Field-Size Dependent Practical Limits of IBA CC01 and CC04 Chambers in Small-Field Dosimetry on Elekta Versa HD

Abstract

Author(s): Ouafae CHFIK, EL Mahjoub CHAKIR, Mohamed OUALI ALAMI, Khalid EL OUARDY, Mohamed elghalmi, Yassine HERRASSI, Khalid NABAOUI

To establish field-size dependent practical limits for the IBA CC01 and CC04 ionization chambers in small-field dosimetry on an Elekta Versa HD linear accelerator. The study was based on an experimental dataset acquired in a water phantom and compared with Monaco Golden Beam Data (GBD) for 6 MV FFF and 10 MV FFF photon beams. Square fields ranging from 1×1 to 5×5 cm² were investigated at SSD = 90 cm, using the 10×10 cm² field as the machine-specific reference. The evaluated dosimetric parameters included percentage depth dose, lateral beam profiles, point-dose response at 10 cm depth, gamma analysis, and corrected TPR20/10. Chamber usability was assessed in terms of practical clinical acceptability across the investigated field-size range. For both energies, measured dose per monitor unit increased progressively with field size, from 0.693 cGy/MU at 1×1 cm² to 1.000 cGy/MU at 10×10 cm² for 6 MV FFF, and from 0.664 to 1.000 cGy/MU for 10 MV FFF. Agreement with GBD remained strong, with point-dose differences ranging from −1.95% to +0.11% for 6 MV FFF and from −1.54% to +0.16% for 10 MV FFF. Minimum gamma pass rates at 2%/2 mm were between 99.2% and 99.6%. Corrected TPR20/10 values increased with field size and were systematically higher for 10 MV FFF than for 6 MV FFF. The combined dosimetric analysis indicated that detector-related limitations were most pronounced in the 1×1 and 2×2 cm² fields, where volume averaging and positioning sensitivity become more important. CC01 is the preferred chamber for the smallest investigated fields, particularly for output-factor and profile measurements. CC04 becomes clinically reliable from about 3×3 cm² upward, where detector-related effects are less pronounced. This work provides a clinically useful framework for detector selection during commissioning and quality assurance of stereotactic small fields on Elekta Versa HD.

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