Session Item

Stereotactic MR-guided adaptive radiation therapy (or SMART) allows for recontouring of both target volumes and organs at risk prior to each fraction. Since SMART allows for optimizing treatment delivery in response to inter-fractional anatomical and volumetrical changes, subsequent fractions are reviewed separately. This approach will incorporate radiation-induced increases in the GTV into planning. However, editing daily contours can potentially reduce target volumes, and result in the unintended use of ‘shrinking field’ radiation. In this study we evaluated time trends in GTV’s (and CTV for prostate cancer), during the course of SMART with daily plan re-optimization. 

All patients treated with five fractions of SMART (total dose 35-50 Gy) within an overall treatment time of 2.5 weeks at a single institute using the MRIDIAN system (ViewRay Inc) were selected for this study. Target volume data for subsequent fractions were obtained for prostate cancer (N=463 pts), pancreatic cancer (N=129 pts), lung tumors (N=40 pts), renal cell cancer (N=79 pts) and adrenal metastases (N=58 pts). An analysis of time trends was performed using an ANOVA test with post hoc analysis. Additionally, in order to investigate the incidence of substantial target volume reduction during the course of SMART, the deviation (%) from fraction 5 relative to fraction 1 was calculated for each patient. 

The changes in GTV’s during the course of MRgRT were only modest, and on average (95% CI) for prostate CTV’s +6.7% (6.0 - 7.3%), for pancreatic cancer +1.3% (0.2 - 2.4%), for lung tumors +1.0% (-1.6 - 3.7%), for renal cell cancer +1.0% (-0.7 - 2.8%), and for adrenal metastases +1.8% (-0.8 - 4.4%). Only the average increase in prostate cancer CTV’s as compared to baseline was statistically significant (p < 0.001). Table 1 shows the rate of patients exhibiting a 10% and 20% decrease of GTV’s/CTV’s (prostate) during the 2.5 weeks of SMART delivery. In general, substantial reductions in GTV’s were uncommon, however, in particular for adrenal metastases a reduction of ≥20% was observed in 10% of the patients.

Figure 1: Average GTV/CTV deviations during the course of 5-fraction SMART delivery per tumor site

Table 1: Rate of GTV/CTV reduction during the course of SMART (fr5 versus fr1) per tumor site

Target volume changes during a 5-fraction course of MRgRT were modest, with only a small but significant increase observed in average prostate CTV’s. A subsequent study will evaluate whether such increase is indicative for acute toxicity. A substantial reduction in GTV’s was observed for some adrenal metastases. Although the clinical relevance remains unclear, clinicians need to be aware of the potential cumulative dosimetric impact of GTV reductions when performing daily plan re-optimization.