Physics Contribution
Accurate Analysis of the Change in Volume, Location, and Shape of Metastatic Cervical Lymph Nodes During Radiotherapy

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Purpose

To establish a method for the accurate acquisition and analysis of the variations in tumor volume, location, and three-dimensional (3D) shape of tumors during radiotherapy in the era of image-guided radiotherapy.

Methods and Materials

Finite element models of lymph nodes were developed based on computed tomography (CT) images taken before the start of treatment and every week during the treatment period. A surface geometry map with a volumetric scale was adopted and used for the analysis. Six metastatic cervical lymph nodes, 3.5 to 55.1 cm3 before treatment, in 6 patients with head and neck carcinomas were analyzed in this study. Three fiducial markers implanted in mouthpieces were used for the fusion of CT images. Changes in the location of the lymph nodes were measured on the basis of these fiducial markers.

Results

The surface geometry maps showed convex regions in red and concave regions in blue to ensure that the characteristics of the 3D tumor geometries are simply understood visually. After the irradiation of 66 to 70 Gy in 2 Gy daily doses, the patterns of the colors had not changed significantly, and the maps before and during treatment were strongly correlated (average correlation coefficient was 0.808), suggesting that the tumors shrank uniformly, maintaining the original characteristics of the shapes in all 6 patients. The movement of the gravitational center of the lymph nodes during the treatment period was everywhere less than ±5 mm except in 1 patient, in whom the change reached nearly 10 mm.

Conclusions

The surface geometry map was useful for an accurate evaluation of the changes in volume and 3D shapes of metastatic lymph nodes. The fusion of the initial and follow-up CT images based on fiducial markers enabled an analysis of changes in the location of the targets. Metastatic cervical lymph nodes in patients were suggested to decrease in size without significant changes in the 3D shape during radiotherapy. The movements of the gravitational center of the lymph nodes were almost all less than ±5 mm.

Introduction

The latest advances in high-precision radiotherapy enable focusing of higher radiation doses to the tumor region while minimizing unwanted radiation exposure to surrounding tissue. Intensity-modulated radiation therapy (IMRT) varies the intensities and profiles of beams from various directions to fit the tumor size and shape. An initial treatment plan prescribing the radiation field and dose in IMRT is made based on information about the tumor location and geometry before treatment starts. If the tumor moves or shrinks during treatment, the surrounding normal tissue may become included in the region exposed to high radiation doses (1). Such potential anatomic changes can be measured and corrected during fractionated radiotherapy by any of several imaging techniques. Image-guided radiotherapy (IGRT) is one such technique for margin reduction caused by setup uncertainties or anatomic movement used to improve the accuracy of dose delivery. Many studies have reported interfractional organ motions and targeting errors in IGRT using daily computed tomography (CT) images 2, 3, 4. The clinical efficacy of IGRT techniques in reducing the treatment margins have been confirmed in head-and-neck cancer 5, 6, 7 and prostate cancer 8, 9, 10.

However, it is still not practical to perform daily modifications to treatment plans to adapt to observed changes in the tumor dimensions. Several practical ways to minimize anatomic uncertainties have been reported. Hansen et al. proposed a strategy of repeated CT imaging and replanning to ensure adequate doses to the target volumes and safe doses to normal tissue during the course of IMRT (11). Smyth et al. verified the dose coverage in clinical target volumes at the time of treatment delivery using a dose distribution overlay technique (12). Several image registration methods have also been proposed for online replanning in IGRT 13, 14, 15, 16. To improve the efficacy of practical solutions, accumulation of precise knowledge about the changes in tumor volumes, locations, and especially the three-dimensional (3D) shape during radiotherapy is critically important. The authors have proposed a method to represent 3D tumor shapes in a two-dimensional (2D) map and evaluate the therapeutic response to radiotherapy (17).

The purpose of this study was to establish an accurate method for the acquisition and analysis of variations in tumor volumes, locations, and 3D shapes during radiotherapy. The 3D tumor geometry was expressed in terms of distances from the tumor center to the tumor boundary. This information is represented in 2D color-scaled maps of the distances from the center, from which the 3D tumor shapes can be visually and quantitatively assessed. Further, by comparison of maps before and after treatment, the therapeutic response can be evaluated quantitatively based on the changes in tumor shape during the course of treatments.

Section snippets

Clinical cases

The study participants were 6 patients with nasopharyngeal cancer and metastatic cervical lymph nodes who were treated at the Hokkaido University Hospital, Sapporo, Japan, between February 2007 and November 2008. The patient characteristics are given in Table 1. Of the 6 patients, 3 had Stage T3N2M0 disease, and 3 patients were diagnosed with Stages T2bN2M0, T2bN1M0, and T1N1M0 disease, respectively. Four tumors were undifferentiated carcinomas and two were squamous cell carcinomas. The average

Results

Details of changes in tumor volumes during the radiotherapy are shown in Table 2. After irradiation with 66 to 70 Gy, the tumor volumes decreased to 0.31 to 8.4 cm3 (to 8.7–39 % of the initial volumes). In some cases, the measured tumor volumes at CT1 were larger than those at CT0, suggesting that tumor volumes increased during treatment planning. Changes in the tumor geometry during treatment are shown in Fig. 3 with the surface geometry maps. These maps show convex regions in red and concave

Discussion

The FE model and surface geometry maps with CT images were shown to be a useful tool to visualize the deformation of tumors during radiotherapy. Variations in the surface geometry map were also informative in indicating changes in tumor size, allowing the conclusion that surface geometry mapping can be a useful tool when comparing the effect of radiation among different tumor types, doses, and treatment types. It may be hypothesized that the surface geometry map may change unevenly if the

Conclusion

The surface geometry map was useful in the accurate evaluation of the changes in volume and 3D shapes of the lymph nodes. The metastatic cervical lymph nodes in nasopharyngeal cancer patients were found to have decreased in size without significantly affecting the 3D morphologic features during radiotherapy. A surface geometry map based on the finite element model was shown to be useful in IGRT, particularly in the accurate recording and in the analysis of changes in tumor volumes and the 3D

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Supported in part by a Grant-in-Aid for Scientific Research (A), MEXT (NO. 18209039) and Grant-in-Aid for JSPS Fellows (NO. 09J02587).

Conflict of interest: none.

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