Calculation of optimal margins between clinical target volume (ctv) and planning target volume (ptv)

The purpose of this study was to estimate the CTV-PTV margin required for prostate cancer cases at the radiotherapy department of Basvatarakam Indo American Cancer Hospital and Research Institute, Hyderabad. Portal image data from patients treated at the radiotherapy departments during the period of 2013-2015 was used to estimate the set-up displacements for each prostate area. By using the acquired images the magnitude of the systematic, i.e. preparatory, and random, i.e. execution, error was determined in the anterior- posterior (AP), superior-inferior (SI) and right-left (RL) direction. The calculated PTV margin is based on the systematic and random errors of the entire patient populations. A total of 29 patients were used for the analysis of prostate treatments. The evaluation of the PTV margin was done for two different matching protocols; CBCT matching and DRR matching. Results show that there is no considerable difference in the margin values evaluated from DRR matching protocol and CBCT matching protocol. This happens due to the fact that both protocols depend on the bony anatomy matching. Larger shifts occurring in longitudinal direction can be attributed to immobilization. The immobilization used in the hospital does not have foot holder or indexer support. Based on ICRU suggestions and recommendations, the minimum PTV margin given to prostate cases in the institute is 0.5 cm in all directions. The observed table correction values pointed out that the PTV margins used in the institute is inadequate to provide sufficient coverage of tumour volume, setup uncertainties and internal organ motion in Prostate cancer cases.