Calcul de dose de radiothérapie à partir de tomographies coniques : état de l’art

Does a patient-specific bladder-filling protocol affect bladder volume and dose in postprostatectomy radiotherapy?

PURPOSE

Our aim is to develop a patient-specific bladder-filling protocol (PSP) using an ultrasound-based bladder scanner (BS) and compare the volumetric and dosimetric parameters with those of the standard filling protocol (SP) in postprostatectomy patients.

METHODS

Twenty postprostatectomy patients who received salvage radiotherapy (72 Gy/36 fx) were included. For PSP, the patient was asked to drink 500 mL of water after emptying his bladder. Bladder volume was measured using BS every 10 min. Each patient's unique time to reach a 150-200 cc volume was used for simulation and treatment. For the SP, the patient was asked about the feeling of having a full bladder. Organs at risk (OAR) were contoured on cone-beam computed tomography (CBCT) scans that were transferred to the treatment planning system (TPS). Treatment plans were applied to CBCTs. Changes in bladder volume and doses for planning computed tomography (PCT) and CBCT were determined.

RESULTS

In the SP, there was no significant difference in mean bladder volume for PCT and CBCT (p = 0.139); however, there was a trend for significance in the mean bladder dose (p = 0.074). In PSP, there was no significant difference in the mean bladder volume or dose for PCT and CBCT (p = 0.139 and p = 0.799, respectively). There was a significant difference in terms of mean CBCT bladder volume between the two protocols (p = 0.007), whereas no significant difference was detected in terms of bladder dose (p = 0.130).

CONCLUSION

With PSP, optimal bladder filling was obtained and maintained throughout the whole treatment course, and it was reproducible in every fraction.

Evaluation of Dose Calculation Based on Cone-Beam CT Using Different Measuring Correction Methods for Head and Neck Cancer Patients

Purpose: To investigate and compare 2 cone-beam computed tomography (CBCT) correction methods for CBCT-based dose calculation. Materials and Methods: Routine CBCT image sets of 12 head and neck cancer patients who received volumetric modulated arc therapy (VMAT) treatment were retrospectively analyzed. The CBCT images obtained using an on-board imager (OBI) at the first treatment fraction were firstly deformable registered and padded with the kVCT images to provide enough anatomical information about the tissues for dose calculation. Then, 2 CBCT correction methods were developed and applied to correct CBCT Hounsfield unit (HU) values. One method (HD method) is based on protocol-specific CBCT HU to physical density (HD) curve, and the other method (HM method) is based on histogram matching (HM) of HU value. The corrected CBCT images (CBCT_HD and CBCT_HM for HD and HM methods) were imported into the original planning system for dose calculation based on the HD curve of kVCT (the planning CT). The dose computation result was analyzed and discussed to compare these 2 CBCT-correction methods. Results: Dosimetric parameters, such as the D_mean, D_max and D_5% of the target volume in CBCT plan doses, were higher than those in the kVCT plan doses; however, the deviations were less than 2%. The D_2%, in parallel organs such as the parotid glands, the deviations from the CBCT_HM plan dose were less than those of the CBCT_HD plan dose. The differences were statistically significant (P < .05). Meanwhile, the V₃₀ value based on the HM method was better than that based on the HD method in the oral cavity region (P = .016). In addition, we also compared the γ passing rates of kVCT plan doses with the 2 CBCT plan doses, and negligible differences were found. Conclusion: The HM method was more suitable for head and neck cancer patients than the HD one. Furthermore, with the CBCT_HM-based method, the dose calculation result better matches the kVCT-based dose calculation.

Impact of Acquisition Protocols on Accuracy of Dose Calculation Based on XVI Cone Beam Computed Tomography

Purpose:

The objective of this work is to study the impact of acquisition protocols on the accuracy of cone beam computed tomography (CBCT)-based dose calculation and to determinate its limits from image characteristics such as image quality, Hounsfield numbers consistency, and restrictive sizes of volume acquisition, compared to the CT imaging for the different anatomy localizations: head and neck (H&N), thorax, and pelvis.

Materials and Methods:

In this work, we used a routine on-board imaging CBCT of the XVI system (Elekta, Stockholm, Sweden). Dosimetric calculations performed on CT images require the knowledge of the Hounsfield unit-relative electron density (HU-ReD) calibration curve, which is determined for each imaging technology and must be adapted to the imaging acquisition parameters (filter/field of view). The accuracy of the dose calculation from CBCT images strongly depends on the quality of these images and also on the appropriate correspondence to the electronic densities, which will be used by the treatment planning system to simulate the dose distribution. In this study, we evaluated the accuracy of the dose calculation for each protocol, as already pointed in many studies.

Results:

As a result, the protocols that give better results in terms of dose calculation are F0S20 for the H&N region and F1M20 for the thoracic and pelvic regions, with an error <2% compared to results obtained with CT images. In addition, the dose distributions obtained with CT and CBCT imaging modalities were compared by two different methods. The first comparison was done by gamma index in three planes (sagittal, coronal, and transverse) with 2%; 2 mm criteria. The results showed good correspondence, with more than 95% of points passed the criteria. We also compared the target volume, the organs at risk (OARs), and the maximum and minimum doses for the three localizations (H&N, thorax, and pelvis) in CT and CBCT imaging modalities using a Rando phantom.

Conclusions:

The choice of the adequate CBCT acquisition protocol and the appropriate phantom to determine the HU-ReD calibration curve provides a better precision in the calculation of dose on CBCT images. This allows improving the results obtained when using the HU-ReD calibration method for dose calculation in adaptive radiotherapy.