Radiation dose-fractionation effects in spinal cord: comparison of animal and human data

Characterizing Proton-Induced Biological Effects in a Mouse Spinal Cord Model: A Comparison of Bragg Peak and Entrance Beam Response in Single and Fractionated Exposures

PURPOSE

Proton relative biological effectiveness (RBE) is a dynamic variable influenced by factors like linear energy transfer (LET), dose, tissue type, and biological endpoint. The standard fixed proton RBE of 1.1, currently used in clinical planning, may not accurately represent the true biological effects of proton therapy (PT) in all cases. This uncertainty can contribute to radiation-induced normal tissue toxicity in patients. In late-responding tissues such as the spinal cord, toxicity can cause devastating complications. This study investigated spinal cord tolerance in mice subjected to proton irradiation and characterized the influence of fractionation on proton- induced myelopathy at entrance (ENT) and Bragg peak (BP) positions.

METHODS AND MATERIALS

Cervical spinal cords of 8-week-old C57BL/6J female mice were irradiated with single- or multi-fractions (18x) using lateral opposed radiation fields at 1 of 2 positions along the Bragg curve: ENT (dose-mean LET = 1.2 keV/μm) and BP (LET = 6.9 keV/μm). Mice were monitored over 1 year for changes in weight, mobility, and general health, with radiation-induced myelopathy as the primary biological endpoint. Calculations of the RBE of the ENT and BP curve (RBEENT/BP) were performed.

RESULTS

Single-fraction RBEENT/BP for 50% effect probability (tolerance dose (TD50), grade II paresis, determined using log-logistic model fitting) was 1.10 ± 0.06 (95% CI) and for multifraction treatments it was 1.19 ± 0.05 (95% CI). Higher incidence and faster onset of paralysis were seen in mice treated at the BP compared with ENT.

CONCLUSIONS

The findings challenge the universally fixed RBE value in PT, indicating up to a 25% mouse spinal cord RBEENT/BP variation for multifraction treatments. These results highlight the importance of considering fractionation in determining RBE for PT. Robust characterization of proton-induced toxicity, aided by in vivo models, is paramount for refining clinical decision-making and mitigating potential patient side effects.

Risk of Myelopathy Following Second Local Treatment after Initial Irradiation of Spine Metastasis

Metastatic lesions of the spine occur in up to 40% of cancer patients and are a frequent source of pain and neurologic deficit due to cord compression. Palliative radiotherapy is the main first-intent local treatment in the form of single-fraction radiotherapy or fractionated courses. Reirradiation is a viable option for inoperable patients where spinal decompression is needed but with an increased risk of radiation-induced myelopathy (RM) and subsequent neurologic damage. This review summarizes reported data on local treatment options after initial irradiation in patients with relapsed spine metastasis and key dosimetric correlations between the risk of spinal cord injury and reirradiation technique, total dose, and time between treatments. The Linear Quadratic (LQ) model was used to convert all the published doses into biologically effective doses and normalize them to EQD2. For 3D radiotherapy, authors used cumulative doses from 55.2 Gy2/2 to 65.5 Gy2/2 EQD2 with no cases of RM mentioned. We found little evidence of RM after SBRT in the papers that met our criteria of inclusion, usually at the median reported dose to critical neural tissue around 93.5 Gy2/2. There is a lack of consistency in reporting the spinal cord dose, which leads to difficulty in pooling data.

Intramedullary spinal nephroblastoma in a mixed breed dog

A 1-year-old male mixed breed dog presented for the evaluation of progressive hindlimb paresis. Neurological examination indicated a spinal cord lesion between the 3rd thoracic and 3rd lumbar vertebrae. Magnetic resonance imaging (MRI) revealed an intramedullary spinal cord lesion located at the level of the 1st and 2nd lumbar vertebrae. Following cytoreductive surgery of the mass, palliative radiation therapy was administered. A diagnosis of nephroblastoma was made based on histological examination. After radiation therapy, the disappearance of the spinal lesion was confirmed by MRI. The dog was improved from gait abnormality and alive at 16 months postoperatively, with slight signs of neurological dysfunction.

Radiation-Induced Myelitis: Initial and Follow-Up MRI and Clinical Features in Patients at a Single Tertiary Care Institution during 20 Years

Myelitis is a rare complication of radiation exposure to the spinal cord and is often a diagnosis of exclusion. A retrospective review of clinical records and serial imaging was performed to identify subjects with documented myelitis and a history of prior radiation. Eleven patients fulfilled the inclusion criteria. All patients had longitudinally extensive cord involvement with homogeneous precontrast T1 hyperintense signal in the adjacent vertebrae, corresponding to the radiation field. T2 signal abnormalities involving the central two-thirds of the cord were seen in 6/11 patients (55%). The degree of cord expansion and contrast enhancement was variable but was seen in 6 (54%) and 5 (45%) patients, respectively. On follow-up, 2 patients developed cord atrophy, while complete resolution was noted in 1. Clinical improvement was noted in 5 patients, with symptom progression in 2 patients. Our results suggest that radiation myelitis is neither universally progressive nor permanent, and some radiographic and clinical improvement may occur.

[Re-irradiation of head and neck cancers: Target volumes, technical evolutions and prospects]

Malignant tumors of the head and neck have a predominantly regional recurrence pattern, with most deaths resulting from this progression. Optimization of re-radiation in recurrence setting is a major objective for these patients. Extensive research has been carried out with the PubMed search engine to find publications dealing with this topic. The first attempts to reirradiate the ORL sphere date back to the 1980s and the first to be performed by intensity modulation conformational radiotherapy (IMRT) date back to the late 1990s. Compared to 3 dimensional conformal radiotherapy, IMRT improves clinical outcomes and reduces toxicity. In IMRT series, associated or not with concomitant chemotherapy, the locoregional control obtained at 2 years was of the order of 45 to 65% and the overall survival of 15 to 60%, depending on predictive factors. Grade 3 acute toxicity occurred on the order of 10 to 30% and late-grade 3 toxicity on the order of 15 to 50%. In a selected population with low volumes tumors, stereotactic re-irradiation at a minimum dose of 35Gy obtained outcome comparable to IMRT. Re-irradiation of head and neck tumors by proton therapy is rare. The toxicity rate appears to be lower than that usually seen after photon therapy. However, we do not have a long follow-up. This technique therefore remains reserved for search protocols and represents a future perspective in these situations.