Radioprotection No 59-2 | Page 68

S . Semghouli et al .: Radioprotection 2024 , 59 ( 2 ), 123 – 130 127 a b Radiotherapy planning CT ( CTDIvol )

Radiotherapy planning CT ( DLP )

Fig . 1 . DRL in terms of ( a ) CTDIvol and ( b ) DLP calculated by this study for radiotherapy planning CT compared to other previous studies .

embolism protocol and 150 mAs for other protocols . The pitch is constant in radiotherapy planning CT , and varies according to the protocol used in diagnostic CT . In reality , the dose has an inverse relationship with the pitch , and doubling the milliampere-seconds ( mAs ) will result in a 100 % increase in dosage . It is therefore advisable to choose the highest pitch value and lowest mAs that correspond to the clinical diagnosis required ( IAEA , 2012 ). However , an understanding of mAs and pitch is not sufficient to determine X-ray tube power in computed tomography ( CT ), as it also relies on factors such as X-ray tube characteristics , X-ray tube voltage , and beam filtering . In contrast , CTDIvol values , measured in a standardised phantom size and with a quality X-ray beam , offer a universal measure to quantify the amount of radiation incident on any patient undergoing any type of examination ( Tipnis , 2016 ).

4.3 Estimation of delivered doses :

For the radiotherapy planning CT scan , the DRL calculated in terms of CTDI vol is 19.37 mGy , which is slightly higher than the DRLs of Slovenia ( 19.2 mGy ) ( Zalokar , 2020 ), and lower than that of Croatia ( 17 mGy ) ( Božanić , 2022 ). The DRL calculated in terms of DLP is 851.9 mGy cm , which is higher than the DRLs of Slovenia ( 832.4 mGy cm ), and lower than that of Croatia ( 865 mGy cm ).

For thoracic diagnostic CT scan , the “ pulmonary embolism ” indication recorded the highest DRL in terms of CTDI vol ( 11.13 mGy ), while the " COPD " indication recorded the lowest DRL value ( 7.37 mGy ). In terms of DLP , the “ Infectious lung disease ” indication recorded the highest DRL value ( 451.9 mGy cm ), and the “ COPD ” indication recorded the lowest DRL value ( 317.78 mGy cm ). These variations in terms of reported dosimetric values would be due to some protocols used for some clinical indications which were not specifically adapted for that clinical indication Public Ukoha et al . ( 2023 ). In addition , DRL in terms of CTDI vol which was reported for pulmonary embolism ( 11.13 mGy ) is less than the DRL reported by Kanal et al . ( 2017 ) ( 19 mGy ), and Foley et al . ( 2012 ) ( 13 mGy ), and higher than that reported by Geryes et al . ( 2019 ) ( 8 mGy ). DRL in terms of DLP calculated for pulmonary embolism ( 417.73 mGy cm ) was less than that reported by Kanal et al . ( 2017 ) ( 557 mGy cm ), and Foley et al . ( 2012 ) ( 432 mGy cm ), and higher than that reported by Geryes et al . ( 2019 ) ( 310 mGy cm ).

In this context , DRL in terms of CTDIvol for infectious lung disease ( 10.26 mGy ) was lower than that reported by the Public Health of England ( 2016 ) ; 12 mGy ) and Salama et al . ( 2017 ( 22 mGy ), and the Danish Health Authority ( 2015 ) ( 13 mGy ). DRL in terms of DLP for this indication ( 451.9 mGy cm ) was higher than that reported by the Public Health of England ( 2016 ) ( 350 mGy cm ) and Salama et al . ( 2017 ) ( 421 mGy cm ), and lower than that calculated by the Danish Health Authority ( 2015 ) ( 500 mGy cm ).

The DRLs of this study for the COPD indications in terms of CTDIvol and DLP were 7.37 and 317.78 mGy cm , respectively . These values are higher than those reported by Geryes et al . ( 2019 ), which were 4.1 mGy and 185 mGy . cm , respectively .

The results of this study show that the DRLs in terms of CTDI vol and DLP calculated in radiotherapy planning CTscans are higher than those calculated in diagnostic CT-scans . The high observed CTDI vol values in radiotherapy planning scans indicate that the employed protocols are not optimized according to the patient ’ s size ( McCollough et al ., 2013 ). Moreover , the magnitude of radiation exposure is contingent upon the dimensions of the patient , exhibiting a substantial association between patient size and absorbed radiation dose , a relationship that can also be observed in thoracic CT scans ( Menke , 2005 ). In fact , a large number of factors can affect the dose and image quality in CT examinations such as radiographic protocol or scan parameter , equipment , image reading condition and patient related factors which can usually be controlled through adjustments in the scan parameters ( IAEA , 2012 ). Figures 1 – 3 show the DRLs in terms of DLP and CTDIvol for radiotherapy planning CT and diagnostic CT compared with other previous studies .

The results show also that the mean effective dose in radiotherapy planning CT-scans is 13 mSv , which is almost three times lower than the effective dose calculated by