Comparison of a simple dose-guided intervention technique for prostate radiotherapy with existing anatomical image guidance methods

Objectives

A simple dose-guided intervention technique for prostate radiotherapy using an isodose overlay method combined with soft-tissue-based corrective couch shifts has been proposed previously. This planning study assesses the potential clinical impact of such a correction strategy.

Methods

10 patients, each with 8–11 on-treatment CT studies (n=97), were assessed using this technique and compared with no intervention, bony anatomy intervention and soft-tissue intervention methods. Each assessment technique used a 4-mm action level for intervention. Outcomes were evaluated using measures of sensitivity, specificity and dosimetric effect, and compared across intervention techniques. Dosimetric effect was defined as the change in dosimetric coverage by the 95% isodose from the no intervention case of an evaluation construct called the verification target volume.

Results

Bony anatomy, soft tissue and dosimetric overlay-based interventions demonstrated sensitivity of 0.56, 0.73 and 1.00 and specificity of 0.64, 0.20 and 0.66, respectively. A detrimental dosimetric effect was shown in 7% of interventions for each technique, with benefit in 30%, 35% and 55% for bony anatomy, soft tissue and dosimetric overlay techniques, respectively.

Conclusion

Used in conjunction with soft-tissue-based corrective couch shifts, the dosimetric overlay technique allows effective filtering out of dosimetrically unnecessary interventions, making it more likely that any intervention made will result in improved target volume coverage.Image-guided radiotherapy (IGRT) aims to improve treatment delivery accuracy by visualising the patient''s anatomy immediately prior to treatment, comparing this with the localisation data, usually a CT scan, and identifying and compensating for inaccuracies in the set up or target position that would compromise treatment efficacy 1]. In cancer of the prostate, potential inaccuracies include misalignments of the patient, e.g. caused by pelvic rotation or skin drag against the treatment couch, or changes in internal anatomy as a result of motion caused by bladder or rectal filling.Planar megavoltage (MV) imaging using electronic portal imaging devices has long been used to verify bony anatomy position 2] and, in recent years, the increased availability of kilovoltage and three-dimensional MV in-room imaging systems has enabled soft-tissue visualisation 3-6]. Image-based correction using translational couch shifts is now routine practice in modern radiotherapy centres, with bony anatomy, fiducial marker and soft-tissue-based assessment protocols being well documented 7-12].In prostate radiotherapy, moving from bony anatomy to soft-tissue-based assessment and intervention changes the approach from a surrogate for target position to tracking the target itself. Logically this should improve treatment accuracy, since the effect of internal motion on prostate position should be directly taken into account. However, clinical intervention strategies assume that any breach of a defined action level always requires a corrective shift and takes no account of the expected dose distribution in the patient.Systematic and random error components of the margin between the clinical target volume (CTV) and the planning target volume (PTV) mean that dosimetric coverage of the CTV will not be compromised if, despite changes in position, it remains within the International Commission on Radiation Units and Measurements (ICRU) 50/62-compliant 95% dose “cloud” 13,14]. In such a case, clinical intervention would not be necessary. Using dose-guided radiotherapy, the coverage of the daily verification CTV (vCTV) could be assessed against the expected daily dose distribution. An informed decision on the need to intervene could then be made based on probable dosimetric coverage, taking account of remaining uncertainties.Such an online dose-guided technique could be performed using a full dose recalculation based on the daily on-treatment anatomy immediately prior to treatment delivery. However, the implementation of any online dose-guided intervention poses a number of logistical problems: it would be time consuming, require prompt access to treatment planning stations, be prone to error because of the short decision time available and is a significant role extension for treatment staff more used to anatomical matching techniques. An alternative technique would be to use a sufficiently accurate surrogate for a full dose calculation, allowing dose-based judgements without the need for a potentially time-consuming calculation while a patient is in the treatment position.A previous paper proposed a dosimetric overlay method for dose-based assessment in image-guided radiotherapy of the prostate 15]. The technique involved the use of an overlay of the treatment plan 95% isodose over an on-treatment verification CT scan, achieved by a quick CT reference point registration between the verification CT scan and the localisation planning scan. The isodose could then be used in lieu of a full recalculation of the dose distribution on the pre-treatment scan and used to assess the adequacy of CTV coverage on that day. The paper showed that the technique was a feasible and acceptable means of assessment for prostate radiotherapy and that uncertainties between a full recalculation and this overlay isodose for a given patient anatomy were quantifiable and reasonable.This paper describes a planning study performed to determine the efficacy of the dosimetric evaluation technique described in Smyth et al 15] compared with both existing bony anatomy and soft tissue-matching and intervention protocols. Issues around future clinical implementation of the dosimetric overlay technique will also be discussed.