Comparing cone-beam CT intensity correction methods for dose recalculation in adaptive intensity-modulated photon and proton therapy for head and neck cancer

Abstract

Background. Adaptive intensity-modulated photon and proton radiotherapy (IMRT and IMPT) of head and neck (H & N) cancer requires frequent three-dimensional (3D) dose calculation. We compared two approaches for dose recalculation on the basis of intensity-corrected cone-beam (CB) x-ray computed tomography (CT) images. Material and methods. For nine H & N tumor patients, virtual CTs (vCT) were generated by deformable image registration of the planning CT (pCT) to the CBCT. The second intensity correction approach used population-based lookup tables for scaling CBCT intensities to the pCT HU range (CBCT LUT ). IMRT and IMPT plans were generated with a commercial treatment planning system. Dose recalculations on vCT and CBCT LUT were analyzed using a (3%, 3 mm) gamma-index analysis and comparison of normal tissue and tumor dose/volume parameters. A replanning CT (rpCT) acquired within three days of the CBCT served as reference. Single fi eld uniform dose (SFUD) proton plans were created and recalculated on vCT and CBCT LUT for proton range comparison. Results. Dose/volume parameters showed minor differences between rpCT, vCT and CBCT LUT in IMRT, but clinically relevant deviations between CBCT LUT and rpCT in the spinal cord for IMPT. Gamma-index pass-rates were found increased for vCT with respect to CBCT LUT in IMPT (by up to 21 percentage points) and IMRT (by up to 9 percentage points) for most cases. The SFUD-based proton range assessment showed improved agreement of vCT and rpCT, with 88 -99% of the depth dose profi les in beam ' s eye view agreeing within 3 mm. For CBCT LUT , only 80 -94% of the profi les fulfi lled this criterion. Conclusion. vCT and CBCT LUT are suitable options for dose recalculation in adaptive IMRT. In the scope of IMPT, the vCT approach is preferable.

Online daily assessment of dose change in head and neck radiotherapy without dose‐recalculation

“…Dose calculations on CBCTs are possible with the results varying between reported studies19, 20, 21 because of inferior image quality and tissue densitometry. Performing reliable analysis of the dose to the target and organs at risk would require contouring of relevant structures on the daily CBCT image.…”

Europium‐155 as a source for dual energy cone beam computed tomography in adaptive proton therapy: A simulation study

“…[22][23][24] After converting Z eff to mean excitation energy with the method proposed by Yang et al 9 the SPR of a pixel can be calculated with Eq. (6).…”

“…Despite the widespread use of CBCT in conventional X‐ray radiotherapy, this technology is only now becoming a standard offering of proton therapy vendors. The most effective utilization of CBCT in a clinical proton therapy environment is a topic receiving increasing interest …”

“…The most effective utilization of CBCT in a clinical proton therapy environment is a topic receiving increasing interest. [3][4][5][6] Due to the dependence of CBCT Hounsfield units (HUs) on the particular scatter environment created by the patient, CBCT image data are not typically used directly for dose calculation. 7,8 Rather, CBCT images are typically only used for geometrical information in online use.…”

Technical Note: Feasibility of MRI‐based estimation of water‐equivalent path length to detect changes in proton range during treatment courses

“…An obstacle to using CBCT as an alternative to regular CT is that the Hounsfield unit (HU) is not sufficiently accurate to calculate the water‐equivalent path length (WEPL) or radiation dose because of high X‐ray scatter. This obstacle has been tackled by correcting the scatter or constructing a virtual CT (vCT, also known as synthetic CT or pseudo‐CT) by deforming planning CT (pCT) onto CBCT …”