# Value of couch height-based positioning in postoperative adjuvant radiotherapy for left-sided breast cancer

## TL;DR

This study shows that using couch height for positioning in radiotherapy improves accuracy and reduces dose errors for left-sided breast cancer patients.

## Contribution

The study introduces couch height-based positioning as a novel method to reduce ventral-dorsal positioning errors in breast cancer radiotherapy.

## Key findings

- Couch height-based positioning reduced positioning errors compared to reference line positioning.
- Dorsal positioning errors over 3mm worsened dose distribution in PTV and OARs.
- Ventral positioning errors over 1.5mm significantly altered PTV dose distribution.

## Abstract

This study aimed to evaluate the value of positioning based on the treatment couch height in radiotherapy for left-sided breast cancer. Sixty patients who had undergone radical mastectomy for left-sided breast cancer were selected, with each patient undergoing positioning based on the treatment couch height (couch height group) and positioning based on the reference skin marking lines (reference line group), to measure corresponding positioning errors. Meanwhile, 20 of 60 patients were randomly selected, and the planning system was used to simulate the changes in radiation doses in planning target volume (PTV) and organs at risk (OARs) along with the changes in positioning errors in dorsal (increasing couch height) and ventral (decreasing couch height), respectively. Compared with the original plan, when the positioning error in the dorsal direction reached 3 mm, Dmean, V30 , and V20 in the ipsilateral lung were increased by 35.12%, 16.35%, and 10.6% respectively, and V50 in PTV was decreased by 0.99% (all p < 0.05); when the positioning error in the ventral direction reached 1.5 mm, V50, V48, and V45 were decreased by 2.07%, 0.58%, and 0.14% respectively. The homogeneity index (HI) was increased by 14.28% (all p < 0.05). There was a statistically significant difference in the positioning errors in the ventral-dorsal directions between the couch height group (0.16 ± 0.14 cm) and reference line group (0.36 ± 0.25 cm) (p < 0.05); the percentages of the absolute positioning errors within 1.5 mm and 3 mm were 52.4%, 88.7% respectively in the couch height group and 29.8%, 54.4% respectively in the reference line group, (all p < 0.05). Dorsal positioning errors greater than 3 mm significantly worsen the dose distribution for both the PTV and OAR, while positioning based on the treatment couch height keeps 88.7% of positioning errors within 3 mm; ventral positioning errors greater than 1.5 mm result in significant changes in the dose within the PTV. Compared to the reference line group, positioning based on the treatment couch height controls 52.4% of positioning errors within 1.5 mm. Therefore, couch height-based positioning demonstrates greater advantages in managing ventral-dorsal positioning errors. This study provides a reference for clinical positioning in postoperative adjuvant radiotherapy of breast cancer.