Study of association between OARs reference points doses and OARs 3D volume doses in 2D intracavitary brachytherapy for cervical cancer

Objective To investigate the relationship between organs at risk (OARs) reference point dose and three dimensional OARs volume dose of two dimensional (2D) brachytherapy for cervical cancer, and to study whether the OARs volume dose can meet the dose constraints of three dimensional (3D)brachytherapy under certain conditions. Methods A retrospective study was carried out on 10 patients with cervical cancer who were treated with CT image guided high dose rate (HDR) brachytherapy. The graphical optimized 2D plan was designed with the prescription dose was 600 cGy. Both the dose of bladder and rectum reference point were lower than 360 cGy. The relationship between the bladder or rectum reference point dose and OARs volume dose was analyzed by Pearson correlation analysis. Through regression analysis, the linear equations were given to describe the connection between the dose of bladder and rectum reference point and their volume dose. Results D1 cm3, D2 cm3, D5 cm3 and Dmean of bladder and rectum correlated positively with the dose of reference point (r=0.559-0.668, P<0.05). The dose of bladder and rectum reference point underestimated D2 cm3 with ratio of 1.404 and 1.181. Total equivalent dose at 200 cGy/fraction (EQD2) dose (external beam radiotherapy plus intracavitary brachytherapy) of bladder and rectum were 8 410.0 and 6 827.0 cGy, which were below the volume dose limit of them. Conclusions The bladder and rectum volume dose could be estimated to a certain extent by the dose of bladder and rectum reference point. If the bladder and rectum reference dose kept to less than 60% of prescription dose in 2D brachytherapy, the bladder and rectum volume dose could be controlled within relatively safe dose range. Due to the lack of dose detection of small intestine and sigmoid, the two dimensional brachytherapy has serious limitations in clinical application. Key words: Cervical cancer; 2D intracavitary brachytherapy; Bladder/rectum reference point; 3D volume dose