Adaptive radiotherapy involves altering the treatment plan according to variations in patient anatomy and set-up. This relies upon an accurate representation of the changing dose distribution within the patient, requiring a full dose recalculation. This work proposes a novel workflow using the planned dose distribution to assess dose coverage in three-dimensional verification CT studies acquired at the time of treatment delivery, using an overlay technique, in lieu of a recalculated dose distribution. The concept has been validated in a pilot study of 10 patients, each with 7-10 on-treatment CT studies. Differences between the geometric shape of the treatment plans for the 95% isodose and the 95% isodose obtained when the planned geometry was recalculated from the verification CT dataset were quantified. Dosimetric coverage of the verification clinical target volume (vCTV) was assessed for both the proposed overlay technique and the recalculated "delivered" dose distribution, and the conclusions on adequacy were compared. Results were consistent with geometric uncertainties of the dose calculation matrix (5 x 5 x 5 mm), suggesting that differences in the geometric shape of the 95% isodose are not significant for normal variations in patients' anatomy. Decisions on adequacy of vCTV coverage were consistent in 80 out of 87 cases, with discrepancies limited to a maximum of three axial slices per study within the range 0.5-4.5 mm (mean, 1.6 mm). The proposed dosimetric overlay technique has been validated and found to be an acceptable method of image-guided radiotherapy of the prostate suitable for effective implementation in the treatment clinic.
Craniospinal irradiation is technically demanding due to the complex shape of the planning target volume (PTV). Radiotherapy treatment techniques have evolved over time as imaging and radiotherapy treatment technology have improved. However, most are variations on a class solution utilizing a prone patient position with two shaped lateral cranial portals and a matched posterior spinal portal with moving junctions. Major areas of difficulty remain with the accurate definition of the PTV and achieving a homogeneous dose within it, especially at the junctions. We describe a three-dimensionally (3D) planned craniospinal radiation technique that permits rapid image acquisition with reduced localization time, simplified spinal PTV definition and standardized cranial PTV definition. Improved dose homogeneity within the PTV is achieved by use of a segmented "field-in-field" technique (forward planned intensity-modulated radiotherapy (IMRT)) in place of customized compensators. This has negated the requirement for constructing physical compensators. Autosequencing for field delivery enables the junction to be "moved" during a single fraction and reduces the overall treatment time, an important consideration when treating very young patients.
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 (n597), 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.
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