Consideration of the likely benefit from implementation of prostate image-guided radiotherapy using current margin sizes: a radiobiological analysis

Abstract

Objective: To estimate the benefit of introduction of image-guided radiotherapy (IGRT) to prostate radiotherapy practice with current clinical target volume-planning target volume (PTV) margins of 5-10 mm.

Methods: Systematic error data collected from 50 patients were used together with a random error of σ=3.0 mm to model non-IGRT treatment. IGRT was modelled with residual errors of Σ=σ=1.5 mm. Population tumour control probability (TCP(pop)) was calculated for two three-dimensional conformal radiotherapy techniques: two-phase and concomitant boost. Treatment volumes and dose prescriptions were ostensibly the same. The relative field sizes of the treatment techniques, distribution of systematic errors and correlations between movement axes were examined.

Results: The differences in TCP(pop) between the IGRT and non-IGRT regimes were 0.3% for the two-phase and 1.5% for the concomitant boost techniques. A 2-phase plan, in each phase of which the 95% isodose conformed to its respective PTV, required fields that were 3.5 mm larger than those required for the concomitant boost plan. Despite the larger field sizes, the TCP (without IGRT) in the two-phase plan was only 1.7% higher than the TCP in the concomitant boost plan. The deviation of craniocaudal systematic errors (p=0.02) from a normal distribution, and the correlation of translations in the craniocaudal and anteroposterior directions (p<0.0001) were statistically significant.

Conclusions: The expected population benefit of IGRT for the modelled situation was too small to be detected by a clinical trial of reasonable size, although there was a significant benefit to individual patients. For IGRT to have an observable population benefit, the trial would need to use smaller margins than those used in this study. Concomitant treatment techniques permit smaller fields and tighter conformality than two phases planned separately.

Figure 1

Grey line indicates the measured beam penumbra data for a 9×9 cm field at a depth of 10 cm and focus-to-skin distance of 90 cm; only one half of the beam is shown. The dose levels and lateral extent of the 2 planning target volumes (PTVs) are shown by the bold horizontal lines indicating that 95% of the isocentric dose is required to enclose PTV2, and 72% is required to enclose PTV1. Note that, if PTV2 is just covered by this beam, PTV1 is covered by a dose in excess of 72%. CTV, clinical target volume.

Figure 2

Results of planning a spherical clinical target volume (CTV) with planning target volume (PTV)1=CTV+5 mm and PTV2=CTV+10 mm with three-dimensional conformal radiotherapy (anterior beam with two wedged laterals) and 15 MV beams (σ_p=4.9 mm). (a) The dose distribution resulting from Phase 1, with the 95% isodose conforming to PTV1. The second distribution (b) represents a treatment in which the 95% isodose is conformed tightly to PTV2 (the middle grey outline) with one set of beams. Note that in the absence of seminal vesicles this also ensures that the 72% isodose encloses PTV1 (the outer grey outline), and so is also suitable as a concomitant boost plan in regions where CTV1=CTV2. The final part (c) represents the composite distribution resulting from a two-phase plan. Note that the 95% isodose is pulled out approximately 3.5 mm from PTV2 and the 72% isodose is grossly too large for PTV1.

Figure 3

Histogram distribution of a patient's systematic moves in the craniocaudal direction. Positive values indicate prostates located inferiorly of the reference position. Dark bars represent observed frequencies. Light bars represent a normal distribution of the same mean and standard deviation.

Figure 4

Correlation between systematic shifts in the craniocaudal and anteroposterior directions for 50 patients. The line indicates the least squares fit.