Dosimetric impact of switching from AAA to Acuros dose-to-water and dose-to-medium for RapidArc plans of nasopharyngeal carcinomas.

PURPOSE: This work aims to evaluate the dosimetric consequences of replacing the Anisotropic Analytical Algorithm (AAA) by Acuros XB (AXB), dose-to-water (Dw) or dose-to-medium (Dm), for RapidArc plans of nasopharyngeal carcinomas (NPC). MATERIALS AND METHODS: Seventeen NPC plans created with AAA (v15.6) were recalculated with AXB (v15.6) Dw and Dm. The dose-volume parameters to the planning target volumes (PTV) and relevant organs at risk (OAR) were compared. The high dose PTV was divided into bone, air and tissue components and the comparison was performed for each of them. RESULTS: AXB Dw revealed no significant differences in the PTVs compared to AAA. Lower values were observed to spinal cord, brainstem, oral cavity and parotids (0.5% to 2.3%), and higher values to cochleas (up to 5.4%) and mandible (up to 6.7%). AXB Dm predicted lower values than AAA for all PTVs and OARs (2.0% to 6.1%). For the bone PTV subvolume, AXB Dw and Dm predicted respectively higher (2.4%) and lower (2.2% to 3.4%) values. No significant differences were noted in air. AXB predicted lower values than AAA in soft tissues (0.4% to 1.6%). The largest difference was found to the mandible V60Gy parameter, with median differences of 6.7% for AXB Dw and -6.0% for AXB Dm. CONCLUSION: Significant dose differences are expected when switching from AAA to AXB in NPC cases. The dose prescriptions and the tolerance limits for some OARs, especially those of high density, may need to be adjusted depending on the selected dose calculation algorithm and reporting mode.

Collision prediction for intracranial stereotactic radiosurgery planning: An easy-to-implement analytical solution.

PURPOSE: Gantry collision is a concern in linac-based stereotactic radiosurgery (SRS). Without collision screening, the planner may compromise optimal planning, unnecessary re-planning delays can occur, and incomplete treatments may be delivered. To address these concerns, we developed a software for collision prediction based on simple machine measurements. MATERIALS AND METHODS: Three types of collision were identified; gantry-couch mount, gantry-couch and gantry-patient. Trigonometric formulas to calculate the distance from each potential point of collision to the gantry rotation axis were generated. For each point, collision occurs when that distance is greater than the gantry head to gantry rotational axis distance. The colliding arc for each point is calculated. A computer code incorporating these formulas was generated. The inputs required are the couch coordinates relative to the isocenter, the patient dimensions, and the presence or absence of a circular SRS collimator. The software outputs the collision-free gantry angles, and for each point, the shortest distance to the gantry or the colliding sector when collision is identified. The software was tested for accuracy on a TrueBEAM® machine equipped with BrainLab® accessories for 80 virtual isocenter-couch angle configurations with and without a circular collimator and a parallelepiped phantom. RESULTS: The software predicted the absence of collision for 19 configurations. The mean absolute error between the measured and predicted gantry angle of collision for the remaining 61 cases was 0.86 (0.01-2.49). CONCLUSION: This tool accurately predicted collisions for linac-based intracranial SRS and is easy to implement in any radiotherapy facility.

Technique alternatives for breast radiation oncology: Conventional radiation therapy to tomotherapy.

Breast conserving radiotherapy uses tangential fields and compensating wedges. This conventional approach can be improved by a field-in-field technique using the linac multi-leaf collimator (MLC). A simplified field-in-field technique that planners can easily achieve and which improves dose uniformity in the breast volume is presented here. Field junction problems are more easily solved by the use of a virtual simulation. A unique isocenter can be set at the junction between the supra-clavicular field and the breast tangential fields. However, careful quality assurance of the treatment planning system must be performed. Tomotherapy has promising clinical advantages: the ability of a tomographic image to correct for random set-up errors, a continuous cranio-caudal delivery which suppresses junction problems, the conformality of the dose distribution throughout the complex volumes formed by the lymph nodes and the breasts. Tomotherapy is a valuable recourse for complex irradiations like bilateral breast or mammary plus axillary irradiation while a field-in-field associated with a unique isocenter technique can be used for majority of the patients.