Central Nervous System

PV QA 2 - Poster Viewing Q&A 2

MO_5_2578 - A Novel Three Dimensional Conformal Radiation Therapy Technique to Decrease the Mean Parotid Gland Dose in Whole Brain Radiation Therapy

Monday, October 22
10:45 AM - 12:15 PM
Location: Innovation Hub, Exhibit Hall 3

A Novel Three Dimensional Conformal Radiation Therapy Technique to Decrease the Mean Parotid Gland Dose in Whole Brain Radiation Therapy
L. Hilal, P. Ramia, H. Mekdash, W. Jalbout, H. Abdul Khalek, B. Shahine, F. B. Geara, Y. Zeidan, D. Mahmoud, T. A. Eid, and B. Y. Youssef; American University of Beirut Medical Center, Beirut, Lebanon

Purpose/Objective(s): Whole brain radiation therapy (WBRT) remains standard for patients with brain metastasis not amenable to stereotactic radiation. Incidental irradiation of the parotid glands can result in xerostomia that could persist for several months after treatment and each additional Gray of mean parotid dose decreases salivary flow at 6 months by around 4%. A novel technique using 3D conformal radiotherapy (3DCRT) was used to decrease the radiation dose to the parotid glands and hence the risk of xerostomia.

Materials/Methods: CT simulations of 20 de-identified patients were randomly selected. For each patient, 3 different WBRT plans were generated. The first using the standard opposed laterals fields with the inferior border at C2, the second using opposed laterals with the inferior border at C1 and the third using a novel proposed technique. In the new technique, we placed the isocenter 0.5-1 cm above the contoured parotid glands. Above the isocenter, 2 upper lateral fields were designed similar to the standard opposed laterals. Below the isocenter, 2 lower fields were used,conformal to the contoured brain, with a 1cm collimation to the edge of the MLC. The lower beams used were a left posterior oblique (angle: 115 degrees) and a right posterior oblique (angle: 245 degrees) such that each beam completely spares one ipsilateral parotid gland. Prescription dose was 30Gy in 10 fractions for all plans. Dose volume histogram data were generated for each of the three plans for every patient. Brain coverage and the means of the right parotid, left parotid and bilateral parotids were compared in each of the 3 plans using dependent samples t-test in IBM SPSS version 23.

Results: In all three techniques of WBRT, brain coverage was adequate with 95% of the dose covering more than 95% of the brain. The average of the mean right parotid dose using C2, C1, and the new technique were 17 Gy ± 4, 12 Gy ± 3, and 6 Gy ± 2. The average of the mean left parotid dose using C2, C1, and the new technique were 18 Gy ± 4, 13 Gy ± 3, and 6 Gy ± 1. The average of the mean bilateral parotids dose using C2, C1, and the new technique were 18 Gy ± 4, 12 Gy ± 3, and 6 Gy ± 1. There was a statistically significant decrease in the mean dose to the parotids, by at least one-half, using our new technique versus the standard WBRT with the inferior border at C2 (P-value<0.0001) and by at least a third using the inferior border at C1 (p-value<0.0001).

Conclusion: Our new 3DCRT technique for WBRT significantly reduces mean parotid gland dose compared to the standard WBRT fields and can significantly decrease the risk of xerostomia and thus improve patients’’ quality of life.

Author Disclosure: L. Hilal: None. H. Mekdash: None. H. Abdul Khalek: None. Y. Zeidan: None. B.Y. Youssef: None.

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