A Ready Reference for Estimating Dose to Pelvic Node Metastases from High Dose Rate Brachytherapy (HDR-BT) in Cervical Cancer

INTRODUCTION: Metastasis to lymph nodes is one of the best predictive indicators of recurrence and death for cervical cancer patients. A dose of greater than 55 Gy is recommended for nodes 2 cm or less for durable control. It is expected that nodes closer to the radiation source will receive a higher dose. In this study we explored the relationship between lymph node location and the Point A dose as a means of assisting the radiation oncologist to determine the required boost dose. METHODS: This retrospective study from 2009 through 2013 included 29 cervical cancer patients receiving high dose rate brachytherapy and had a total of 60 metastases to pelvic lymph nodes. The lymph nodes were mapped and contoured in the treatment planning system.. The external beam and brachytherapy doses and percentage of brachytherapy point A dose received by each lymph node were calculated. RESULTS: The median doses from brachytherapy and external beam radiation to the lymph nodes were 5.5 Gy (range, 1.4-12.4 Gy) and 49.6 Gy (range, 42.5-62.9 Gy), respectively. The median total dose for all lymph nodes was 56.5 Gy (range, 46.5-66.3 Gy). The dose from brachytherapy accounted for 9.97% of the total dose to the lymph node. The location of the lymph node affected the dose received. The common iliac nodes, which were furthest from the uterus, received 3.18 Gy (11.09 % of point A); the internal iliac nodes received 4.29 Gy (16.43% of point A); and the external iliac nodes, which were closest to the uterus, received 6.05 Gy (21.75% of point A). As expected, nodes closer to the uterus received higher doses of brachytherapy radiation. DISCUSSION AND CONCLUSION: The common iliac nodes received the smallest fraction of the brachytherapy dose and thus need the greatest external boost dose. Also the internal iliac lymph nodes above the sacroiliac joint, the external iliac lymph nodes anterior to the acetabular line, and the external iliac nodes lateral to the external iliac artery received a smaller amount of the brachytherapy dose than the rest of the nodes in their group. This will require them to have a higher boost dose from external beam compared to other nodes in their group. The results of this study provide radiation oncologists a reference for determining which nodes require an external beam boost dose and the optimal boost dose for those nodes. Improved optimization of the boost dose should lead to better local control and outcomes for patients.