Pencil Beam Scanning Intensity Modulated Proton Therapy for Head and Neck Cancers Involving Skull Base

Introduction: Head and neck cancers directly involving skull base or infiltrating through perineural invasion are a significant challenge to any radiotherapy intervention due to the presence of organs at risk in close proximity to tumor target and the expectation of tumor control and preservation of function. Proton therapy (PT) offers a dosimetric advantage over photons providing adequate target dose coverage combined with reduced dose to organs at risk (OAR) due to the finite range of protons and the steep radiation dose gradients between target and adjacent tissues. PT delivery techniques have evolved from passively scattered broad beam to actively scanned pencil beam capable of providing a highly conformal dose to the target. Furthermore, the use of intensity-modulated proton pencil beams (IMPT) allow for even greater sparing of adjacent organs with a sharp dose fall off between targets and OAR. With IMPT, multiple fields each individually delivering an inhomogeneous dose to the target, are jointly delivered to meet target prescription and OAR constraint requirements. Our experience with the treatment of these tumors in a group of patients treated with IMPT with will be presented.

Material and Methods: Tumors of the nasal cavity and paranasal sinuses, nasopharynx, and adenoid cystic carcinoma arising from salivary and lacrimal glands were treated using IMPT. All patients had skull base involvement. Attention was paid to issues specifically related to proton therapy: presence of metal artifacts, choice of beam angles, robustness of treatment to range and setup uncertainty, monitoring of patients during treatment with weekly CT scans for the variable filling of sinus cavities or changes in anatomy that would affect dose to target and OAR, and adaptive planning as necessitated by changes in anatomy. Radiation treatments were performed using either a sequential boost to 70 – 75.6 GyE at 2 or 1.8 GyE per fraction or by a simultaneous integrated boost technique to 70.8 GyE at 2.36 GyE per fraction with attention paid to the dose per fraction delivered to adjacent organs at risk. Chemotherapy was added as per the indication.

Results: All patients completed their prescribed course of treatment and tolerated the treatment well. Nutrition was maintained in all by oral intake except for one patient who had a palatal fenestration and obturator and required PEG-tube feeding during the end of treatment. Skin and mucosal reactions were minimal when compared with passively scattered proton therapy, none greater than erythema and dry desquamation (Grade 2). With a minimum follow up of 6 months, local control was achieved in all patients. One patient developed intracranial metastasis at 4 months. There were no ocular or auditory dysfunction nor any temporal lobe changes noted on follow up MRI.

Conclusion: IMPT is well tolerated by patients, provides excellent tumor coverage while sparing critical tissues. However, attention to details of treatment planning and careful monitoring of patients during treatment is important to ensure the safety of the treatment as well as a good clinical outcome.