Sparing the Submandibular Gland (SMG) When Treating Neck Level IB in Oral Squamous Cell Carcinoma (OSqCCA)

Abstract

Patients with OSqCCA frequently have the ipsilateral SMG resected at neck dissection and require post-operative radiation (PORT) bilaterally. Contralateral level IB is often at risk and is included within the low risk clinical target volume (CTV). The NRG contouring atlas includes the SMG within the level IB target contour, and such inclusion is common practice. However, the lymphatic system develops prior to encapsulation of the parotid but after encapsulation of the submandibular and sublingual glands. At completion of embryogenesis, the parotid has lymph nodes and lymphatics within the capsule, whereas the submandibular and sublingual glands do not. Consequentially, surgical series report no metastases within the SMG even when level IB is involved. The SMG provides ≈ 66% of unstimulated saliva and SMG doses relate to xerostomia, as such, we hypothesized we could treat level IB while sparing the SMG without comprising target coverage. Twelve patients with OSqCCA undergoing neck dissection with indications for PORT including the contralateral level IB within the planning target volume (PTV) were retrospectively reviewed and re-planned. Neck nodal volume (CTVs) contouring, including level IB, were in accordance with the NRG contouring atlas but excluded the SMG. PTVs consisted of 3 mm uniform expansions of CTVs. The contralateral neck PTVs were planned to 54 Gy (PTV54). Dose requirements were per RTOG 1008: goal PTV54 D95%> 54 Gy, with an allowable variation of >48.6 Gy. The dose constraint for the SMG was mean dose < 39 Gy, found previously to preserve SMG saliva (Murdoch-Kinch CA et al, IJROBP 2008). Treatment plans were created utilizing volumetric modulated arc therapy. Pearson correlation coefficients assessed linear dependence between variables. Mean SMG volume was 8.6 cc and mean dose was 38.9 Gy (interquartile rage [IQR] 38.3-39.0). Mean PTV54 dose was 56.3 Gy (IQR 56.1-56.4). Median PTV54 D95% was 53.0 Gy (range 52.5-54.6 Gy), with all cases meeting our pre-specified allowable coverage goal. When assessing the portion of the PTV associated with level IB only (PTV_IB), mean PTV_IB dose was 53.7 Gy and median PTV_IB D95% was 43.3 Gy (range 42.5-44.8). Median D95% to CTV_IB was 50.2 Gy. SMG sparing resulted in an approximate 10-20% underdosing of the part of the PTV corresponding to level IB, as a small portion of the PTV volume lies within the SMG (the portion underdosed). The PTV54 D95%(Gy) was not correlated with PTV54 volume or SMG volume (both p >0.7), and PTV_IB D95%(Gy) was not correlated with PTV_IB or SMG volume (both p>0.15). The SMG can be spared to a mean dose < 39 Gy without compromising PTV coverage in patients receiving contralateral PORT including level IB for OSqCCA, with slight underdosing of the level IB PTV where it overlaps with the SMG. These principles also apply to cases where bilateral levels IB are targeted with intact SMGs.