Evaluation of the dosimetric and radiobiological parameters in four radiotherapy regimens for synchronous bilateral breast cancer

This study is to investigate the optimal treatment option for synchronous bilateral breast cancer (SBBC) by comparing dosimetric and radiobiological parameters of intensity‐modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) plans using single and dual isocenters.

Twenty patients with SBBC without lymph node involvement were selected retrospectively. Four treatment plans were generated for each patient using the Eclipse treatment planning system (Varian Medical System, Palo Alto, CA, USA) following two delivery techniques with two isocenter conditions—IMRT using a single isocenter (IMRT_Iso1), VMAT using a single isocenter (VMAT_Iso1), IMRT using dual isocenters (IMRT_Iso2), and VMAT using dual isocenters (VMAT_Iso2). A dose of 42.56 Gy in 16 fractions was prescribed for the planning target volume (PTV). All plans were calculated using the Acuros XB algorithm and a photon optimizer for a 6‐MV beam of a Vital Beam linear accelerator. PTV‐related dosimetric parameters were analyzed. Further, the homogeneity index, conformity index, and conformation number were computed to evaluate plan quality. Dosimetric parameters were also measured for the organs at risk (OARs). In addition, the equivalent uniform dose corresponding to an equivalent dose related to a reference of 2 Gy per fraction, the tumor control probability, and the normal tissue complication probability were calculated based on the dose–volume histogram to investigate the radiobiological impact on PTV and OARs.

IMRT_Iso1 exhibited similar target coverage and a certain degree of dosimetric improvement in OAR sparing compared to the other techniques. It also exhibited some radiobiological improvement, albeit insignificant. Although IMRT_Iso1 significantly increased monitor unit compared to VMAT_Iso1, which is the best option in terms of delivery efficiency, there was only a 22% increase in delivery time. Therefore, in conclusion, IMRT_Iso1, the complete treatment of which can be completed using a single setup, is the most effective method for treating SBBC.

Stereotactic body radiotherapy optimization to reduce the risk of carotid blowout syndrome using normal tissue complication probability objectives

Purpose

To determine the possibility of further improving clinical stereotactic body radiotherapy (SBRT) plans using normal tissue complication probability (NTCP) objectives in order to minimize the risk for carotid blowout syndrome (CBOS).

Methods

10 patients with inoperable locally recurrent head and neck cancer, who underwent SBRT using CyberKnife were analyzed. For each patient, three treatment plans were examined: (1) cone‐based without delineation of the ipsilateral internal carotid (clinical plan used to treat the patients); (2) cone‐based with the carotid retrospectively delineated and spared; and (3) Iris‐based with carotid sparing. The dose–volume histograms of the target and primary organs at risk were calculated. The three sets of plans were compared based on dosimetric and TCP/NTCP (tumor control and normal tissue complication probabilities) metrics. For the NTCP values of carotid, the relative seriality model was used with the following parameters: D₅₀ = 40 Gy, γ = 0.75, and s = 1.0.

Results

Across the 10 patient plans, the average TCP did not significantly change when the plans were re‐optimized to spare the carotid. The estimated risk of CBOS was significantly decreased in the re‐optimized plans, by 14.9% ± 7.4% for the cone‐based plans and 17.7% ± 7.1% for the iris‐based plans (p = 0.002 for both). The iris‐based plans had significant (p = 0.02) reduced CBOS risk and delivery time (20.1% ± 7.4% time reduction, p = 0.002) compared to the cone‐based plans.

Conclusion

A significant improvement in the quality of the clinical plans could be achieved through the delineation of the internal carotids and the use of more modern treatment delivery modalities. In this way, for the same target coverage, a significant reduction in the risk of CBOS could be achieved. The range of risk reduction varied depending on the proximity of carotid artery to the target.

Radiobiological Comparison of Teardrop Technique for Breast Cancer Radiotherapy Treatment Planning on a Tomotherapy System

Breast cancer is one of the most common cancer worldwide with large morbidity. In Mexico, it is the first cause of death by cancer in women. Radiotherapy has proven to be a great tool to control such ailments and TomoTherapy is a relatively new technology to accomplish it. To obtain good clinical outcomes, tight dosimetric constraints are placed on organs at risk (OARs) to maximize tumor control and minimize normal tissue complication probabilities. The teardrop technique helps meeting these constraints by placing a virtual block over parts of the ipsilateral lung and the heart but it contributes to lengthen the treatment time. In this work, we present our experience in using this technique and compare its radiobiological estimations with similar plans without it. Ten patients diagnosed with breast cancer were planned twice, with and without the teardrop technique. Dose-volume histograms were obtained and analyzed to get uncomplicated tumor control probability (UTCP) and optimization estimator (fEUD) parameters. Classical dosimetrical parameters for planning target volumes (PTVs): conformity index, homogeneity index, and coverage were also recorded and statistically described. Several dosimetrical parameters for OARs were recorded and analyzed. The UTCP parameter had a mean value of 0.968 ± 0.023 when no block was used and 0.966 ± 0.022 with the teardrop. The fEUD parameter values were: 0.515 ± 0.049 without blocks and 0.541 ± 0.057 with the teardrop. Optimization of every plan was stopped only after all constraints were met, and it was easier to accomplish this goal with the teardrop technique. The teardrop technique permitted a 5% gain in fEUD. The teardrop technique was observed to have a net radiobiological benefit with little impact on patient scheduling.