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A theoretical investigation of optimal target‐dose conformity in gamma knife radiosurgery a)
Author(s) -
Petti Paula L.,
Kunwar Sandeep,
Larson David A.
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3582945
Subject(s) - toxicity , nuclear medicine , radiosurgery , medicine , medical prescription , dosimetry , conformity , volume (thermodynamics) , therapeutic index , surgery , radiation therapy , pharmacology , drug , physics , psychology , social psychology , quantum mechanics
Purpose: The purpose of this paper is to suggest guidelines for target‐dose conformity in gamma knife stereotactic radiosurgery (GKSRS) by taking into account factors that have been linked to GKSRS complications. We also suggest an explanation for the failure of previous studies to find a correlation between improved conformity index and reduced risk of GKSRS toxicity, where the conformity index, C S , is defined as the ratio of the prescription volume, V P , to the target volume, V T .Methods: Previous investigations have shown that symptomatic toxicity in GKSRS is correlated with the volume of nontarget tissue receiving the prescription dose, D P . In this study, we formulated the volume of nontarget tissue, V NTD , receiving dose D ≤ D P as a function of the target volume, prescription volume, and prescription dose. We verified the model for D = 12–15 Gy by comparing V NTD calculated from the model versus V NTD calculated directly for 114 tumors in 63 consecutive patients treated at our institution. Once verified, we used this formulation of V NTD to calculate the volume of nontarget tissue receiving doses between 12 and 15 Gy from published data reported for patients experiencing varying degrees of GKSRS toxicity. Next, assuming that the V NTD values calculated for those patients who had either no toxicity or mild neurological symptoms in the published study represented safe levels of normal tissue irradiated to the dose in question, we substituted these V NTD values into an equation expressing C S in terms of V NTD , V T , and D P , and examined how C S varied as a function of V T and D P .Results: The R 2 value for the correlation between V NTD calculated directly or calculated with the proposed formula for V NTD ranged from 0.98 to 0.99, indicating that the formula accurately models the behavior of the nontarget volume receiving dose D . Applying this formulation of V NTD to historical data suggested that the requirements V NT15 ≤ 2.2 cm 3 , V NT14 ≤ 2.6 cm 3 , V NT13 ≤ 3.1 cm 3 , and V NT12 ≤ 3.8 cm 3 minimize the risk of severe complications following GKSRS. Imposing these criteria imply that as the target size increases, delivering a given prescription dose requires increasing target‐dose conformity. For tumor sizes >5 cm 3 C S must be ≤1.2 to restrict V NTD to the values listed above. For very small targets, on the other hand, nearly any reasonable conformity index will lead to acceptable values of V NTD . These observations may explain why previous investigations failed to show a correlation between improved conformity and decreased toxicity in GKSRS, because in these earlier studies the range of conformity indices represented was not wide enough, in particular C S values <1.3 were not represented for large tumors.Conclusions: Our model suggests that for target volumes ≥ 3 cm 3 , high levels of target‐dose conformity ( C S < 1.3) are required for typical GKSRS prescription doses in order to limit V NTD to levels associated with either no toxicity or mild neurological symptoms in a previous investigation.