# The effect of incidental dose to pelvic nodes in bladder-only irradiation in the era of IMRT: a dosimetric study

## TL;DR

This study examines the incidental radiation doses to pelvic lymph nodes in bladder cancer patients treated with IMRT, finding that doses vary but remain below levels needed to eradicate microscopic disease.

## Contribution

The study provides a detailed dosimetric analysis of pelvic lymph node exposure during bladder-only IMRT in the modern treatment era.

## Key findings

- Incidental nodal doses with bladder-only IMRT are heterogeneous across different lymphatic regions.
- Doses remain below generally accepted thresholds for microscopic disease eradication in bladder cancer.
- Obturator lymphatics received the highest mean dose, while distal common iliac lymphatics received the lowest.

## Abstract

While three-dimensional radiotherapy (RT) causes high incidental nodal doses in bladder-only irradiation for muscle-invasive bladder cancer (MIBC), the impact on pelvic lymphatics is unclear in the era of intensity-modulated RT (IMRT). This study evaluates incidental doses to pelvic lymphatics in MIBC patients treated with IMRT.

The data of 40 MIBC patients treated with bladder-only IMRT and concurrent chemotherapy were retrospectively evaluated. The pelvic lymphatics were contoured on initial simulation images and incidental nodal doses were evaluated. The Statistical Package for the Social Sciences (SPSS) version 23.0 (IBM, Armonk, NY, USA) was used for statistics.

Median RT dose to the bladder was 60 Gy in 30 fractions. In dosimetric analysis, median values of mean dose (Dmean) of the obturator, presacral, external iliac, internal iliac, and distal common iliac lymphatics were 33 Gy (range 4–50 Gy), 3 Gy (range 1–28 Gy), 9.5 Gy (range 3–41 Gy), 7.5 Gy (range 2–14 Gy), and 1 Gy (range 0–15 Gy), respectively. The Dmean of the obturator lymphatics was significantly higher (p < 0.001) and the Dmean of the distal common iliac lymphatics was significantly lower (p < 0.001) than all remaining lymphatic stations. The Dmean of the external iliac lymphatics was significantly higher than that of the presacral lymphatics (p < 0.001), but the difference with the internal iliac lymphatics was not statistically significant (p = 0.563).

The incidental nodal doses with bladder-only IMRT are heterogeneous and remain below the generally accepted doses for microscopic disease eradication for bladder cancer.