Effect of Titanium Hardware on Radiation Dosimetry in Postoperative Proton Versus Photon Spinal Stereotactic Body Radiation Therapy after Surgical Decompression for Spinal Cord Compression

We hypothesized that titanium hardware caused larger differences in dosimetric parameters between photon and proton therapy as post-operative spinal stereotactic body radiation therapy (SBRT) after surgical decompression for spinal cord compression. The volumetric modulated arc therapy (VMAT) plans (by Eclipse Treatment Planning System version 10.0) of two prospective-recruited patients treated with post-operative spinal SBRT after surgical decompression with titanium implantation for spinal cord compression were generated with application of electron densities of 4.50 g/cm3 (titanium) and 1.0 g/cm3 (water) to the titanium and its associated metallic artefacts based on their respective Hounsfield units (Hus) before treatment optimization. An override with electron density of 1.82 g/cm3 (cortical bone) was applied to the titanium implant to the same set of CT images including the target volumes and organs-at-risk followed by dose re-calculation without optimization. The same sets of planning CT images were re-planned by algorithms for proton therapy after conversion from HUs to the relative stopping powers (RSPs) for the titanium hardware and metallic artefacts followed by dose re-calculation without re-optimization after density and RSP override with those for cortical bone. Planning target volume (PTV) to the involved spine was given 24 Gy in 2 fractions and the dose to planning volume of spinal cord (PRV_SC) was limited to 17 Gy in 2 fractions. Doses to the clinical target volume (CTV), PTV, spinal cord (SC) and planning volume for spinal cord (PRV_SC) were compared before and after density override and between protons and photons. An average increase in dose by 6.5% (range -0.1% – +17.0%), 7.9% (range -0.7% – +23.9%), 8.8% (range -21.2% – +33.3%) and 6.6% (range -7.1% – +22.3%) for protons and an average corresponding increase in dose by 1.7% (range -1.4% – +5.4%), 1.5% (range -1.4% – +2.5%), 2.2% (range -7.0% – +15.4%) and 1.1% (range -7.1% – +5.9%) for CTV, PTV, SC and PRV_SC respectively for photons were observed after density override from 4.5g/cm3 to 1.82g/cm3. In general, larger percentage differences in dosimetric parameters were demonstrated after density override for protons compared to photons. Titanium hardware caused larger dosimetric variations in protons after density override, probably due to its larger RSP against protons compared to photons. Density override with the electron density of individual artificial material should be seriously considered when treatment is delivered by protons.