These patients with locally advanced NPC might benefit from replanning.Part four: Assessment of hypothamalus/pituitary gland dose in the treatment of advanced T-staged nasopharyngeal carcinoma with IMRT versus 2D-RTObjective: To determine the radiation dose to the HT/PG in advanced T-staged NPC patients treated with IMRT and to compare it to the dose delivered by a two-dimensional technique.Methods: Eleven patients with T3/4 NPC who were treated with IMRT were selected for this analysis.Two dimensional plans were reconstructed for comparison.The HT/PG were retrospectively contoured,and the dose-volume histograms(DVHs)of the targets and the HT/PG were compared between IMRT and 2D treatment plans.Results: With IMRT,the dose delivered to 95% of the tumor volume was significantly higher than that of 2D-RT,reflecting the greater conformity of this technique.The mean doses to the HT/PG were 28.55 Gy and 58.76 Gy respectively with IMRT vs 34.75 Gy and 68.83 Gy respectively with 2D-RT plans.After the application of HT/PG sparing,it was possible to reduce the mean dose to 22.74 Gy and 50.74 Gy in the IMRT replans.Conclusions: IMRT achieves the superiority of clinical target volume coverage and dose homogeneity over 2D-RT,but the HT/PG doses remain high.HT/PG sparing IMRT replanning significantly decreases the doses,with minimal impact on the target and other OARs.