Parotid Gland Dose In Whole-Brain Radiation Therapy Patients

Mean parotid gland (PG) doses as low as 15 Gy in head and neck cancer patients have been reported to decrease salivary production by nearly 50%. Few studies have evaluated the clinical/quality of life (QOL) effects of PG dose in patients receiving whole brain radiation therapy (WBRT). There is no consensus about treating PGs as organs at risk (OAR) in these patients. We evaluated PG dose, risk of gland toxicity and dosimetric impact of reducing mean parotid dose in patients treated with WBRT at our institution. We conducted an IRB approved, retrospective review of 20 consecutive patients treated with WBRT using opposing lateral fields encompassing brain parenchyma, skull and spinal cord to the lower level of the C1 vertebral body. Brain parenchyma and bilateral PGs were contoured on each slice of the simulation CT. Dose volume histograms, gland mean dose and target volume coverage were analyzed. Parotid complication risk was assessed using the Normal Tissue Complication Probability (NTCP) calculated by Eisbruch et al. After evaluating the original plans, treatment fields were modified to block as much of the bilateral PGs as possible without blocking brain parenchyma. Resulting dosimetric effects on gland mean dose and target volume coverage were evaluated. Patients received 30 Gy / 10 fractions (16 patients), 30 Gy/ 15 fractions (1 patient) or 35 Gy/ 14 fractions (3 patients). Mean gland volumes are 18.0 cc [range 5.0-35.9 cc] and 19.2 cc [range 6.3-37.4 cc] for right and left parotids, respectively. The average mean dose to the right and left parotids are 11.5 Gy [range 2.4-18.4 Gy] and 11.9 Gy [range 3.8-20.9 Gy], respectively; 70% (29/40) of gland mean doses are >10Gy. Complication risk for all glands was ≤ 5%. For the modified WBRT fields, the average mean dose to the right and left parotids are 6.2 Gy [range 2.4-14.4 Gy] and 6.1 Gy [range 2.0-17.5 Gy], respectively, with an average mean gland dose decrease of about 50% [range 3.8-80%]. 98% of the target volume received a mean of 99.4% [range 99.4-100.7%] of the prescribed dose compared to 99.2% [range 97.4-100.7%] after field modification. Mean PG doses in this study are less than the recommended threshold of 26 Gy. NTCP based risk of salivary complications in this population is low. However, loss of PG function of about 5%/ 1 Gy of mean dose has been reported, suggesting that mean doses encountered in this study are significant with potential to impact salivary function and thereby affect QOL in these patients. Modifying treatment fields to block portions of bilateral PGs can substantially reduce mean parotid dose without compromising target volume coverage. Further study is required to determine the clinical and QOL impact of these changes.