Purpose: It is commonly believed that it is crucial for beam(s) used for stereotactic treatment to have small penumbra. This belief resulted in the use of either small diameter radioactive sources, thereby sacrificing dose rate, or linear accelerators of over-focused electron beam on the target, thereby shortening target life. While sound when a single isocenter is used, it may not be true when multiple isocenters overlap to provide proper dose coverage, especially when inverse planning is used in modern practices. This study investigates dosimetric effects of penumbra on SRS/SBRT plan quality to provide an insight of future technique development.
Methods: SRS/SBRT treatment plans using focal spots of different penumbras were simulated using Monte-Carlo. Radiation sources were uniformly distributed, covering a 2Ï€ solid angle in a half-spherical arrangement. The source diameters ranged from 5mm to an ideal 0mm in decrements of 1mm, yielding reduced penumbras in conjunction with adjusted circular collimators for constant FWHM at isocenter depth. Treatment plans for two spherical PTVs of 2cm and 3cm diameters were optimized. The resulting dose conformity, fall-off, and PTV DVH were analyzed and compared.
Results: 12 optimized treatment plans using different focal spot penumbra for 2 different-sized PTVs were generated. As penumbra reduces from 5mm source to a point, Paddick conformity index changes from 0.73 to 0.80 respectively for 2cm PTV and 0.75 to 0.79 for 3cm PTV. DVHs showed no significant differences in PTV volume. Râ‚…â‚€ ranged from 4.7 to 5.0 and 3.6 to 3.8 for 2cm and 3cm PTVs respectively. D(max, 2cm) were all below 25.6% for both PTVs.
Conclusion: As focal spot overlapping is optimized computationally for high conformity of PTV, the effect of quality (uniformity and penumbra) of each individual focal spot becomes less significant on plan quality. Therefore, penumbra is a minor concern for future SRS/SBRT technique development.
Not Applicable / None Entered.