C. Tsai

SU‐GG‐T‐33: An Investigation on the Acquisition and Modification of Cone‐Beam Computed Tomography Images for Head and Neck Cancer

Purpose: To determine the optimal parameters in the acquisition of cone‐beam computed tomography(CBCT) for good image quality and to modify the CT numbers for accurate dose calculation in head and neck cancer.Method and Materials: Systems of fan‐beam CT (FBCT) and CBCT were used to acquire images. A program was developed to establish the averaged CT numbers for different materials, and plot them as a function of density. CT numbers of CBCT were modified to match those of FBCT. The modified CBCTimages were used for dose calculation. Three CT phantoms were scanned by FBCT and CBCT for image uniformity, spatial resolution,CT numbers, and for comparing doses from orthogonal beams. A Rando phantom was scanned for head and neck regions by both FBCT and CBCT, and planned with intensity modulated radiation therapy(IMRT). In addition, the CBCTimages at the first and sixth weeks of radiotherapy of a nasopharyngeal cancer patient treated with an IMRT plan were calculated for dose comparison. Results: With 360°acquisition of CBCTimages, and high‐resolution reconstruction, the uniformity of CT number distribution was improved and the otherwise large variations of CT numbers for background and high‐density materials were reduced significantly. The dose difference between FBCT and CBCT was <2% in CT phantoms. In both the Rando phantom and the patient, the dose‐volume histograms were similar between FBCT and CBCT. The corresponding isodose curves covering 90% of the prescribed dose on FBCT and CBCT were within 2 mm. The dosimetric difference on CBCTimages between the first and sixth weeks were more from the setup errors due to body weight loss. Conclusion: The protocol developed for CBCT acquisition and modification produced good quality images which can be used in dose calculation for re‐planning purposes.

Poster — Wed Eve—41: VMAT for Prostate Cancer: Superior Organ Sparing to IMRT and Delivery Efficiency to Helical Tomotherapy

Volumetric modulated arc therapy (VMAT) has been developed for the better dose conformity and shorter treatment time. This study is aimed to compare the dosimetric and treatment parameters between step‐and‐shoot intensity modulated radiation therapy(IMRT), helical tomotherapy (HT), and volumetric modulated arc therapy (VMAT). Eight patients with localized prostate cancer with the dosimetric plans of 5‐field IMRT, HT, and VMAT for their 78‐Gy definitive treatment formed the basis of the study. Treatment target was prostate and proximal seminal vesicles. The goals were minimum clinical target volume (CTV) dose ≥ 78Gy, and 95% of planning target volume (PTV) ≥ 78Gy in 39 fractions. Target comparison showed PTV V95% of 99.95±0.05% for VMAT, 99.84±0.21% for IMRT, and 99.39±0.49% for HT; maximum dose to PTV was 82.44±0.29Gy, 82.45±0.54Gy, 82.80±0.72Gy, respectively. The rectum V65Gy was 11.18±1.52% for VMAT, 13.77±1.68% for IMRT, and 8.44±1.13% for HT, respectively. The bladder V65Gy was 13.25±8.22% for VMAT, 16.51±11.05 % for IMRT, and 10.63±4.47% for HT, respectively. Total monitor units were 304.3±42.3 for VMAT, 335.8±19.0 for IMRT, and 3073.4±532.6 for HT, respectively. Treatment time was 2.47±0.10 minutes for VMAT, 3.71±0.29 minutes for IMRT, and 3.64±0.60 minutes for HT, respectively. The VMAT plans in all 8 patients met the quality assurance criteria of 3% and 3mm. In conclusion, VMAT is a practical technique with comparable target coverage, better organ sparing than IMRT, and superior treatment efficiency to HT in prostate cancer patients.