Application Of The Method Of CT Value Correction For CBCT Image Based On Patient-specific Region Segmental Hu Correction Table

PurPoseTo make replanning and dose calculation based on CBCT image for thoracic patientavailable, Li had proposed a patient-specific HUCBCTcorrection method and hadinvestigated the efficacy of this method including both phantom and a set of CBCT imageof a NSCLC patient. We continued to investigate the efficacy of this method includingpatients’data for NSCLC, esophagus cancer, breast cancer and nasopharynx cancer，andincluding series datas of CBCT image belonging to one patient.MethodsOne set of FBCT images（CTp）and one set of CBCT images（CBCTp）of patientswere acquired before reatment with scanning techniques commonly used for routine clinicin this study. During the treatment, more sets of CBCT images （CBCT1，CBCT2，CBCT3…CBCTn）were acquired after positioning but before treatment in each fraction.The procedure of HU values correction for CBCT as following: Firstly, got the modifiedCBCT (mCBCT), including mCBCTp and mCBCT1，mCBCT2，mCBCT3…mCBCTn.The mCBCTp images were determined by deformable registering of FBCTp and CBCTpimages.The mCBCT1，mCBCT2，mCBCT3…mCBCTn images were determined bydeformable registering of FBCTp and CBCT1，CBCT2，CBCT3…CBCTn images. ThemCBCT images have both the anatomical structures of the corresponding CBCT imagesand the HU values of CTp.Secondly, the mCBCTp and CBCTp volumetric images, weresegmented to a number of iso-size correction areas (CAs). Then, matched the HU values ofthe CBCTp to that of the mCBCTp, one FBCT-CBCT HU value look-up table was determined for each CA. Thirdly, the FBCT-CBCT HU value look-up tables were used tocorrect the CT numbers of the CBCT1，CBCT2，CBCT3…CBCTn images. The CTnumbers of the voxels were corrected one by one according to the corresponding CAs, thena CT-value-corrected CBCT was obtained, named corrected CBCT (cCBCT1，cCBCT2，cCBCT3…cCBCTn). Lastly, make the same three dimensional conformal radiationtherapy plans on the cCBCTn images, the corresponding mCBCTn images, and thecorresponding CBCTn images. The dose distribution was calculated combined withHU-RED calibration table predefined in treatment planning system (TPS) for FBCT, andthen followed by a treatment replanning. The dose distributions of cCBCTn images, thecorresponding mCBCTn images, and the corresponding CBCTn images were compared.Some parameters like V30, V50of organs at risk were compared too.ResultsThe NSCLC, esophagus cancer, breast cancer and the nasopharynx cancer patients’results are with one accord. When the smallest correction area was （16,16,6）（,32,32,12）and（64,64,24）, the performance of CT number correction are very good. The dosedistributions in the plans of cCBCTn images and the corresponding mCBCTn imagesshowed perfect agreement. parameters are approximate. After comparing the single-field,3-field, and5-field plans, we found that the more fields there are, the better theperformance of the correction. The dose distributions in the plans using a15MV beamshowed better agreement than plans using a6MV beam. The time consuming of correctionis longer for the smaller correction area group. For instance,（16,16,6）is more than2min,but（64,64,24）is just few seconds. Fortunately, all of the time consuming is acceptable.ConclusionWhen the smallest correction area was （16,16,6）,（32,32,12）and（64,64,24）, theperformance of CT number correction are perfect. The time consumings of all methods areacceptable. This method of CT value correction for CBCT image based on patient-specificregion segmental HU correction table is applicable to clinical use.