Organs in plain sight-parotid gland doses in short-course palliative whole-brain radiotherapy

Palliative whole-brain radiotherapy (WBRT) is a useful treatment modality for patients diagnosed with brain metastasis. Our study aimed to document parotid doses using a short course of 20 Gy in five fractions (SC-WBRT) and compare them with the known xerostomia parameters.

We retrieved dosimetric parameters for 59 patients who received SC-WBRT. The contouring of parotids was done retrospectively. The data was stratified depending on the lower edge of the planning target volume (PTV) border, up to either the first cervical vertebra (C1) or the second cervical vertebra (C2). The biologically effective dose (BED) of the parotid dose constraints from the literature was calculated and corrected to obtain the dose for a five-fraction treatment. These were used as a reference for assessing the parotid doses of our cohort.

With SC-WBRT, the mean doses of either gland (11.46 Gy) and of total parotids (11.48 Gy) were statistically significantly less than the BED corrected constraints of 12.82 Gy (p < 0.01) and 15.31 Gy (p < 0.01). Patients with PTV to C1, compared to those with a PTV to C2, had significantly lower mean doses of either gland (9.44 Gy vs 12.76 Gy, p < 0.01) and total parotid dose (9.44 Gy vs 12.78 Gy, p < 0.01). Patients with PTV to C1 had doses significantly lower than all the reference dose parameters.

Routine delineation and documentation of parotid dosimetry would enable parotid sparing, irrespective of the dose regimen used. SC-WBRT provides better parotid sparing dosimetrically which may reduce the related toxicities even in palliative settings. Limiting the lower border of the PTV to C1 will further decrease the parotid doses.