Geometric factors influencing dosimetric sparing of the parotid glands using IMRT

To determine the relationship between the parotid volume, parotid-planning target volume (PTV) overlap, and dosimetric sparing of the parotid with intensity-modulated radiation therapy (IMRT).

Parotid data were collected retrospectively for 51 patients treated with simultaneous boost IMRT. Unresectable patients received 54 or 59.4 Gy to subclinical disease, 70 Gy to gross disease. Patients treated postoperatively received 54, 60, and 66 Gy to low-risk, high-risk, and tumor bed regions. Volume and mean dose of each gland and gland segments outside of and overlapping the PTV were collected. Proximity of each gland to each PTV was recorded.

Dosimetric sparing (mean dose 21% overlap (p = <0.0001). Among spared glands, the median mean dose in the overlap region was 55.0 Gy in glands with < or =21% overlap, but only 45.4 Gy when overlap >21%. Median mean dose was 25.9 Gy to glands overlapping PTV(54) or PTV(59) alone and 30.0 Gy to those abutting PTV(70) (p < 0.001). Although proximity to PTV(70) was associated with higher parotid dose, satisfactory sparing was achieved in 24 of 43 ipsilateral glands.

Dosimetric sparing of the parotid is feasible when the parotid-PTV overlap is less than approximately 20%. With more overlap, sparing may result in low doses within the overlap region, possibly leading to inadequate PTV coverage. Gland proximity to the high-dose PTV is associated with higher mean dose but does not always preclude dosimetric sparing.