Abstract
Purpose: To gain insight into radiosurgical brainstem dose tolerance by considering the radiobiological implications of the bath and shower experiments. Methods: Clinical brainstem dose tolerance for radiosurgery is compared to the two extreme cases of hypofractionated whole brain irradiation and trigeminal neuralgia radiosurgery. In 2003 and 2009, experimental results were published wherein a low dose to a large volume (bath) was combined with a high dose to a small volume (shower), for the cases of rat cervical spinal cord and rat parotid gland. Clinically the uniqueness of each patients' tumor requires a customized mixture of bath and shower doses to the adjacent critical structures, whereas in the animal experiments it was possible to systematically vary the bath dose and the shower dose and to statistically analyze the outcomes to study the interactions in detail, although human response may differ. Results: Hypofractionated whole brain irradiation results in essentially uniform bath dose to the brainstem at a single fraction dose equivalent of about 10Gy, according to the LQ model. For radiosurgery, Timmerman's 2008 limit allows 1cc of brainstem to exceed 10Gy in a single fraction. Therefore since the volume of brainstem receiving the bath dose is reduced, it allows a higher shower dose to be received by a small volume, and the corresponding TG101 limit allows a max dose of 15Gy. In the case of trigeminal neuralgia, the bath dose to brainstem is much lower, and some institutions have allowed an extremely high shower dose of 45Gy in a single fraction to the maximum point dose in brainstem, with surprisingly infrequent side effects under conditions of meticulous targeting and dosimetric accuracy. Conclusion: Clinically the tradeoff between doses to large and small volumes is already implicitly taken into account, but comparison to the radiobiological research can help provide insight. Dr. Grimm developed and holds intellectual property rights to the DVH Evaluator software tool which is an FDA‐cleared product in commercial use
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