To assess the radiation-induced optic neuropathy (RION) prevalence, following high dose pencil beam scanning proton therapy (PBS PT) to skull base and head and neck (H&N) tumours Treatment of aggressive intracranial tumours often require high radiation doses, the delivery of which can be limited by the dose constraints to neighboring organs at risk (OARs), such as the optic nerves and the chiasma. Exceeding the radiation tolerance of these optic structures can result in radiation-induced optic neuropathy (RION); which presents usually as painless monocular loss of vision in presence of optic nerve injury or bilateral vision loss if the optic chiasma is involve.
Beam scanning proton therapy
RION is in essence due to ischemia to the optic apparatus caused by the occlusive obstruction of the blood supply; hence leading to optic atrophy and consequential partial loss of function. In order to reduce the risk of RION particle beam therapy using protons has been employ to deliver high radiation doses to tumours in close vicinity to the optic apparatus. Particles have a finite range in tissue and a zero exit dose rendering them an advantageous treatment option in such challenging cases.
Furthermore, using pencil beam scanning (PBS); a delivery technique clinically developed at the XXX; optimises proximal dose delivery compared to passive scattered protons. patients receiving PBS PT with curative intent for a tumour of the base of skull or the head and neck (H&N) between 1999 and 2014 and who received ≥45GyRBE to the optic apparatus. This dose level was chosen because it is very unlikely that patients present any visual toxicity under this cut-off dose.
Passive scattered protons
Patients who are diagnose with a tumour of the optic apparatus are not includ in the study; neither patients who had received chemotherapy prior or concomitant to PT. Hence, 216 consecutive patients were identified. Of these; 44 (20.9%) patients presented with visual deficits (visual acuity and/or visual field deficits) prior to PT. The type of tumours is summarized.
The median patient age at PT was 47 years (interquartile range (IQR); 35.8-60), with most (83.3%) patients being less than 65 years old; with slightly more females than males All but one patient with meningioma underwent biopsy prior to PT and most of them underwent a partial resection (n=201; 93.1%), only 14 (6.5%) patients underwent total resection.
This is follow by adjuvant PT in 164 patients (75.9%) while the other patients (n=52; 24.1%) received salvage proton therapy following tumour recurrence. The dose/volume metrics on the optic apparatus are detail. The median postoperative volume of the residual disease as identify on the planning computed tomography and postoperative MRI; and defining the gross tumour volume (GTV), was 25.6 cm3 (range 0.0 – 205.3). When a dose boost is plan this is deliver sequentially.