To evaluate the inter-fraction variation in interstitial high-dose-rate (HDR) brachytherapy. To assess the positional displacement of catheters during the fractions and the resultant impact on dosimetry.

Although brachytherapy continues to be a key cornerstone of cancer care, it is clear that treatment innovations are needed to build on this success and ensure that brachytherapy continues to provide quality care for patients. The dosimetric advantages offered by HDR brachytherapy to the tumour volume rely on catheter positions being accurately reproduced for all fractions of treatment.

A total of 66 patients treated over a period of 22 months were considered for this study. All the patients underwent computer tomography (CT) scan and three-dimensional treatment planning was carried out. Brachytherapy treatment was delivered by the HDR afterloading system. On completing the last fraction, CT scan was repeated and treatment re-planning was done. The variation in position of the implanted applicators and their impact on dosimetric parameters were analysed using both the plans.

For all breast-implant patients, the catheter displacement and D90 dose to clinical target volume were <3 mm and 3%, respectively. The displacement for carcinoma of the tongue, carcinoma of the buccal mucosa, carcinoma of the floor of mouth, carcinoma of the cervix, soft-tissue sarcoma and carcinoma of the lip were comparatively high.

Inter-fraction errors occur frequently in interstitial HDR brachytherapy. If no action is taken, it will result in a significant risk of geometrical miss and overdose to the organs at risk. It is not recommended to use a single plan to deliver all the fractions. Imaging is recommended before each fraction and decision on re-planning must be taken.