Evaluation of hippocampus dose for patients undergoing intensity-modulated radiotherapy for nasopharyngeal carcinoma

Abstract

Objective: To evaluate the dose received by the hippocampus among patients undergoing intensity-modulated radiotherapy (IMRT) for nasopharyngeal cancer.

Methods: 10 patients with biopsy-proven, locally advanced nasopharyngeal cancer constituted the study population. The total prescribed dose to the planning target volume (PTV) was 70 Gy (D95%) delivered in 2.12-Gy daily fractions using IMRT. Using established anatomical guidelines, MRI co-registration and the assistance of a board-certified neuroradiologist, the right and left hippocampi were delineated on axial imaging from the CT scan obtained at simulation for each patient beginning at the most anterior portion of the lateral ventricle. IMRT treatment plans were generated without dose-volume constraints to the hippocampus. A range of dose-volume statistics was calculated.

Results: The mean hippocampus volume was 6.01 ± 2.61 cm(3). The mean V20 was 72.2%; V40 was 22.0%; V50 was 10.2%; and V60 was 5.5%. The average mean, minimum and maximum hippocampus doses were 30.27 Gy (range, 19.08-47.99 Gy); 17.54 Gy (range, 11.66-33.17 Gy); and 54.95 Gy (range, 35.59-75.57 Gy), respectively. The hippocampus received a maximum dose exceeding 70 Gy in 30% of cases.

Conclusion: Our dosimetric analysis suggests that, for patients undergoing IMRT for nasopharyngeal cancer, the hippocampus routinely receives significantly high doses.

Advances in knowledge: The hippocampus receives a fair amount of incidental radiation during treatment for nasopharyngeal cancer. Given the importance of this structure with respect to memory and neurocognitive function, consideration should be given to identifying the hippocampus as a critical organ at risk in the IMRT optimization process.

Figure 1.

Visual illustration of the contoured hippocampus in (a) axial, (b) coronal and (c) sagittal views on CT image. The hippocampus contour is represented bilaterally. Starting from the centre, planning tumour volume and isodose lines (95%, 90%, 80%, 50% and 23%) are represented by the progressive contours shown. Maximal point dose for this patient was 75.57 Gy. Of interest, a significant portion of both ipsilateral and contralateral hippocampus contours received incidental radiation from intensity-modulated radiotherapy. HPC, hippocampus; PTV 54, low-dose planning target volume; PTV 70, high-dose planning target volume.

Figure 2.

Hippocampus dose characteristics of 10 nasopharyngeal carcinoma patients contoured for the present study. The individual minimum point dose (diamond), mean dose (triangle) and maximum point dose (square) of the bilateral hippocampus are depicted for each of the 10 patients. The median doses as well as quartile lines have been demarcated. The median minimum point dose was 17.54 Gy. The median mean dose was 31.11 Gy. The median maximum point dose was 52.93 Gy. Note that each data set has been arranged by ascending values, and data points in each column do not necessarily coincide with the same patient.

Figure 3.

A graphical dose–volume histogram (DVH) depicting the hippocampus as well as planning target volumes (PTVs) and other selected critical avoidance structures for the case patient who underwent intensity-modulated radiotherapy (IMRT) with and without hippocampus sparing. The solid and dashed lines represent the IMRT plans with and without designation, respectively, of the hippocampus (*) as an avoidance structure to be used in inverse planning and optimization. The PTV 70, PTV 54, optic chiasm and brainstem are shown and labelled.

Figure 4.

Isodose distributions in the (a) coronal and (b) axial planes for a patient who underwent hippocampus-sparing intensity-modulated radiotherapy. Starting from the centre, the bolded contours progressively represent the 66-, 60-, 54- and 40-Gy isodose lines. The hippocampus and brainstem are depicted using the thin contours.