dose-to-medium vs. dose-to-water: dosimetric evaluation of dose reporting modes in acuros xb for prostate, lung and breast cancer

Purpose: Acuros XB (AXB) dose calculation algorithm is available for external beam photon dose calculations in Eclipse treatment planning system (TPS). The AXB can report the absorbed dose in two modes: dose-to-water (D_w) and dose-to-medium (D_m). The main purpose of this study was to compare the dosimetric results of the AXB_D_m with that of AXB_D_w on real patient treatment plans.

Methods: Four groups of patients (prostate cancer, stereotactic body radiation therapy (SBRT) lung cancer, left breast cancer, and right breast cancer) were selected for this study, and each group consisted of 5 cases. The treatment plans of all cases were generated in the Eclipse TPS. For each case, treatment plans were computed using AXB_D_w and AXB_D_m for identical beam arrangements. Dosimetric evaluation was done by comparing various dosimetric parameters in the AXB_D_w plans with that of AXB_D_m plans for the corresponding patient case.

Results: For the prostate cancer, the mean planning target volume (PTV) dose in the AXB_D_w plans was higher by up to 1.0%, but the mean PTV dose was within ±0.3% for the SBRT lung cancer. The analysis of organs at risk (OAR) results in the prostate cancer showed that AXB_D_w plans consistently produced higher values for the bladder and femoral heads but not for the rectum. In the case of SBRT lung cancer, a clear trend was seen for the heart mean dose and spinal cord maximum dose, with AXB_D_w plans producing higher values than the AXB_D_m plans. However, the difference in the lung doses between the AXB_D_m and AXB_D_w plans did not always produce a clear trend, with difference ranged from -1.4% to 2.9%. For both the left and right breast cancer, the AXB_D_m plans produced higher maximum dose to the PTV for all cases. The evaluation of the maximum dose to the skin showed higher values in the AXB_D_m plans for all 5 left breast cancer cases, whereas only 2 cases had higher maximum dose to the skin in the AXB_D_m plans for the right breast cancer.

Conclusion: The preliminary dosimetric results from our clinical study showed that the selection of either D_m or D_w in AXB is less likely to produce significant dosimetric differences in the clinical environment. However, the difference between the AXB_D_m and AXB_D_w calculations depends on the disease site, and even for the same type of disease (e.g., lung cancer), the results are patient specific.