Type of publication:
*Pettit L.; *Welsh A.G.; *Williams M.T.; *Puzey C.H.
Radiotherapy and Oncology; Apr 2018; vol. 127
Purpose or Objective: Parotid sparing IMRT has been shown to reduce incidence of xerostomia, leading to
recovery of salivary function and subsequently improve quality of life. It is usual to contour the whole parotid gland (WPG) which is considered a parallel organ. It can be challenging to meet dose constraints of the WPG even with VMAT. The general consensus in the U.K. has been to use a simple planning constraint of mean < 24 Gy to the WPG. However, this is not always achievable, especially with large tumours. Previous work suggests if only part of the parotid was spared this may be enough for preservation of saliva function. Material and Methods A retrospective dosimetric analysis of ten previous unselected patients who had received bilateral radical radiotherapy 65 Gy in 30 fractions for squamous cell carcinoma of the oropharynx that were identified from ARIA. Demographics were recorded on an excel spreadsheet. The deep lobe (DL) and superficial lobe (SL) of the ipsilateral (IL) and contralateral (CL) parotid gland were contoured on each CT planning slice. Mean dose to the deep lobe (DL) and superficial lobe (SL) was calculated from the original plan with the volume of the WPG. V40 and D50 were recorded. All treatment was replanned using a SL tolerance of V40 < 33% and D50 < 50% without compromise to the PTV’s or change to other OAR. The DL of the parotid was not considered an organ at risk (OAR) for the re-plan. Results 10 patients were identified. 8 male, 2 female. All had squamous cell carcinoma of the oropharynx, 7 had tonsilar primaries, 2 base of tongue and 1 posterior pharyngeal wall. 7 were positive for p16. 9 also received concurrent platinum based chemotherapy. As expected the mean volume of the IL and CL parotid were similar (32.8 cc IL (7.8 cc DL, 24.7 cc SL, 31.9 cc CL (7.6 cc DL, 23.7 cc SL). On average, the DL accounted for 24.8% of the IL parotid and 25.1% of the CL parotid. Average IL SL mean dose was significantly reduced from 36.1 Gy to 33.9 Gy, average CL SL mean dose reduced from 28.3 Gy to 25.5 Gy (p = 0.02 t-test). Conclusion Our retrospective study confirmed that tolerances to the superficial lobe only are relatively easy and practical to meet. Previous work suggests that D50 may be a more reliable predictor of recovery of parotid function than mean dose to whole gland. Following this retrospective study our department will change dose constraints to superficial lobe V40 < 33% and D50 < 50% and no longer consider the deep lobe an OAR. Prospective data will investigate preservation of salivary function using D50/V40. (Figure Presented).