The effect of significant tumor reduction on the dose distribution in intensity modulated radiation therapy for head-and-neck cancer: a case study

Abstract

We present a unique case in which a patient with significant tissue loss was monitored for dosimetric changes using weekly cone beam computed tomography (CBCT) scans. A previously treated nasopharynx patient presented with a large, exophytic, recurrent left neck mass. The patient underwent re-irradiation to 70 Gy using intensity modulated radiation therapy (IMRT) with shielding blocks over the spinal cord and brain stem. Weekly CBCT scans were acquired during treatment. Target contours and treatment fields were then transferred from the original treatment planning computed tomography (CT) to the CBCT scans and dose calculations were performed on all CBCT scans and compared to the planning doses. In addition, a "research" treatment plan was created that assumed the patient had not been previously treated, and the above analysis was repeated. Finally, to remove the effects of setup error, the outer contours of 2 CBCT scans with significant tumor reductions were transferred to the planning scan and dose in the planning scan was recalculated. Planning treatment volume (PTV) decreased 45% during treatment. Spinal cord D05 differed from the planned value by 3.5 +/- 9.8% (average + standard deviation). Mean dose to the oral cavity and D05 of the mandible differed from the planned value by 0.9 +/- 2.1% and 0.6 +/- 1.5%, respectively. Results for the research plan were comparable. Target coverage did not change appreciably (-0.2 +/- 2.5%). When the planning scan was recalculated with the reduced outer contour from the CBCT, spinal cord D05 decreased slightly due to the reduction in scattered dose. Weekly imaging provided us the unique opportunity to use different methods to examine the dosimetric effects of an unusually large loss of tissue. We did not see that tissue loss alone resulted in a significant effect on the dose delivered to the spinal cord for this case, as most fluctuation was due to setup error. In the IGRT era, delivered dose distributions can be more readily determined during treatment, and this information can be useful in deciding whether replanning is necessary.

Fig. 1

Target volume as a function of time since simulation, as measured from the 8 CBCT scans and the 2 simulation scans. Treatment began on day 10.

Fig. 2

Corresponding axial CT slices from the beginning and the end of treatment. The original target volume is delineated by the yellow contour.

Fig. 3

Variation of spinal cord D05 between CBCT and plan for the clinical plan and research plan. The curves are separated at approximately 40 days since the patient was replanned at that time due to a loose mask.

Fig. 4

Comparison of a right lateral beam’s-eye view as seen in the planning scan and a CBCT scan. The C1–C2 region was used for the setup registration and matches well; however, it can be seen that the head rotation is slightly different in the 2 images. Superiorly, the brain stem appears farther from the field edge in the CBCT scan. Inferiorly, the cord is closer to the field edge in the CBCT scan.

Fig. 5

Variation of D05 to the mandible vs. time for the clinical plan and research plan.

Fig. 6

Variation of D05 of the target volume vs. time for the clinical plan and research plan.

Fig. 7

A patient in whom tumor regression may potentially move the brain stem into the high-dose region. The gross tumor is delineated by the blue contour and the brain stem is delineated by the yellow contour.