Date Approved

8-4-2025

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Allied Health Sciences

First Advisor

Lee Culp, M.S., CMD, R.T.(T.)

Second Advisor

Kristen Vu, M.S., CMD, RT(T)

Academic Year

2024/2025

Abstract

Purpose:

The parotid glands are essential to salivary function and patient quality of life, yet they are not routinely prioritized as organs at risk (OARs) during whole-brain radiotherapy (WBRT). With growing recognition of long-term side effects like xerostomia in patients with prolonged survival after WBRT, parotid-sparing techniques warrant investigation. This study compares radiation dose to the parotid glands using three different 3D conformal WBRT planning approaches: field-in-field (FiF) with blocking, electronic tissue compensation (eCOMP) without blocking, and eCOMP with blocking.

Methods and Materials:

A retrospective study was conducted on 20 patients previously treated with 3D conformal WBRT. Original eCOMP plans were re-evaluated, and new FiF plans were created using the same field borders, planning parameters, and CT simulation datasets. Parotid glands were delineated post hoc using automated contouring. Dose metrics including mean dose (Dmean), maximum dose (Dmax), and volumetric coverage at 12.30Gy and 20Gy (V12.3, V20) were analyzed for both left and right parotid glands. Statistical comparisons between techniques were conducted using multivariate analysis of variance (MANOVA) with Bonferroni correction.

Results:

Parotid gland dose was significantly reduced in both FiF with blocking and eCOMP with blocking plans compared to the original eCOMP plans without blocking. For the left parotid, FiF with blocking yielded a Dmean of 5.86Gy and eCOMP with blocking 5.58Gy, compared to 16.51Gy for eCOMP without blocking (p < .0001). Right parotid values showed a similar trend, with both blocking techniques achieving significantly lower Dmean and V20 values. FiF with blocking and eCOMP with blocking both met or fell below QUANTEC thresholds for parotid dose, while eCOMP without blocking did not.

Conclusion:

Incorporating blocking into 3D conformal WBRT—whether via FiF or eCOMP—substantially reduces parotid gland dose. These findings support the inclusion of parotid-sparing strategies in standard WBRT planning, particularly for patients with potential for longer survival. As xerostomia is closely linked to quality of life, future prospective studies are needed to evaluate the clinical impact of these dosimetric improvements and to establish formal guidelines for parotid sparing in WBRT.

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