# Comparison of dose calculation approaches and clinical dose–response in image-guided head&neck brachytherapy of the oral cavity

## TL;DR

This study compares different dose calculation methods in head and neck brachytherapy and finds that model-based methods better predict toxicity risks than traditional methods.

## Contribution

The study identifies specific dose thresholds linked to toxicity using model-based dose calculations in oral cavity brachytherapy.

## Key findings

- Osteoradionecrosis is correlated with bone dose ≥ 59.3 Gy using model-based calculations.
- Soft tissue necrosis is linked to tissue dose ≥ 87.7 Gy with model-based methods.
- Larger target volumes are associated with higher rates of soft tissue necrosis and mucositis.

## Abstract

•Osteoradionecrosis is correlated to bone D2ccm ≥ 59.3 Gy.•Soft tissue necrosis is correlated to tissue D5ccm ≥ 87.7 Gy.•Target volumes ≥ 10.2–11.8ccm are associated with increased rates of soft tissue necrosis and mucosits.•Dose thresholds for toxicity differed between TG-43 and model-based dose calculation.•Model-based dose calculation differed partly substantially from TG-43 dose calculations.

Osteoradionecrosis is correlated to bone D2ccm ≥ 59.3 Gy.

Soft tissue necrosis is correlated to tissue D5ccm ≥ 87.7 Gy.

Target volumes ≥ 10.2–11.8ccm are associated with increased rates of soft tissue necrosis and mucosits.

Dose thresholds for toxicity differed between TG-43 and model-based dose calculation.

Model-based dose calculation differed partly substantially from TG-43 dose calculations.

Model-based dose calculation algorithms (MBDCAs) are increasingly applied in brachytherapy, but their considerations in dose–response analyses is still lacking. This study aimed to assess correlations between both TG-43 and MBDCA dosimetry and reports on clinical outcomes for oral cavity brachytherapy.

We considered 158 patients with oral cavity cancer treated in our institution between 2012 and 2021. Survival outcomes and toxicity (soft tissue necrosis, osteoradionecrosis, mucositis, xerostomia) were reported for a median follow-up of 80 months (2–152 months). All clinical, TG-43 based treatment plans were re-calculated using a MBDCA integrated into our planning system. Differences considering several target volume, tissue, and bone dose parameters were evaluated. Parameter correlations with clinical outcomes and thresholds associated with increased toxicity were investigated.

Cumulative local recurrence, soft-tissue necrosis, osteoradionecrosis, mucositis, and xerostomia rate were 21 %, 22 %, 28 % and 79 % considering all patients. Substantial differences between MBDCA and TG-43 were observed, especially regarding high-dose areas with changes up to 19 %. A number of dose-toxicity correlations were observed, as for osteoradionecrosis (1.6 % vs. 10.3 % for bone D2ccm ≥ 59.3 Gy), soft tissue necrosis (16 % vs. 32 % for tissue D5ccm ≥ 87.7 Gy), and local recurrence (13 % vs. 25 % for dose non-uniformity ratio < 0.29) (using the MBDCA). Target volumes ≥ 10.2–11.8ccm were associated with increased rates of soft tissue necrosis and mucositis. Dosimetric thresholds and correlations differed between TG-43 and MBDCA.

For oral cavity brachytherapy, several important dosimetric thresholds associated with increased toxicity were determined. MBDCAs for corresponding dose calculations should be advanced and used in clinical practice.