eContour

54 y/o M with cT2N0M0 (stage IIA) Merkel cell carcinoma (MCC) of the right parietal scalp s/p wide local excision with negative margins, sentinel lymph node biopsy (SLNB), and lymph node dissection with two positive sentinel lymph nodes in the right neck with micrometastatic disease, no extracapsular extension (ECE); pT2N1a(sn)M0 (stage IIIA)

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Technique:

  • Postoperative radiation using VMAT

Dose:

  • The PTV primary, encompassing the postoperative tumor bed, received 56 Gy in 28 fractions. Following resection of the primary MCC, dose recommendations range between 50-56 Gy for negative resection margins (per NCCN 2.2022).
  • The right elective neck (CTV56),encompassing right neck level Ib (submandibular), level 2 upper jugular), level 3 (middle jugular), level 4a (lower jugular) and level 8 (parotid) received 56 Gy in 28 fractions. If multiple involved lymph nodes and/or ECE after SLNB and LN dissection, dose recommendations range between 50-60 Gy (per NCCN 2.2022).

Simulation:

  • Tumor bed and an additional margin of 5 cm were wired to represent the high-risk CTV and covered with 1 cm of bolus. Patient was immobilized in a short thermoplastic head-mask.
  • Please note that we used a margin of 5 cm because even though the primary is in the head and neck, it was feasible based on distance to nearby critical structures. 

Contours Per

  • Cover areas at high-risk of disease/recurrence based on initial location of primary tumor and surgical pathology.
  • Post-op RT contours as per Grégoire et al. Radiotherapy and Oncology 2014 and Biau et al. Radiotherapy and Oncology 2019. Though these contouring atlases and nodal recommendations were developed for mucosal surface SCC of the H&N, their use in defining nodal stations for patients at risk is likely still reasonable for MCC.
  • PTV primary: post-op tumor bed in the right parietal scalp area with a 5 cm margin.
  • CTV56: right elective neck, encompassing right neck level Ib (submandibular), level 2 (upper jugular), level 3 (middle jugular), level 4a (lower jugular) and level 8 (parotid).
  • PTV56: CTV56 + 3mm.

OARs:

  • Globes, larynx, esophagus, mandible, brain, brainstem, spinal cord, pharyngeal constrictor muscles, oral cavity, left parotid and left submandibular gland

Additional notes:

  • Merkel Cell Carcinoma has a high rate of microsatellitosis and nearby in-transit lymphatic metastases.
  • NCCN 2.2022 recommend wide margins (5 cm) around the primary site whenever possible, with studies recommending at least 2 cm in the head and neck. See Veness et al. IJROBP 2010, Lok et al. Cancer 2012.
  • Bilateral nodal irradiation was not administered to reduce morbidity given that there was no clinical lymphadenopathy, and surgical pathology only showed evidence of microscopic disease in the ipsilateral neck with no extracapsular extension. The tumor was also mostly lateralized to the right scalp. NOTE: There is limited evidence regarding dosing for Merkel Cell Carcinoma.
  • The randomized, but underpowered, clinical trial by Jouary et al. showed a potential benefit to elective nodal radiation, therefore our coverage extended to the low neck in addition to the cervical nodal stations that harbored positive sentinel lymph nodes.

 

 

  • Though genomically distinct, UV-associated and Polyoma virus-associated Merkel Cell Carcinomas do not have different treatment strategies. See NCCN 2.2022, Horny et al. Cancers 2021.
  • Acceptable elective nodal irradiation doses are between 50-56Gy, see Foote et al. IJROBP 2010.
  • Nodal observation is a reasonable option for negative sentinel lymph node biopsy unless there are concerns about the accuracy of the biopsy. If multiple involved lymph nodes and/or ECE, consider dose escalation to 60Gy (per NCCN 2.2022). 
  • There are current national clinical trials investigating the role of adjuvant immunotherapy in locoregionally advanced Merkel Cell Carcinoma, see NCT04291885, NCT03712605, NCT03271372. 
  • Phase II data suggest an extremely high response to Ipilimumab plus Nivolumab in patients with metastatic MCC. No major benefit was seen with SBRT (24Gy in 3 fractions) in addition to dual agent immunotherapy. See Kim et al. The Lancet 2022.