Prostate IMRTEvaluation of the ‘dose of the day’ for IMRT prostate cancer patients derived from portal dose measurements and cone-beam CT
Section snippets
Methods and materials
Reconstruction of the actually delivered 3D dose distribution to a patient in a certain treatment fraction consists of two main parts: (1) derivation of the fluence that was delivered for each beam direction during treatment and (2) definition of the patient geometry at the time of treatment using a CBCT scan. For each treatment beam the delivered fluence is obtained by comparing the portal dose image (PDI) measured during treatment with the corresponding predicted PDI. To eliminate the impact
Validation of dose reconstruction
Within the 10 × 10 cm2 treatment field, with either a 6% or 3% fluence offset in the central 2 × 2 cm2 region of the field, the differences between the reconstructed and forward calculated dose distributions at 10 cm depth in a solid water phantom were less than 1%. When repeating this measurement for the lung phantom, the overall agreement was within 2%.
For one of the IMRT fields, the gamma distribution of observed portal dose differences between the predicted PDI, based on the unmodified IMRT
Discussion
In this work, a method to derive the delivered ‘dose of the day’ was described, based on the actually delivered fluence during treatment and the patient geometry as derived from a CBCT scan. Application of this method to 10 prostate cancer patients showed an overall good agreement between the planned and reconstructed 3D dose distributions. However, for 2 patients large deviations were found in two fractions that were most likely related to differences in rectal filling with respect to the
Conclusion
A method for calculation of the actually delivered dose to the patient was developed and validated. Application of the method to a group of 10 prostate cancer patients showed that generally DVH parameters remained within the planning constraints. The isocenter dose was found to be within −0.4 ± 1.0% (1 SD) and the difference between the planned and delivered dose was within 3%/3 mm for 98.1% of the pixels with a dose higher than 50% of the prescribed dose. A large degradation of the dose delivery
Acknowledgements
This work was financially supported by the Dutch Cancer Society (Grant DDHK 2004-3107). The authors thank Elekta-CMS for providing a research version of XIO to perform the dose reconstruction.
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