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 Impact of the 160 MLC on IMRT technique for head and neck cancer treatment.

Engy M. Abdelatif1, Inas M. El-Badry2, Sabbah I. Hammoury1, Amr H. Hussein2 and Ehab I. Mohamed3

1Ayadi ElMostakbal Oncology Center, Alexandria, Egypt; Departments of 2Cancer Management and Research and 3Medical Biophysics, Medical Research Institute, Alexandria University, Alexandria, Egypt

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Purpose: This study's objective was to evaluate the impact of 160 versus 58 MLC of two linear accelerator machines on head and neck IMRT treatment plans. 

Patients and Methods: Two IMRT radiotherapy plans ((one plan with 160 MLC and the other one with 58 MLC) using 7 fields and 70 Gy prescribed dose) were designed for everyone from the 15 patients. Doses for target and organs at risk were collected from dose volume histogram and compared. Dosimetric properties for MLC thickness effect; scattered dose, interleaf leakage and tongue and groove were measured for each machine using radiochromic films EBT3.

Results: The mean doses of PTV (D95% and D98%) for IMRT plan using 160 MLC were significantly (p < 0.001) higher than those for the IMRT plan using 58 MLC. The same was found for homogeneity and uniformity indexes and dosimetric parameters for OARs. The IMRT plan using 160 MLC reduced the doses significantly (p < 0.001) compared to the IMRT plan using 58 MLC to the brainstem, spinal cord, right and left parotid, optic chiasm, left optic nerve, and right and left eye. Scattered radiation doses on films irradiated from 160 MLC were higher than those from 58 MLC linear accelerator due to interleaf leakage.

Conclusion: IMRT plan using 160 MLC can achieve a better dose coverage to the PTV than that using 58 MLC since its 1 cm leaf width covers a part of the PTV to spare a specific OAR. Despite being scattered, the interleaf leakage radiation doses from the 160 MLC machine are higher than from the 58 MLC machine. However, the 160 MLC is still recommended for the IMRT treatment as these doses are very small compared with the l60 MLC advantages of PTV homogeneous dose coverage and OARs sparing.