Intensity-modulated proton therapy for oropharyngeal cancer reduces rates of late xerostomia

Abstract

Background and purpose: To determine rates of xerostomia after intensity-modulated radiotherapy (IMRT) or intensity-modulated proton therapy (IMPT) for oropharyngeal cancer (OPC) and identify dosimetric factors associated with xerostomia risk.

Materials and methods: Patients with OPC who received IMRT (n = 429) or IMPT (n = 103) from January 2011 through June 2015 at a single institution were studied retrospectively. Every 3 months after treatment, each patient completed an eight-item self-reported xerostomia-specific questionnaire (XQ; summary XQ score, 0-100). An XQ score of 50 was selected as the demarcation value for moderate-severe (XQs ≥ 50) and no-mild (XQs < 50) xerostomia. The mean doses and percent volumes of organs at risk receiving various doses (V5-V70) were extracted from the initial treatment plans. The dosimetric variables and xerostomia risk were compared using an independent-sample t-test or chi-square test.

Results: The median follow-up time was 36.2 months. The proportions of patients with moderate-severe xerostomia were similar in the two treatment groups up to 18 months after treatment. However, moderate-severe xerostomia was less common in the IMPT group than in the IMRT group at 18-24 months (6% vs. 20%; p = 0.025) and 24-36 months (6% vs. 20%; p = 0.01). During the late xerostomia period (24-36 months), high dose/volume exposures (V25-V70) in the oral cavity were associated with high proportions of patients with moderate-severe xerostomia (all p < 0.05), but dosimetric variables regarding the salivary glands were not associated with late xerostomia.

Conclusion: IMPT was associated with less late xerostomia than was IMRT in OPC patients. Oral cavity dosimetric variables were related to the occurrence of late xerostomia.

Keywords: Intensity-modulated proton therapy; Intensity-modulated radiotherapy; Oropharyngeal cancer; Xerostomia.

Fig. 1.

The proportions of patients with moderate-severe xerostomia (XQs ≥50) in both treatment groups (IMRT and IMPT) at the indicated times after treatment. The total numbers of patients in each group are shown under the graph. The p values are for comparison of the proportions of patients with moderate-severe xerostomia in the two groups (chi-square test). The error bars indicate 95% confidence intervals.

Fig. 2.

The numbers of patients with XQ scores at 0–6 months after treatment and at the indicated subsequent times. (A) Four hundred thirteen patients had XQ scores at 0–6 months, but only 142, 107, 145, 148, and 181 of those patients also had XQ scores at 6–9, 9–12, 12–18, 18–24, and 24–36 months, respectively. (B) The proportions of the 142 patients with moderate-severe xerostomia (XQs ≥50) in both treatment groups who had XQ scores at both 0–6 and 6–9 months. (C-F) The proportions of the (C) 107 patients with XQ scores at 0–6 and 9–12 months, (D) 145 patients with XQ scores at 0–6 and 12–18 months, (E) 148 patients with XQ scores at 0–6 and 18–24 months, and (F) 181 patients with XQ scores at 0–6 and 24–36 months in the IMRT and IMPT groups. Fewer patients in the IMPT group than in the IMRT group had moderate-severe xerostomia (XQs ≥50) at 18–24 months (p = 0.035) and 24–36 months (p = 0.034) after treatment.