3699 - Dosimetric Results for Organs at Risk (OAR) from Radiotherapy Dose De-Escalation to the Subclinical Region in HPV-Mediated Oropharyngeal Cancer (HPV+ OPC): A Retrospective Analysis

Purpose/Objective(s): Our standard treatment for HPV+ OPC is 30Gy to the subclinical region followed by a 40Gy cone down to the gross tumor. With this approach, we have not observed any failures in the subclinical region. The dosimetric consequence, however, is not known. This study aims to retrospectively assess the impact on the surrounding OARs when delivering only 30Gy to the subclinical region.

Materials/

Methods: We conducted a retrospective analysis of dosimetric data for HPV+OPC patients treated with 30Gy to the subclinical region then a cone down of 40Gy to all gross disease. Included patients had T1-4, N0-3 disease (AJCC 7^th edition). All patients were treated with intensity-modulated radiotherapy with concurrent systemic therapy. Dosimetric data on the head/neck OARs were collected, analyzed, and compared to historical published data.

Results: 171 consecutive patients were included in this study. 69 patients had N2c disease, and 102 had unilateral disease (T1-2/N1-2b) or no gross neck nodes (T3-4 disease). For non-N2c disease, the mean ipsilateral parotid gland dose was 20.3 Gy (SD 7.3), and mean contralateral parotid gland dose was 8.6 Gy (SD 2.6). Mean contralateral submandibular gland dose was 28.9 Gy (SD 9.7). Mean oral cavity dose was 38.8 Gy (SD 10.6), and mean larynx dose was 22.2 Gy (SD 10.7). Patients treated for N2c disease had expectedly higher mean doses for all OARs (Table 1).

Conclusion: Our dosimetric results broadly showed that radiotherapy dose de-escalation in the subclinical region for OPC resulted in low OAR doses well below established dose constraints, particularly for the parotid gland, larynx, and contralateral submandibular gland. When comparing this data to published dosimetric data from other institutions (Richetti et al, IJROBP 2011), our OAR doses were significantly lower for the contralateral parotid gland, contralateral submandibular gland, and larynx. Prospective studies evaluating toxicities of subclinical dose de-escalation are warranted. Abstract 3699 – Table 1: OAR dosimetric results of subclinical dose de-escalated radiotherapy for HPV+ OPC