Comparison between margin-growing algorithms in radiotherapy software environments

Dependence of volume calculation and margin growth accuracy on treatment planning systems for stereotactic radiosurgery

OBJECTIVE

Uncertainties in radiotherapy target structures are partly dependent on differences between volume calculation and margin growing methods in treatment planning systems (TPS). These uncertainties are exacerbated with very small structures such as those common in stereotactic radiosurgery.

METHODS

Data from a national commissioning programme for SRS was used to assess variation in reported volumes for six benchmark cases, including malignant and benign indications. Reported volumes were compared both with and without any margins added according to local practice.

RESULTS

137 plans were submitted, with a total of 311 structures and covering seven TPS. For volumes < 1 cm³ agreement was within 0.05 cm³, and for volumes > 1 cm³ agreement was within 5%. Systematic differences were seen between TPS, partly because of different methods for calculating the end slice volume. About one third of structures had a margin added, of 1-2 mm. Most TPS over-grew the volumes, compared to the approximation of a perfect sphere, especially Pinnacle and Eclipse.

CONCLUSION

Differences between volume calculation methods may lead to 5-10% variation in reported volumes from different TPS. This should be taken into account when comparing multicentre studies, and it is recommended that a minimum volume of 0.05 cm³ be used for any near-point doses to allow more consistent comparisons. When margins are added to small structures, there may be up to 40% difference to nominal margin size. Such differences are still small compared to interobserver variation in delineation. Advances in knowledge: This study quantifies the potential uncertainties in clinical volume calculation and margin growth with small radiosurgical targets.

A simple method of evaluating margin-growing accuracy in image-guided radiation therapy

OBJECTIVE

In order to not add extra uncertainty to radiotherapy planning process, accurate margin algorithm is necessary, therefore we propose a centre-shift method to estimate its accuracy.

METHODS

A series of spherical phantoms are used with different CT slice separations (SSs) and pixel sizes (PSs). They are grown by different margins, and displaced geometric centres provide a window on accuracy. Volume difference between pre- and post-expansion is also calculated to double-check the accuracy.

RESULTS

The measured margin nearly varies as multiples of PS in the transaxial plane; in the superior-inferior direction, it is approximately equal to SS when the ideal margin is smaller than SS. A sphere's volume of <1 cm(3) is underestimated by 3-70% for all PSs, and 2-100% for SS of ≤5 mm but overestimated up to 112% for >5 mm. For volumes of >1 cm(3), relative volume error decreases, and it is nearly zero for >100 cm(3). The dependence of margin accuracy on SS and PS is largely eliminated by volume difference method.

CONCLUSION

We have proposed a simple method to estimate margin-growing accuracy and suggested corrective action to minimize the variation.

ADVANCES IN KNOWLEDGE

One big difference from the previous results is that SS and PS both influence the accuracy of margin growth and volume calculation in Eclipse(™) treatment planning system (Varian Medical System, Palo Alto, CA).