Measurements of patient's setup variation in intensity modulated radiation therapy of head and neck cancer using electronic portal imaging device

Abstract

The introduction of IMRT poses new challenges for delivering intended target dose and minimizing dose and toxicity to critical normal structures. For head and neck cancer, reproducible patient positioning throughout the whole treatment course is particularly important due to the proximity of many critical organs. Deviations from the planned irradiation geometry during a treatment session may be systematic or random. Systematic errors occur if the mean irradiation geometry in the fractionated treatment differs differs from the geometry in the treatment plan. The mean deviations are then called systematic errors. Fraction-to-fraction variations around the mean deviation are called random errors. The purpose of this study is to measure the interfraction setup variation of patient undergoing IMRT of head and neck cancer. The data is used to define adequate treatment clinical target volume to planning target volume (CTV-to-PTV) margin. During March to November 2006, the data was collected from 9 head and neck cancer patients treated with dynamic IMRT using 6 MV X-ray beam from Varian Clinac 23EX. Weekly portal images of setup fields which were anterior-posterior and lateral portal images were acquired for each patient with an amorphous silicon EPID, Varian aS500. These images were matched with the reference images from Varian Acuity simulator using the VARiS Vision software, version 7.3.10 Six anatomical landmarks were selected for comparison. The displacement of portal image from the reference image was recorded in X (Left-Right, L-R), Y (Superior-Inferior, S-I) direction for anterior field and Z (Anterior-Posterior, A-P), Y (S-I) direction for lateral field. The systematic and random error for individual and population were calculated. Then the population-based margins were obtained. The total of 168 images ( 27 simulation images and 141 portal images) and 564 match points were evaluated. The results showed that the systematic error ranged from 0 to 7.5 mm and the random error ranged from 0.3 to 4.8 mm for all direction. The population-based margin ranged from 2.4 to 4.9 mm (L-R), 3.9 to 5.0 mm (S-I) for anterior field and 3.4 to 4.7 mm (A-P), 2.6 to 3.7 mm (S-I) for the lateral field. The difference in population-based margins along S-I axis between anterior field and lateral field were observed because the clavicles chosen for anterior field at the shoulder level were less stable than anatomical landmarks chosen for lateral field i.e. skull bones, C1 and C4. These margins were comparable to the margin that prescribed at King Chulalongkorn Memorial Hospital (5-10 mm) for head and neck cancer. These results showed that the population-based margin is less than 5 mm, thus the margin provides sufficient coverage for all of the patients.