Evaluation of the glottic surface dose in three-dimensional conformal radiotherapy for early-stage glottic cancer using a treatment planning system.

BACKGROUND

In radiotherapy for early-stage glottic cancer, evaluating the target surface dose at the glottic air-tissue boundary is crucial, as buildup effect can cause underdosing. The accuracy of dose evaluation in the surrounding tissues is affected by both the dose calculation algorithms and the accuracy of the Hounsfield unit values in the glottic air cavities.

PURPOSE: The objective of this study is to investigate the impact of dose calculation algorithms and material override on glottic surface dose calculations in three-dimensional conformal radiotherapy (3DCRT) for glottic cancer.

METHODS

We retrospectively included three patients with early-stage glottic cancer treated with 3DCRT. Treatment planning based on the collapsed cone convolution (CCC) algorithm in the treatment planning system with a 1-mm dose grid was conducted using a prescribed dose of 65 Gy in 26 fractions. The contours of the glottic air cavities and the surrounding glottic tissues were delineated for material override to air and water, respectively to assign correct materials in dose calculation. Each treatment plan was initially calculated using CCC without material override (CCC_w/o) and recalculated using CCC with material override (CCC_w) as well as photon Monte Carlo (pMC) algorithm with and without material override (pMC_w and pMC_w/o, respectively). A 1-mm glottic surface dose (D99%) was evaluated using CCC_w/o, CCC_w, pMC_w, and pMC_w/o.

RESULTS

PMC_w predicted a ∼13.0% reduction in the glottic surface dose compared with the prescribed dose. CCC_w/o, CCC_w, and pMC_w/o overestimated the dose by ∼10.0% compared with pMC_w. The difference between CCC_w/o and pMC_w/o was minimal (0.6%); pMC_w/o significantly overestimated (by 10.8%) the dose compared with pMC_w, indicating the significant impact of material override in pMC.

CONCLUSION

Monte Carlo dose calculations with material override are essential for the accurate surface dose calculation in 3DCRT for glottic cancer. Without appropriate material override, both CCC and pMC overestimate the surface dose.