ESTRO 2025 - Abstract Book

S4058

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

References: Mariados N et al, Hydrogel Spacer Prospective Multicenter Randomized Controlled Pivotal Trial: Dosimetric and Clinical Effects of Perirectal Spacer Application in Men Undergoing Prostate Image Guided Intensity Modulated Radiation Therapy. Int J Radiat Oncol Biol Phys. 2015 Aug 1;92(5):971-977. doi: 10.1016/j.ijrobp.2015.04.030. Epub 2015 Apr 23. PMID: 26054865.

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Poster Discussion Dosimetric impact of positioning accuracy on vulva surface-guided radiotherapy patients hao liang radiotherapy, peking union medical college hospita, Beijing, China

Purpose/Objective: Vulvar cancer is a relatively rare tumor. Frogged-leg is the common treatment position for vulvar radiation therapy. The treatment accuracy and reproducibility are challenging as it is a superficial lesion and highly mobile. This study is to explore the target coverage of frogged-leg vulvar radiotherapy patients using surface guided radiation therapy (SGRT). Material/Methods: 16 vulvar radiotherapy patients who were positioned using lasers (Group 1 – 8 patients) and SGRT (Group 2 – 8 patients) were selected retrospectively. All these patients were treated using CT-based linear accelerator with daily fan-beam CT (FBCT). PTV and CTV contours were copied to FBCT images after the image rigid registration was done. Auto-contouring was applied on rectum, bladder and small bowel and verified by the oncologist. Lastly the original CT plan was copied to the non-shifted FBCT images for PTV D 95 , CTV D 100 , PTV V 45 and CTV V 45 dose calculations. A total of 400 FBCT images were analyzed using t-test and Mann-Whiteney U test.

Results: The positioning errors for Group 1 and 2 were (0.12±0.37)cm, (0.17±0.45)cm, (-0.21±0.34)cm,

(0.39±1.41)°,(0.22±0.67)°, (0.73±1.28)°and (0.06±0.25)cm, (0.11±0.28)cm, (0.07±0.31)cm, (0.29±1.1)°, (0.31±0.54)°, (- 0.04±0.61)°in lateral, longitudinal, vertical, roll, yaw and pitch directions. P<0.05 in vertical, roll and pitch directions. The median and the interquartile range (IQR) of the target coverage were tabulated below.

Group 1

Group 2

Index

p-value

median IQR

Median

IQR

PTV D 95

96.75% 95.71%-98.22%

98.43%

97.52%-98.87%

<0.05

CTV D 100

98.91% 97.74%-99.32%

99.84%

98.87%-99.98%

<0.05

PTV V 45

98.93% 97.85%-99.10%

99.45%

99.03%-99.79%

<0.05

CTV V 45

99.67% 99.23%-99.75%

99.89%

99.23%-100.00%

<0.05

Conclusion: Surface guided radiotherapy is beneficial to vulvar frogged-leg patients especially in rotational directions. The target coverage using SGRT positioning is superior to traditional laser-based positioning. Daily FBCT could be potentially reduced with the implementation of SGRT in daily positioning.

Keywords: vulvar cancer ， Dosimetric ， surface-guided