Prostate IMRT
Evaluation of the ‘dose of the day’ for IMRT prostate cancer patients derived from portal dose measurements and cone-beam CT

https://doi.org/10.1016/j.radonc.2010.05.015Get rights and content

Abstract

Purpose

High geometrical and dosimetrical accuracies are required for radiotherapy treatments where IMRT is applied in combination with narrow treatment margins in order to minimize dose delivery to normal tissues. As an overall check, we implemented a method for reconstruction of the actually delivered 3D dose distribution to the patient during a treatment fraction, i.e., the ‘dose of the day’. In this article results on the clinical evaluation of this concept for a group of IMRT prostate cancer patients are presented.

Materials and methods

The actual IMRT fluence maps delivered to a patient were derived from measured EPID-images acquired during treatment using a previously described iterative method. In addition, the patient geometry was obtained from in-room acquired cone-beam CT images. For dose calculation, a mapping of the Hounsfield Units from the planning CT was applied. With the fluence maps and the modified cone-beam CT the ‘dose of the day’ was calculated. The method was validated using phantom measurements and evaluated clinically for 10 prostate cancer patients in 4 or 5 fractions.

Results

The phantom measurements showed that the delivered dose could be reconstructed within 3%/3 mm accuracy. For prostate cancer patients, the isocenter dose agreed within −0.4 ± 1.0% (1 SD) with the planned value, while for on average 98.1% of the pixels within the 50% isodose surface the actually delivered dose agreed within 3% or 3 mm with the planned dose. For most fractions, the dose coverage of the prostate volume was slightly deteriorated which was caused by small prostate rotations and small inaccuracies in fluence delivery. The dose that was delivered to the rectum remained within the constraints used during planning. However, for two patients a large degrading of the dose delivery was observed in two fractions. For one patient this was related to changes in rectum filling with respect to the planning CT and for the other to large intra-fraction motion during treatment delivery, resulting in mean underdosages of 16% in the prostate volume.

Conclusions

A method to accurately assess the ‘dose of the day’ was evaluated for prostate cancer patients treated with IMRT. To correct for observed dose deviations off-line dose-adaptive strategies will be developed.

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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