Volumetric Image Based Dosimetric Analysis of Organ at Risk in Intracavitary Brachytherapy for the Treatment of Carcinoma of Cervix

Radiation therapy is the main stay for the treatment of the cervical cancer. Normal organs such as bladder, rectum, sigmoid colon and bowel loops also get significant dose during treatment of carcinoma of cervix which often results late toxicity. The purpose of this study is to analyze the classical point based dosimetry and CT image based volumetric dosimetry and correlate the doses received by the organ to the toxicity profile of patients.

Sixty high dose rate intracavitary brachytherapy applications were performed for the treatment of carcinoma of cervix. The CT based treatment planning was done for every application. Point doses were measured acm3ording to classical method of dosimetry of ICRU-38 and volumetric dose parameters were computed from dose volume histogram of CT guided planning and correlated with the toxicity profile of the patients.

The average volume of rectum, sigmoid colon and bowel loops were found to be 60.34 cm³, 22.19 cm³ and 270.82 cm³. The average, median and 2 cm³ volume doses for rectum 289 ± 121 cGy, 263 ± 113 cGy and 884 ± 444cGy and for sigmoid colon 409± 211 cGy, 366 ± 185cGy and 693 ± 371 cGy resp. and for bowel loops 240 ± 169 cGy, 153 ± 59 cGy and 870 ± 222 cGy. The average and median sigmoid colon point doses were higher than rectum average doses (p= 0.000) and median doses (p =0.001) but 2 cm³ volumetric doses of sigmoid colon are less than rectum 2 cm³ volumetric doses (p= 0.013). For bowel loops the 2 cm³ volumetric doses were much higher than average doses (p= 0.000) due to its large volume. The recto-sigmoidal toxicity profile were evaluated for sigmoidal max doses and rectum 2 cm³ volumetric dose profile. There is a poor correlation between rectum 2 cm³ volumetric dose and sigmoid 2 cm³ volumetric doses.

Three dimensional CT scan image based dosimetry facilitates the evaluation of dose of bowel loop and sigmoid colon and 2D radiograph based point dosimetry cannot provide this critical information as often these organs may be in high dose gradient. Acm3ording to dose toxicity profile, sigmoidal dose represent an important role for dose constrains but till now no standard protocol has been formed for reporting the sigmoidal doses. This study attract the attention for reporting the sigmoidal and bowl loop doses. This study demonstrates the possibility and role of volumetric imaging and dosimetry for improvement in dose constraints and eventual reduction in toxicity.