The relationship between ventricular volume and whole-brain irradiation dose in central nervous system germ cell tumors.

BACKGROUND: Advanced irradiation techniques, including intensity-modulated radiation therapy (IMRT), aim to limit irradiation to adjoining tissues by conforming beams to a well-defined volume. In intracranial germinomas, whole-ventricular IMRT decreases the volume of irradiation to surrounding parenchyma. This study examined the relationship between ventricular volume and radiation dose to surrounding tissue. PROCEDURE: We retrospectively reviewed age, sex, ventricular and brain volume, ventricular dose, and volume of brain that received 12 Gy (V12) for patients diagnosed with germ cell tumors at our institution treated with whole-ventricular IMRT between 2002 and 2016. Variables were assessed for correlation and statistical significance. RESULTS: Forty-seven patients were analyzed. The median whole-ventricular irradiation dose was 24 Gy with a median boost dose of 30 Gy. The median ventricular volume was 234.3 cm3 , and median brain volume was 1408 cm3 . There was no significant difference between mean ventricular volume of suprasellar versus pineal tumors (P = .95). The median V12 of the brain, including the ventricles, was 58.9%. The strongest correlation was between ventricular volume and V12, with an r2 (coefficient of determination) of .47 (P < .001). Multiple regression analysis indicated that total boost dose and boost planning target volume significantly predicted V12 (P < .001). CONCLUSIONS: Although whole-ventricular IMRT limited irradiation to surrounding tissue in our cohort, a significant percentage of the brain received at least 12 Gy. This study suggests that there is a positive correlation between ventricular volume and the volume of brain parenchyma receiving at least 12 Gy with an important contribution from the boost phase of treatment.

A discrete stage-structured model of California newt population dynamics during a period of drought.

We introduce a mathematical model for studying the population dynamics under drought of the California newt (Taricha torosa), a species of special concern in the state of California. Since 2012, California has experienced a record-setting drought, and multiple studies predict drought conditions currently underway will persist and even increase in severity. Recent declines and local extinctions of California newt populations in Santa Monica Mountain streams motivate our study of the impact of drought on newt population sizes. Although newts are terrestrial salamanders, they migrate to streams each spring to breed and lay eggs. Since egg and larval stages occur in water, a precipitation deficit due to drought conditions reduces the space for newt egg-laying and the necessary habitat for larval development. To mathematically forecast newt population dynamics, we develop a nonlinear system of discrete equations that includes demographic parameters such as survival rates for newt life stages and egg production, which depend on habitat availability and rainfall. We estimate these demographic parameters using 15 years of stream survey data collected from Cold Creek in Los Angeles County, California, and our model captures the observed decline of the parameterized Cold Creek newt population. Based upon data analysis, we predict how the number of available newt egg-laying sites varies with annual precipitation. Our model allows us to make predictions about how the length and severity of drought can affect the likelihood of persistence and the time to critical endangerment of a local newt population. We predict that sustained severe drought will critically endanger the newt population but that the newt population can rebound if a drought is sufficiently short.