SU‐E‐P‐21: Impact of MLC Position Errors On Simultaneous Integrated Boost Intensity‐Modulated Radiotherapy for Nasopharyngeal Carcinoma

Purpose:To investigate the impact of MLC position errors on simultaneous integrated boost intensity‐modulated radiotherapy (SIB‐IMRT) for patients with nasopharyngeal carcinoma.Methods:To compare the dosimetric differences between the simulated plans and the clinical plans, ten patients with locally advanced NPC treated with SIB‐IMRT were enrolled in this study. All plans were calculated with an inverse planning system (Pinnacle3, Philips Medical System₎. Random errors −2mm to 2mm₎,shift errors₍ 2mm,1mm and 0.5mm) and systematic extension/ contraction errors (±2mm, ±1mm and ±0.5mm) of the MLC leaf position were introduced respectively into the original plans to create the simulated plans. Dosimetry factors were compared between the original and the simulated plans.Results:The dosimetric impact of the random and system shift errors of MLC position was insignificant within 2mm, the maximum changes in D95% of PGTV,PTV1,PTV2 were‐0.92±0.51%,1.00±0.24% and 0.62±0.17%, the maximum changes in the D0.1cc of spinal cord and brainstem were 1.90±2.80% and −1.78±1.42%, the maximum changes in the Dmean of parotids were1.36±1.23% and −2.25±2.04%.However,the impact of MLC extension or contraction errors was found significant. For 2mm leaf extension errors, the average changes in D95% of PGTV,PTV1,PTV2 were 4.31±0.67%,4.29±0.65% and 4.79±0.82%, the averaged value of the D0.1cc to spinal cord and brainstem were increased by 7.39±5.25% and 6.32±2.28%,the averaged value of the mean dose to left and right parotid were increased by 12.75±2.02%,13.39±2.17% respectively.Conclusion:The dosimetric effect was insignificant for random MLC leaf position errors up to 2mm. There was a high sensitivity to dose distribution for MLC extension or contraction errors.We should pay attention to the anatomic changes in target organs and anatomical structures during the course,individual radiotherapy was recommended to ensure adaptive doses.