ESTRO 2024 - Abstract Book

S1830

Clinical - Mixed sites, palliation

ESTRO 2024

64

Digital Poster

Dosimetric Comparison of Monte Carlo and FSPB Algorithm for MLC-Cyberknife Lung and Brain Cases

Wing Sze WONG, Tin Lok CHIU, Siu Ki YU

Hong Kong Sanatorium & Hospital, Medical Physics Department, Hong Kong, Hong Kong

Purpose/Objective:

Multileaf collimator (MLC) technology was introduced to the CyberKnife system for clinical use in 2015 with only finite size pencil beam (FSPB) algorithm for the dose calculation. Monte Carlo (MC) algorithm was clinically available three years later. It is believed that the MC algorithm provides a more accurate dose calculation than FSPB by addressing the tissue heterogeneity effects, especially for lung tumors. Most research focuses on comparing these two algorithms by the dosimetric parameters of the planning target volume (PTV) (e.g. target dose coverage, maximum target dose, and mean target dose). Limited study has been done on investigating the effects on low dose regions and dose falloff outside the PTV, which are also critical for the oncologist to accept the treatment plans. Faster dose falloff can reduce normal tissue toxicity and spare organs at risk nearby. In this study, the plan quality metrics, dose falloff metrics and dose-volume constraints computed by MC and FSPB algorithms were compared. Twelve MLC-equipped CyberKnife SBRT cases with a single lung lesion and ten SRS cases with multiple brain lesions were computed with FSPB and MC algorithms. All plans computed with the MC algorithm were set with 1% uncertainty and the same monitor units with their corresponding FSPB plans. In part (a), the plan quality metrics were investigated in terms of maximum target dose (D max ), mean target dose (D mean ), and minimum dose delivered to 95% of the PTV (D 95% ) for lung cases. Conformity index (CI), homogeneity index (HI) and target coverage were also studied. In part (b), dose falloff outside the PTV was compared. Gradient index (GI) and Radiation Therapy Oncology Group (RTOG) quality matrix R 50% (ratio of volume receiving 50% of the prescription dose to PTV volume), R 10% (ratio of volume receiving 10% of the prescription dose to PTV volume) and D max,2cm (maximum dose at 2 cm from PTV in any direction) were studied for lung cases. In part (c), dose-volume constraints were evaluated with lung volume receiving 20Gy (V 20Gy ), mean lung dose (D mean, lung ) and dose-volume constraints for lungs listed in the American Association of Physicists in Medicine Task Group 101 for lung cases. For the brain cases, the mean brain dose (D mean, brain ) computed by MC and FSPB algorithms were compared. Wilcoxon Signed-Rank Test with a significance level of 0.05 was performed to evaluate the significant differences for all the above comparisons. Material/Methods:

Results: