PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s)::It has been a constant concern for clinicians on the realistic dose differences computed with an early-available finite size pencil beam algorithm (FSPB) for the treatment plans using a multileaf collimators equipped robotic radiosurgery system (RR-MLC) from the doses computed by an industry well-accepted algorithm, such as Monte Carlo dose algorithm (MC). This study was designed to quantify these dose disparities in the RR-MLC treatment plans for selected anatomic sites in using both FSPB and MC.
Materials/Methods:A total of forty RR-MLC stereotactic ablative radiation therapy (SART) plans computed with FSPB were retrospectively reviewed and compared with MC computed results, including plans for detached lung cancer (or tumors fully surrounded by lung tissues,n=10), non-detached lung cancer (or tumor touched the chest wall or mediastinum, n=10), intracranial(n=10), and pancreas lesions(n=5). MC was commissioned with an uncertainty of 0.2%. All the selected plans were recomputed in the Precision™ planning system for both algorithms with the identical set of contours, optimizations, and MU’s. The dosimetry for planning tumor volume (PTV) and major organ at risks (OAR) was compared with each paired plan. A “normalized deviation” (N_dev=(MC-FSPB)/FSPB) and paired two sample t-tests were used for quantitative analysis.
Results:AS shown in table 1,FSPB plans of RR-MLC overestimates D95 of PTV by averaging 18.9% when compared with the MC computed plans in detached lung cases, and 17.0% in non-detached lung cases. These dose differences appear much less in intracranial and pancreas cases, the differences of D95 of PTV are 1.3% and 2.0% respectively. The lung SART plans encompass the situation with the largest tissue density heterogeneity among all the groups, especially those for detached lung tumors where the disadvantage of FSPB shows largest in dose computations. There is less significant difference (1.3-4.9%) between FSPB and MC calculated maximum dose to OAR’s in each case. Table 1: Dosimetry Comparisons of RR-MLC Plans Computed with FSPB and MC for Selected Sites
|PTV Doses||Non-Detached lung||Detached lung||Pancreas||Intracranial|
Conclusion:The results from this investigation suggest a significant dose difference between lung SART plans computed with FSPB and MC, in which FSPB overestimates D95 of the lung PTV by about 17% to 18.9% and slightly underestimates the hot spots in the surrounding OAR by about 5% or less. The dose differences between plans using the two algorithms are about 2% of less in SART plans for intracranial and pancreatic lesions. These results shall provide a useful guide in evaluating RR-MLC plans when MC is not used.
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