Dosimetric evaluation of beam block techniques in helical tomotherapy for extranodal nasal-type NK/T-cell lymphoma

The aim of the study was to evaluate the dosimetric effects of different virtual beam block combinations on target coverage, organ at risk (OAR) sparing, and delivery efficiency in helical tomotherapy (HT) for extranodal nasal-type NK/T-cell lymphoma (ENKTCL-NT). In addition, normal tissue complication probability (NTCP) modeling was used to assess the biological impact on OARs and to inform optimal treatment planning strategies. Twenty-six patients with stage I-II ENKTCL-NT were retrospectively analyzed. Three virtual beam block configurations for the lens and eyeball-B-OFF, B-LEN, and B-L&E-were generated. Dosimetric parameters of the target volume, physical and biological indices of OARs, monitor units (MUs), and beam-on time (BOT) were compared among the plans. Dose delivery accuracy was verified using the ArcCHECK phantom. All 3 plans achieved clinically acceptable target coverage without significant differences in target dosimetric parameters. Compared with B-OFF, B-LEN significantly reduced the maximum dose (D_max), mean dose (D_mean), equivalent uniform dose (EUD), and NTCP of the bilateral lenses (all p < 0.05) but did not significantly affect the eyeball dose (p > 0.05). B-L&E provided further protection: relative to B-LEN, it reduced the D_max of the left and right lenses by 3.4% and 5.9%, D_mean by 5.0% and 4.9%, and median NTCP by 10.3% and 21.4%, respectively (all p < 0.05). It also significantly lowered the D_max and D_mean of the eyeballs (p < 0.05). No significant differences were observed among the 3 plans for the brainstem, spinal cord, optic chiasm, or optic nerves (p > 0.05). Compared with B-OFF, B-L&E increased the MUs by 17.9% and BOT by 58 s (19.0%). Gamma passing rates (3%/2 mm) exceeded 90% for all the plans, meeting the American Association of Physicists in Medicine Task Group No. 218 (AAPM TG-218) clinical criteria. The virtual beam block technique in HT effectively reduces radiation exposure to the lens and eyeball while maintaining target coverage in ENKTCL-NT. The B-L&E configuration offers the greatest OAR protection, and B-LEN may be preferable when shorter treatment times are required.