2712 - Postoperative Radiotherapy in Buccal-Alveolar Complex Carcinoma: Mapping Failure Patterns for Improved Target Delineation

Purpose/Objective(s): Bucco-alveolar complex (BAC) cancers exhibit distinct locoregional failure patterns despite multimodal treatment. Failure assessment is further complicated by radiotherapy laterality, target delineation uncertainties, and dose inhomogeneity. This study uses a geometric-dosimetric approach to evaluate recurrence sites, correlating them with surgical flap proximity and radiation dose distribution, aiming to refine postoperative radiotherapy (PORT) protocols.

Materials/Methods: Eighty-six patients with squamous cell carcinoma of the (BAC) underwent surgery followed by PORT between 2018 and 2021. All had pathological N0–N2b disease. Patients were categorized into ipsilateral (I/L) and bilateral (B/L) irradiation groups, and baseline characteristics were compared.

Failure patterns were analyzed using focal point and dosimetric methods. Recurrences were classified based on

1. Dose received:

• Type A: High-risk volumes + =95% prescribed dose.
• Type B: High-risk volumes +<95% prescribed dose.
• Type C: Low-risk volumes + =95% prescribed dose).
• Type D: Low-risk volumes + <95% prescribed dose).
• Type E: Extraneous recurrence beyond target volumes.

2. Anatomical location:

• Anterior margin
• Posterior margin/parotid bed
• Supranotch/masticator space
• Pterygopalatine fossa
• Infranotch/pterygoid plates
• Posteromedial flap margin

3. Distance from the flap margin:

• Within the flap
• =5mm from flap
• 5–10mm from flap
• 10mm from flap

Results:

With a median follow-up of 18 months (range: 0–65), median RFS was 26 months (range: 16.26–35.74). Median OS was not reached.

Thirty-three (38.4%) received I/L RT, while 53 (61.6%) underwent B/L RT. LR recurrence occurred in 28 patients (32.6%), with 5 failures in the I/L RT group and 23 in the B/L RT group.

Recurrence patterns:

• Type A: 42.8%
• Type B: 14.3%
• Type C: 17.9%
• Type D: 0%
• Type E: 25%

Failures predominantly occurred at anterior flap margins within 5mm. ECE and pathological N staging were key predictors for B/L neck RT. These findings underscore the importance of precise target delineation and dose optimization in PORT.

Conclusion: This study highlights relapse patterns in BAC carcinoma following surgery and PORT. Findings suggest high-dose region failures stem primarily from intrinsic radioresistance rather than dose inhomogeneity or underdosing. Extraneous recurrences in the non-irradiated neck indicate potential risk stratification gaps. Contralateral nodal failures post-B/L elective nodal RT were consistent with previous studies, pointing to poor disease biology and inaccurate risk assessment as major recurrence factors. I/L neck RT did not increase nodal failures, supporting potential de-escalation in select cases. A high incidence of anterior margin failures suggests conservative target volume selection to mitigate mucositis in the lip region may lead to inadequate surgical bed clearance. This highlights the need for more comprehensive treatment volumes to ensure full surgical flap coverage and reduce marginal recurrences.