OBJECTIVE: A new dosimetric variable, dose-dropping speed (DDS), was proposed and used to evaluate normal tissue sparing among stereotactic radiosurgery (SRS) plans with different prescription isodose lines. METHODS: 40 plans were generated for 8 intracranial SRS cases, prescribing to isodose levels (IDLs) ranging from 50% to 90% in 10% increments. Whilst maintaining similar coverage and conformity, plans at different IDLs were evaluated in terms of normal tissue sparing using the proposed DDS. The DDS was defined as the greater decay coefficient in a double exponential decay fit of the dose drop-off outside the planning target volume (PTV), which models the steep portion of the drop-off. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. RESULTS: Among all plans, the DDS was found to be the lowest for the prescription at 90% IDL and the highest for the prescription at 60% or 70%. The beam profile slope change in the penumbra and its field size dependence were explored and given as the physical basis of the findings. CONCLUSION: A variable was proposed for SRS plan quality evaluation. Using this measure, prescriptions at 60% and 70% IDLs were found to provide best normal tissue sparing. ADVANCES IN KNOWLEDGE: A new variable was proposed based on which normal tissue sparing was quantitatively evaluated, comparing different prescription IDLs in SRS.
Brain Neoplasms/surgery , Radiosurgery/methods , Humans , Radiotherapy Dosage , Treatment Outcome
PURPOSE: TBI treatment delivery MU and patient dose estimation are calculated manually at our institution. This study was to verify the accuracyof MU calculation and dose estimation of bilateral TBI by application of tissue heterogeneity correction. METHODS: Twelve TBI patients were simulated from neck to thigh in bilateral TBI position. CT images were imported into the treatment planning system (Philips, Pinnacle3). Treatment dose was prescribed to the midpoint at the level of the umbilicus. Treatment distance was 519 cm. Both 6MV and 23 MV opposite lateral beams delivered 200 cGy to the dose prescription point with a 40 ×40 cm2 field size and 45o collimator angle. A 1 cm thick spoiler was placed about 15 cm from patient skin. Adaptive convolution superposition with and without heterogeneity correction was used for calculation of MUs and doses at the midpoints of the shoulder, chest, abdomen, and pelvis. RESULTS: Monitor units calculated with heterogeneity correction were 1.1% and 0.9% smaller on average than those without heterogeneity correction for 6MV and 23MV beams respectively. The maximum deviations of MU were 3.8% and 2.8% smaller. Average percentage differences of point doses with and without heterogeneity corrections were -0.2%, 17.0%, -0.3%, and -2.7% at the levels of shoulder, chest, abdomen, and pelvis for 6MV beam and 0.4%, 11.0%, 0.2%, and -1.7% for 23MV beam. Discrepancy of doses to the points at the shoulder level varied from -6.8% to 8.9% for 6MV beam and from -1.6% to 5.1% for 23MV beam. CONCLUSIONS: Bilateral TBI MU calculation errors caused by ignoring tissue inhomogeneity would be less than 4% and 3% for 6MV and 23 MV beam. Dose estimation is less accurate using 6MV beam and the inaccuracy could be more than 8% for shoulder midpoint and 4% for pelvis midpoint.
