Measurement-based dosimetric comparison of kilovoltage and megavoltage X-ray systems for static-field 3D conformal radiotherapy in companion animals.

IMPORTANCE: Kilovoltage (kV) X-ray radiotherapy systems have been introduced as a potential treatment option for companion animal radiotherapy, particularly for superficial targets. On the other hand, the lack of experimental validation of dose delivery accuracy has limited their broader clinical adoption. OBJECTIVE: A measurement-based dosimetric comparison was made between kV and megavoltage (MV) X-ray systems. METHODS: A 300 kV X-ray radiotherapy system and a 6 MV linear accelerator were evaluated for percent depth dose, penetration depth, beam profiles, and field size output factors. Eight static-field plans were generated for targets at varying depths and tissue densities in an anthropomorphic head phantom. Point doses were measured with thermoluminescent dosimeters and compared with treatment planning system-calculated doses. The calculated-to-measured absolute percentage differences were compared between the kV and MV systems using the Wilcoxon signed-rank test with a bootstrap 95% confidence interval. RESULTS: Compared with the MV beam, the kV X-ray beam exhibited a negligible buildup distance, higher field-size dependence, a pronounced heel effect, and substantially lower penetration depth. Despite these dissimilarities, the absolute differences between the calculated and measured point doses were 0.1%-4.0% and 3.1%-6.5% for the kV and MV systems, respectively, across different target depths and tissue densities. The differences were significantly smaller for kV (median paired difference, -1.9 percentage points; 95% CI, -3.8 to -0.7; p = .0078). CONCLUSIONS AND RELEVANCE: Under the evaluated static-field phantom conditions, the commissioned kV treatment-planning model showed calculated-to-measured differences of 0.1%-4.0%. Although these values were lower than those for MV, this finding does not establish higher dosimetric accuracy for the kV system because measurement-related uncertainties may have contributed.