The surface area effect: How the intermediate dose spill depends on the PTV surface area in SRS

How to estimate R50% for cranial SRS/SRT cases with overlapping 50% isodose volumes: A proposed system

Inevitably in clinical stereotactic cranial single isocenter multiple target cases treated with linac‐based multi‐leaf collimated (MLC) delivery, one encounters planning target volumes (PTVs) in close proximity with overlapping 50% isodose clouds (IDC50%). In such cases, it is very difficult to separate the IDC50% attributable to each individual target and, thus, assess the intermediate dose conformality or R50%. Such scenarios happen regardless of what metric is used to measure intermediate dose spill. Now that universal standards for intermediate dose spill have been proposed, it is important to have a consistent method for apportioning these overlapping IDC50% volumes to allow comparison with the proposed standards when multiple PTVs have overlapping IDC50%. We propose a systematic method for apportioning the IDC50% of multiple targets with overlapping IDC50% based on the relative surface area of each target volume; we call this the fair value estimate (FVE). This FVE system of apportionment is tested for reasonableness by comparing the apportionment of multiple target single isocenter stereotactic treatment with widely spaced targets where the IDC50% can be obviously assigned to demonstrate that the FVE results are very similar to the actual R50% results. We then demonstrate how the FVE system would be applied to cases with overlapping IDC50%. We propose this FVE system for consideration by the cranial stereotactic community for apportioning the intermediate dose spill when that intermediate dose spill overlaps among multiple targets.

Analysis of R50% location dependence on LINAC-based VMAT cranial stereotactic treatments

Stereotactic radiosurgery (SRS) and stereotactic radiation therapy (SRT) techniques are used to deliver high doses per fraction to various types of intra-cranial targets. LINAC-based solutions are growing in prevalence due to recent advances in technologies such as high-definition multi-leaf collimators and volumetric arc therapy radiation delivery. A wide variety of clinical pathologies including intracranial metastases, meningioma, glioblastoma, arteriovenous malformation, acoustic neuroma, and trigeminal neuralgia have been successfully treated using SRS/SRT techniques. These lesions can be in virtually at any location within the cranium. Several publications have shown a wide dispersion of intermediate dose conformality (intermediate dose spill) indices such as the Paddick Gradient Index or R50% for lesions of a specific volume. A complete explanation of this dispersion is lacking but location has been suggested as a contributing factor. While prior studies of PTV location in SRS/SRT are retrospective in nature, we have conducted a prospective study to ascertain the potential effects of location within the cranium on plan intermediate dose conformality as measured by R50% while controlling for lesion volume, lesion shape, prescription (Rx) dose, and Rx isodose surface. Lesion volumes utilized in this study are consistent with metastatic disease presentation. Results indicate only a weak relationship between intermediate dose conformality as measured by R50% and the lesion location when considering nine different, strategically placed lesions. Close proximity to critical structures can reduce the degree of conformality, but the effect appears to be minimal. Single isocenter multiple target cases were studied in addition to single target plans. All critical structure doses observed in this study were found to be within the recommendations of AAPM Task Group report 101. Lesion location does not appear to be a significant contributing factor to the observed variation of dose conformality seen in several SRS/SRT publications.