Do Anatomic Changes During Radiation Impact Normal Tissue and Target Dosimetry? An Analysis of Calculated Cumulative Dose in Head and Neck Patients

Abstract

To analyze the magnitude of the dosimetric impact of anatomic changes occuring in H&N cancer patients throughout a course of radiotherapy. Eight H&N cancer patients treated definitively on a helical tomotherapy unit constituted the study population. Intensity modulated radiation therapy was planned to be delivered. Planning was performed using the following parameters: 1) Prescription gross target volume (GTV) dose: 64.0 to 74.4 Gy delivered at 2.12–2.00 Gy per fraction; 2) 54–57 Gy to cover ≥95% of low risk PTV; 3) 60 Gy to cover ≥95% of high risk PTV; and 4) <30% of parotid glands receiving 26 Gy. In cases with ipsilateral disease, planning was prioritized to spare the contralateral parotid. Daily MVCT imaging was performed on all cases as a part of routine image guided radiation therapy. At total of 331 daily megavoltage CT scans were evaluated. Daily MVCT scans were processed through a novel deformable image registration (DIR) algorithm to register daily megavoltage CT images to the original planning simulation kilovotage CT image. The deformable registration process was verified by reviewing the adequacy of automatically generated contours on the daily megavoltage CT images. Cumulative dose volume histograms for target volumes and organ at risk volumes were generated and the relative deviation of these DVHs from dosimetry conceived during initial treatment planning was carefully analyzed. The DIR algorithm adequately predicted the location of target and organs at risk volumes in >98% of daily MVCTs reviewed. The average D95 of the high dose target volumes was 68.8 Gy at initial planning versus 67.5 Gy after DIR was applied. In all cases, the difference between planned and actual delivered dose for the target areas was less than 3%. The average parotid dose was higher than initially planned (Fig 1); i.e.113% of planned dose. During the initial planning process in all patients, the dose to at least 1 parotid was kept <25–30 Gy. However, after cumulative DVH evaluation, in 50% of patients, average parotid dose changed from 25–30 Gy (at initial planning) to exceed 30 Gy. Authors have described anatomic changes during a course of radiation therapy1. Using a novel deformable registration algorithm, the impact of these changes on actual delivered doses was evaluated. Anatomic changes throughout the course of treatment had little effect on delivered target doses either to the primary or nodal areas. However, the actual delivered parotid doses were substantially higher than what was planned, exceeding the significant threshold of 25–30 Gy in spared parotids in 50% of cases.1.Barker IJROBP 2004