The impact of IGRT for prostate radiotherapy on dosimetry and the traditional workflow practice of focus to skin distance measurements

Purpose: To assess the relevance of focus to skin distance (FSD) measurements for patients with intraprostatic gold seeds using an image guided radiation therapy (IGRT) protocol. The second aim of this study was to analyse the frequency and nature of isocentre shifts made during a course of treatment for patients with prostate cancer. Additionally, the impact of isocentre shifts on dosimetry, relative to the traditional method of basing dosimetric recalculations on FSD measurements was assessed. Methods and materials: Ten patients underwent prostate radiotherapy with intraprostatic gold seeds and an IGRT protocol. FSD measurements were taken on a daily basis pre-intervention and also post-intervention if an isocentre shift was made. Analysis of systematic and random isocentre shifts were made and compared to measured FSDs. An individual case study was carried out assessing the dosimetric impact of multiple isocentre shifts throughout a course of treatment and analysed against measured FSDs. Results: Ten patients received radiotherapy to the prostate (78 Gy/39 fractions). 390 treatment sessions were available for analysis, inclusive of 2340 measured FSDs. Measured FSDs were out of tolerance (greater than 1 cm difference from planned measurement) less than 2% of the total treatment sessions. Of the isocentre shifts made, 66% (160/242) were random and 34% (82/242) were systematic. The individual case study revealed 72% of treatment sessions required an isocentre shift with FSDs being outside tolerance for one session. Conclusions: FSD measurements in the era of intraprostatic gold seed IGRT have been reduced in importance. IGRT has improved the identification of systematic and random errors therefore allowing better visualisation of dosimetric impact these errors may cause. Our data suggest that FSD measurements for men undergoing IGRT for prostate cancer have minimal dosimetric impact.