Comparison of Discrete Spot Scanning and Passive Scattering Craniospinal Proton Irradiation
J Stoker*, R Amos, Y Li, W Liu, P Park, N Sahoo, X Zhang, X Zhu, M Gillin, MD Anderson Cancer Center, Houston, TXSU-E-T-693 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
To compare plan robustness, dose variations at field junctions, and overall dose conformity to target between passive scattering and discrete spot scanning (DSS) proton craniospinal irradiation.
A DSS treatment plan was generated for three craniospinal CT image sets. A forward planning approach was used to generate treatment plans for the cranium and lower spine, which exhibited dose tapering of about 10 Gy/cm towards the thoracic spine to facilitate the low-gradient field junctions. In-house software accomplished optimization of discrete spot weights for the thoracic-level field, which was then imported into the Varian Eclipse clinical treatment planning system for dose calculation. Planning was guided using clinical targets defined for craniospinal irradiation with passively scattered proton beams.
Robustness analysis was performed by varying the position of each beam isocenter +/-3 mm along each cardinal axis, simulating setup errors, as well as adjusting the CT number to relative stopping power curve to mimic +/-3.5% range variation, and then analyzing the resulting dose distribution.
Dose profiles along the craniospinal axis were used to evaluate intrafraction dose variation at field junctions. Dose volume histograms (DVH) were generated for each robustness element, and then combined to create composite DVH illustrating a range of possible delivered dose.
Results of robustness analysis indicate that field isocenter shifts along the craniospinal axis can result in dose intrafraction variations at the junction in excess of 25% for passive scattering proton plans. This value is markedly reduced for the same shifts applied to DSS plans using a tapered-dose field matching technique.
This work demonstrates the potential for improved robustness of proton craniospinal irradiations using a DSS delivery method, as well as significant decreases in clinic expenses. The use of apertures to define the sagittal plane field edge for DSS delivery improves the dose to target.