Room: AAPM ePoster Library
Purpose: evaluate the correlation between tumour volume and plan quality metrics like gradient index (GI), conformity index (CI), modified gradient index (mGI), heterogeneity index (HI) and quality of coverage (QC) for stereotactic radiotherapy (SRT) cases.
Methods: brain metastasis cases were treated in Truebeam STx linear accelerator using SRT technique. Planning was done in Eclipse treatment planning system V13.7 using non-coplanar rapid arc with grid size 1.5 mm for 6FFF beam. Plan metrics like GI, CI, mGI, HI and QC were calculated using the formula:
G I= Volume of half prescription dose / Volume of prescription dose
CI= prescribed isodose volume / prescribed dose
mGI = GI * CI
HI = Dose received by 5% of tumour volume / Dose received by 95% of tumour volume;
QC = minimum dose in the target / prescribed dose;
The volume of the tumour is divided into four categories as less than 1cc, between 1cc and 2 cc, between 2 cc and 5 cc and greater than 5 cc.
Results: GI for less than 1 cc volume is greater than 7 ; between 1cc and 2 cc volume is between 5 and 6; between 2 cc and 5 cc and for greater than 5 cc is between 4 and 5. The same trend had been observed in mGI also because mGI value depends on GI value.
There is no correlation between volume of tumour and CI, HI and QC of the plan.
Conclusion: result shows that gradient index value depends on the volume of tumour. For very small volumes like less than 1 cc, we can accept greater than 7 for GI. For large tumour volume we can restrict GI between 4 and 6. Remaining indices has to be accepted as defined by the protocol.