ESTRO meets Asia 2024 - Abstract Book

S324

Physics – Motion management and adaptive radiotherapy

ESTRO meets Asia 2024

40

Digital Poster

Evaluation of set-up errors using CBCT in Head and Neck cancer patients treated with IMRT in UMMC

Rangasamy Ramachandran 1 , Nur Fadhlina Abdul Satar 2 , Zulaikha Jamalludin 3 , Wei Loong Jong 4 , Jasmin Loh Pei Yuin 5 1 Radiotherapy & Oncology, Hospital Sultan Ismail, Johor Bahru, Malaysia. 2 Radiotherapy & Oncology, University Malaya Medical centre, Kuala Lumpur, Malaysia. 3 Radiotherapy & Oncology, University Malaya Medical Centre, Kuala Lumpur, Malaysia. 4 Radiotherapy & Oncology, Icon Cancer Centre, Singapore, Singapore. 5 Radiotherapy & Oncology, Beacon Hospital, Kuala Lumpur, Malaysia

Purpose/Objective:

Background: Intensity modulated radiation therapy (IMRT) is a primary modality of advanced radiotherapy treatment in head and neck (H&N) cancers, which delivers a highly conformal radiation dose to the target, sparing organs at risk (OARs). Due to the steep dose gradient produced by IMRT, set up errors increases the risk of marginal miss of the tumour or unnecessary irradiation of normal tissue. Image guided radiotherapy (IGRT) is introduced to detect and measure this error. This study analyses set up errors in the H&N IMRT cancer patients in UMMC using cone beam computed tomography (CBCT) and suggest planning target volume (PTV) safety margins.

Material/Methods:

320 pre-treatment CBCT images of 40 H&N cancer patients treated with IMRT were analyzed retrospectively. Set up errors of each of the 8 CBCT images of the 40 patients were collected to evaluate population systematic and random errors. In addition, frequency of displacements and displacements of more than 3mm based on weeks of treatments were analyzed separately. Adequate PTV margins were calculated for the entire population cohort of patients using the Van Herk formula (2.5∑ + 0.7r).

Results:

The population systematic errors (∑) in vertical, longitudinal and lateral direction was 1.0, 0.8 and 0.9 mm, respectively. The population random error (σ) for the corresponding direction was 1.8,1.4 and 1.3 mm, respectively. The margin from CTV to PTV should be at least 3.8 mm in the vertical, 3.0 mm in the longitudinal and 3.2 mm in the lateral direction. Displacements larger than 3 mm were the highest recorded in the first and last three weeks of RT treatments. The PTV margin recommended for nasopharynx subsite in this study is 3.2-3.4 mm and the non-nasopharynx cohort is 3.2-4.4 mm

Conclusion:

In summary, the population systematic and random errors were less than 2 mm, and the calculated PTV margin is within 5 mm in the vertical, longitudinal, and lateral direction for the H&N cancer patients in UMMC. Variation of displacements of more than 3 mm was more significant in the first and last week of RT treatment. The calculated PTV margin of the non-nasopharynx was larger than the nasopharynx cohort based on our local H&N cancer population group of patients. Both cohorts recorded a PTV margin of within 5 mm, respectively.