Thermoluminescence Dosimetry Of Kilo-voltage Cone Beam CT Based On Pediatric Anthropomorphic Phantom For Image Guided Radiotherapy

Acute lymphoblastic leukemia,central nervous system tuomors and lymphoma are the most common cancer types in children.Hodgkin lymphoma is highly sensitive to radiation.With the development of the technology,radiotherapy plays an important role in pediatric cancer treatment.Due to the small size and poor compliance,positioning accuracy is even more important for pediatric radiotherapy.Radiotherapy guided by kilo-voltage cone beam CT(kV-CBCT)can increase the positioning accuracy,hence may deliver higher dose to the tumor target,reduce the normal tissue complication probability and increase the radiation therapy gain ratio.Unlike the individualized precise therapeutic beams,the additional dose from the larger imaging fields to more normal organs and tissues has become an important public health problem.The carcinogenic risk associated with the diagnostic CT radiation dose has been concerned by the academic community and the public.However,the scan imaging frequency and dose per scan of CBCT in radiotherapy are higher than those of diagnostic CT.The cumulative radiation dose and risk can not be ignored.Due to the smaller size,children will receive a higher radiation dose than adults if scanned by the same protocol,and the differences will be further amplified by repeated imaging.Due to the higher radiosensitivity and the longer life expectancy,the risk of secondary cancer caused by low dose irradiation is much higher than the adults.Therefore,it is clinically desirable to understand the additional imaging radiation dose and the associated risk of this special group in radiotherapy.Firstly,the weighted CT dose index(CTDIw)was measured based on Varian kV-CBCT using different standard scan parameters.Using CTDIW as the standard,the inter-machine similarities and differences of the X-ray beam properties and scanning parameters were evaluated.Secondly,the absorbed dose to important organs were measured using thermoluminescence dosimeter(TLD)using the standard scan parameters based on a 5 years old pediatric anthropomorphic phantom.The effective dose and associated radiation risk were calculated thereafter.Dose measurement were performed using two methods.Firstly,a pelvic anthropomorphic phantom was irradiated by the same X-ray beam twice,and the dose to the central volume of interest was measured by a CT chamber and by TLDs respectively:yielding the calibrated factor for the TLDs.Secondly,the TLDs were sent to a secondary standard dosimetry laboratory to be calibrated the air kerma.The TLDs from the same batch were placed between two tissue-equivalent inserts and placed into the pre-drilled organ cavities of the pediatric phantom.Using three standard protocols,the absorbed dose to 20 organs were measured according to the two coefficients and the corresponding method,based on which the corresponding effective dose and radiation risk were calculated.The results of this study showed that:(1)The CBCT tube voltage can differ slightly from the nominal tube voltage,but the dosimetric stability is acceptable.(2)Using standard head,low-dose thorax,and pelvis protocols,the whole body effective dose which the TLD conversion coefficient calibrated by the CT chamber was 0.63 mSv,6.85 mSv,and 19.3 mSv respectively,suggesting 210,1710,4830 secondary cancer indicence per 100,000 population because of 33,25,25 scans.Using standard head,low-dose thorax,and pelvis protocols,the whole body effective dose which the TLD conversion coefficient calibrated by the secondary standard dosimetry laboratory was 0.55 mSv,6.36 mSv,and 18.53 mSv respectively,suggesting 180,1590,4630 secondary cancer indicence per 100,000 population because of 33,25,25 scans.This work demonstrated that the effective dose of pelvic protocol was higher than that of the thorax and head protocols,suggesting a higher risk of radiation damage and secondary cancer.The imaging dose and carcinogenesis risk of kV-CBCT for pediatric patients are considerable.The special conditions of children should be considered collectively during the treatment planning,where appropriate imaging modality and frequency should be selected to acquire accurate positioning at cost of minimum radiation risk.