Nasopharyngeal carcinoma (NPC) patients with parapharyngeal space (PPS) involvement are routinely given boost irradiation in Hong Kong. The current technique that employs a single field has many limitations in terms of dose distribution. This study is aimed to compare the dose distribution between the newly designed 3-dimensional conformal radiotherapy (3DCRT) and conventional techniques for the boost treatment of PPS so as to determine the more optimal treatment.Fifteen NPC patients with unilateral PPS involvement were recruited. Their CT images were loaded into the FOCUS planning system for treatment planning. The planning target volume (PTV) and seven organs at risk (OARs) including the spinal cord, brain stem, optic chiasm, mandible, temporal lobe, temporo-mandibular (TM) joint and lens were outlined for dose assessment. The conventional and 3DCRT plans were then generated for each patient and the dose distributions were compared using dose parameters derived from the dose volume histograms (DVHs).The 3DCRT technique provides better target coverage and significantly better dose to the planning target volume than the conventional technique. The 3DCRT treatment plans gives better sparing of the ipsilateral TM joints, mandible and lens, but it is less effective to spare spinal cord, brain stem, optic chiasm and temporal lobe. Nevertheless, the total doses to these OARs remain within the clinically defined thresholds and are clinically acceptable.

Purpose: To evaluate the speed up and image quality for digitally reconstructed radiograph (DRR) calculations using a graphic processing unit (GPU) comparing with a central processing unit (CPU). Method and Materials: We developed an application for DRR image calculations using a faster incremental Siddon ray tracing algorithm. The implementation of the core integration loop was the same for the CPU and the GPU except that single‐precision operations were used on the GPU with CUDA 1.2. The GPU runs on NVIDIA GeForce GTX 280M with 1 GB video memory. To validate the correctness of the algorithm, we employed a clinical three‐field plan as an example. The CT volume image and the RT plan were exported from a commercial treatment planning system and read by our application to generate the DRR images. Results: By comparing visually with the bony structures from our DRR images and the commercial software generated DRR images, we have confidence in the correctness of our fast ray tracing implementation on both CPU and GPU. The accuracy of the GPU calculations comparing with the CPU calculation is well acceptable as only about 3% of the total pixels have less than 0.1% pixel values difference from the CPU calculations for the test fields. Conclusions: The DRR calculations using GPU is more than an order of magnitude faster than using a general purpose CPU. The image quality by the two methods is very similar where the difference comes from the float point operation type. This accelerated DRR computation has potential to reduce the patient‐positioning time during the 2D/3D registration where inline DRR calculations are required in the iteration.

Purpose: To develop a two‐dimensional (2D) dose calculation software for helical tomotherapy with absolute dose validation capability. Methods: This in‐house software takes advantage of archived patient treatment planning documents, initial coordinates of red and green lasers, central coronal plane slice numbers of the Cheese phantom, and clinical dosimetric functions. A tomotherapy DQA plan has to be generated and archived first. The software performs an independent absolute dose check to the tomotherapy plan. The software calculates the 2D dose map of the central coronal plane of the Cheese phantom, and the 2D map is narrowed into a 3.2cm×3.1cm rectangular on the plane to save calculation duration. Doses of points on the plane are calculated with 1 mm resolution, so that a total of 992 points are calculated for each patient case. The center of the 2D square is the cross point of red lasers, which is the center of the Cheese phantom by default. The 2D dose map calculated is thereafter compared with the planned one, which is a 3D dose matrix from the DQA plan. Gamma index is then calculated with 3mm and 5% criteria. Results: The newly developed software has been tested on ten tomotherapy patients (cases of 1 pancreas, 2 brain, 1 stomach, 1 lung, 1 head & neck, 3 prostate, and 1 esophagus). The gamma distributions are all passed with a minimum ratio of 95%. Conclusions: An in‐house independent dose calculation software for helical tomotherapy has been developed to independently validates 2D doses with gamma index. Gamma in this software is calculated by comparing the 2D calculation to 3D doses, which makes the introduced method more superior than normal IMRT or tomotherapy QA. Validation of the software on more cases and more tomotherapy units is necessary.