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IMPROVED DOSE DISTRIBUTION FOR ELECTRON FLASH RADIOTHERAPY WITH OPTIMIZED-THICKNESS BOLUS AND INTENSITY MODULATION
Abstract
Background and Aims
In electron radiotherapy, heterogenous dose distribution is a common problem resulting from complex surfaces and tissue inhomogeneities. For the high-dose single-fractions commonly used in FLASH radiotherapy (FLASH-RT), this must be carefully considered. In this study we evaluate Bolus Electron Conformal Therapy (BECT) for FLASH-RT. The specific aim was to evaluate if optimized-thickness bolus and intensity modulation can reduce dose heterogeneities in high-dose single-fraction treatments to be used in a veterinary clinical trial in canine cancer patients.
Methods
In the treatment planning system electronRT (.decimal®, LLC, Sanford, Florida, USA), CT-based treatment planning can be utilized to design and subsequently fabricate individualized 3D bolus in machinable wax. Furthermore, the beams can be intensity modulated using tungsten pins in the electron block cut-out. To evaluate if optimized-thickness bolus and intensity modulation could reduce cold- and hotspots and improve the target dose homogeneity, treatment plans employing these two methods were created in a canine cancer patient case with a simulated superficial nasal tumor. The prescribed dose was 30 Gy to 95% of the CTV, and a circular field with a diameter of 6.5 cm was used.
Results
By adding optimized-thickness bolus to the canine patient treatment plan, the maximum relative dose was decreased from 143% to 116%, and the CTV homogeneity index (HI) was reduced from 0.30 to 0.15 (Figure 1). By adding intensity modulation, the HI could be further reduced to 0.10.
Conclusions
We have evaluated the potential of optimized-thickness bolus and intensity modulation for FLASH-RT. We found that unwanted hot spots in heterogenous tissue could effectively be reduced in a canine patient treatment plan. Next, we will verify that these plans can be accurately delivered. This technique will help us avoid overdosing and possibly related toxicity in future clinical studies with electron FLASH-RT.