Effect of CT contrast on volumetric arc therapy planning (RapidArc and helical tomotherapy) for head and neck cancer.

The objectives of the study were to evaluate the effect of intravenous contrast in the dosimetry of helical tomotherapy and RapidArc treatment for head and neck cancer and determine if it is acceptable during the computed tomography (CT) simulation to acquire only CT with contrast for treatment planning of head and neck cancer. Overall, 5 patients with head and neck cancer (4 men and 1 woman) treated on helical tomotherapy were analyzed retrospectively. For each patient, 2 consecutive CT scans were performed. The first CT set was scanned before the contrast injection and secondary study set was scanned 45 seconds after contrast. The 2 CTs were autoregistered using the same Digital Imaging and Communications in Medicine coordinates. Tomotherapy and RapidArc plans were generated on 1 CT data set and subsequently copied to the second CT set. Dose calculation was performed, and dose difference was analyzed to evaluate the influence of intravenous contrast media. The dose matrix used for comparison included mean, minimum and maximum doses of planning target volume (PTV), PTV dose coverage, and V45Gy, V30Gy, and V20Gy organ doses. Treatment planning on contrasted images generally showed a lower dose to both organs and target than plans on noncontrasted images. The doses for the points of interest placed in the organs and target rarely changed more than 2% in any patient. In conclusion, treatment planning using a contrasted image had insignificant effect on the dose to the organs and targets. In our opinion, only CT with contrast needs to be acquired during the CT simulation for head and neck cancer. Dose calculations performed on contrasted images can potentially underestimate the delivery dose slightly. However, the errors of planning on a contrasted image should not affect the result in clinically significant way.

Dosimetric coverage of the external anal sphincter by 3-dimensional conformal fields in rectal cancer patients receiving neoadjuvant chemoradiation: implications for the concept of sphincter-preserving radiation therapy.

BACKGROUND: We evaluated the anatomic location of the external anal sphincter (EAS) to pelvic bony landmarks related to 3-dimensional conformal radiotherapy (3DRT) and studied the dosimetric coverage of the EAS in patients undergoing neoadjuvant chemoradiation for rectal cancer. METHODS: Sixty-four consecutive rectal cancer patients treated with neoadjuvant chemoradiation were included. All patients were treated in a prone position on a bellyboard by 3DRT. The inferior border of the RT fields was at least 3-5 cm inferior to the gross tumorous volume (GTV) or at the inferior border of the obturator foramen (IBOF), whichever was more inferior. The EAS was contoured and dose distributions were determined using dose-volume histograms. RESULTS: In 53 out of 64 cases (82.8%), the EAS was completely inferior to the IBOF. In the remaining 11 cases, the EAS was either overlapping the IBOF (10 cases; 15.6%) or completely superior to the IBOF (1 case; 1.7%). The average mean dose delivered to the EAS was 2795 cGy. Lower mean doses were delivered to the EAS when the center of the EAS was located more distant from the GTV. CONCLUSIONS: Meticulous planning to define the inferior border of the RT field is recommended to avoid irradiating the EAS.