Authors (including presenting author) :
Li Y(1), Chan PC(1), Cheung KWA(1)
Affiliation :
(1)Department of Oncology, Tuen Mun Hospital
Introduction :
Removal of loose teeth after head and neck treatment planning leads to changes in dose distribution. Especially, T3/4 nasopharyngeal carcinoma (NPC) treatment plans are resulted with marginal acceptable dose to organs at risk.
Objectives :
To assess the applicability of original treatment plans for the patient who has removed loose teeth after treatment planning.
Methodology :
Fifteen T3/4 NPC patients undergoing volumetric modulated arc therapy (VMAT) were included in this retrospective study. Their upper and lower teeth were contoured as upper/lower left, anterior and right portions. Those teeth were overridden the densities to 0g/cm3 respectively to simulate removal of teeth and recalculation of plans were performed. Dosimetric comparisons of brainstem (BS), spinal cord (SC), chiasm, optic nerves, temporal lobes, temporomandibular (TM) joints and brachial plexus were carried out among original plan and the 6 recalculated plans. Correlation tests were performed to find the relationship between the distance from (1) the primary tumor to 5mm planning organ at risk volume (PRV) of SC or (2) the planning target volume (PTV) receiving 54Gy (PTV54) to 5mm PRV of SC and maximum dose (Dmax) of 5mm PRV of SC. Also, the correlation tests between their volumes and Dmax of 5mm PRV of SC were investigated.
Result & Outcome :
All plans showed significant different (p< 0.05, Wilcoxon Signed-Rank T-Test) when compared Dmax of bilateral optic nerves, temporal lobes and TM joints with that of original plan. For the Dmax of BS, chiasm and left brachial plexus, only the plans with removal of low anterior teeth showed no significant difference. For SC, only the plans with removal of upper teeth showed with significant differences. There was no correlation (p>0.05, Spearman rank correlation) found between the distance from primary tumor/PTV54 to 5mm PRV of SC and Dmax of 5mm PRV of SC. Their volumes showed no correlation with Dmax of 5mm PRV of SC. 2 out of 15 cases exceeded doses in Dmax of 5mm PRV of SC and all were related to upper portions (5000.5-5010.5cGy). The upper teeth were further divided into 6 groups for investigation. 1 of the cases showed no exceeded dose while another exceeded in 2 portions (5000.1-5002cGy). The results alert us the need of overriding removed teeth densities to estimate the dose changes or considering computed tomography images acquisition and re-planning again.
