To determine the dosimetric impact of inter-fraction anatomical movements in prostate cancer patients receiving proton therapy.
Methods and Materials
For each of the 10 patients studied, 8 CT scans were selected from sets of daily setup CT images that were acquired from a cohort of prostate cancer patients. The images were acquired in the treatment room using the CT-on-Rails system. First, standard proton therapy and IMRT plans were designed for each patient using standard modality-specific methods. The images, the proton plan, and the IMRT plan were then aligned to the 8 CT images based on skin marks. The doses were recalculated on these 8 CT images using beam from the standard plans. Second, the plans were redesigned and evaluated assuming a smaller CTV-to-PTV margin (3 mm). The images and the corresponding plans were then realigned based on the center of volume of the prostate. Dose distributions were evaluated using isodose displays, dose-volume histograms, and target coverage.
For the skin-marker alignment method, four of the 10 IMRT plans were deficient while three of 10 proton plans were compromised. For the alignment method based on the center of volume of the prostate, only the proton plan for one patient was deficient, while three out of the 10 IMRT plans were suboptimal.
A comparison of passively scattered proton therapy and highly-conformal IMRT plans for prostate cancer revealed that the dosimetric impact of inter-fractional anatomical motions was similar for both modalities.
Keywords: Proton therapy, Image-guided radiotherapy, IMRT, Organ motion