Hypofractionated Protontherapy in Chordomas and Chondrosarcomas of the Skull Base

Inclusion Criteria for 5 fractions:

* With a baseline classification on the Karnofsky performance status scale ≥ 70%.
* With confirmed histological diagnosis of chordoma or chondrosarcoma of the skull base.
* Who have signed the specific informed consent of the protocol, agreeing to participate in it.
* Completion of magnetic resonance imaging with vascular assessment ruling out pre-existing vascular pathology (stenosis or atherosclerosis), including 3D T1 black-blood sequences, pre-contrast 3D TOF, 3D T2 with fat suppression, and perfusion sequences.
* With a maximum tumor size of 50 cc.
* Whose relationship to organs at risk (OARs) allows compliance with the necessary dose restrictions to receive hypofractionated proton therapy in 5 fractions.
* Patients included in the study must meet dosimetric parameters that include:
* Tumor CTV coverage of at least D95\>90%.
* Correct compliance with the dose restrictions, at least in the nominal scenario, for critical organs (optic pathway, brain stem and spinal cord) according to the guidelines published and available in the literature:

Dose contnstraints for 5 fractions:

Optic Nerves: D0.03cc ≤ 25 GyRBE, V23.5 \< 0.5cc. Chiasm:D0.03cc ≤ 25 GyRBE, V23.5 \< 0.5cc. Brainstem:D0.03cc ≤ 31 GyRBE,V23 \< 0.5cc. Spinal Chord: D0.03cc ≤ 30 GyRBE, V23 \< 035cc. Right and left temporal lobes: D0.03 cc ≤ 35 GyRBE, V30 ≤ 5.5 cc.

Inclusion Criteria for 25 fractions:

* With a baseline classification on the Karnofsky performance status scale ≥ 70%.
* With confirmed histological diagnosis of chordoma or chondrosarcoma of the skull base.
* Who have signed the specific informed consent of the protocol, agreeing to participate in it.
* Not considered candidates for the 5-fraction protocol due to tumor size exceeding 50 cc and/or the presence of vascular pathology (stenosis or atherosclerosis) identified on MRI with vascular sequences.
* Tumor relationship to organs at risk allows compliance with the dose constraints required to receive hypofractionated proton therapy delivered in 27 fractions.
* Patients included in the study must meet dosimetric parameters that include:
* Tumor CTV coverage of at least D95\>90%.
* Correct compliance with the dose restrictions, at least in the nominal scenario, for critical organs (optic pathway, brain stem and spinal cord) according to the guidelines published and available in the literature:

Dose constraints for 25 fractions:

Optic nerves: D0.03 cc ≤ 54.7 GyRBE. Optic chiasm: D0.03 cc ≤ 54.7 GyRBE. Brainstem: Surface: D0.03 cc ≤ 57.9 GyRBE. Core: D0.03 cc ≤ 54 GyRBE. Spinal cord: D0.03 cc ≤ 54 GyRBE. Right and left temporal lobes: V65 \< 1.7 cc, V60 ≤ 5.5 cc.

Treatment planning with a minimum of 5 beams. In general, the use of a class solution with 6 beams will be proposed, including 2 lateral beams with gantry angles between 20° and 80°, depending on tumor location; 2 posterior oblique beams; and 2 anterior oblique beams. The latter four beams may include a couch rotation of at least 20° relative to the two lateral beams, with a minimum angular separation of at least 30° between ipsilateral oblique beams. Depending on individual patient characteristics, this class solution will be adapted to adjust specific gantry and couch angles for each field.

If this solution is not feasible due to patient-specific characteristics (surgical constraints, tumor location or laterality, etc.), a 5-beam solution will be evaluated, including 2 posterior oblique beams and 2 anterior oblique beams, in addition to a coronal field with the couch at 270° and a gantry angle between 40° and 90° depending on tumor location, or other configurations that increase the number of ipsilateral oblique beams with a minimum inter-beam separation of at least 30°.

Evaluation of Linear Energy Transfer (LET) and biological dose:

For each treatment plan, the LET distribution obtained from the treatment planning system (TPS) will be evaluated, with particular attention to regions where LET values exceed 5 keV/μm, aiming to minimize such values. Equivalent biological dose distributions based on recognized models in the literature may also be assessed to support decision-making regarding the suitability of a given treatment plan

Exclusion Criteria:

* Patients with distant metastases.
* Patients who have received previous irradiation in the same location.
* Patients whose clinical or dosimetric characteristics do not meet the inclusion criteria.
* Patients who are simultaneously participating in another study that may affect the results of this protocol.