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How Low Should You Go: Choice of Minimum Dose Prescription in Cranial Radiosurgery.

Roberge D, Leclerc-Champagne C, Doucet R, Seuntjens J - Cureus (2015)

Bottom Line: Different minimum doses were calculated using three calculation methods for 20 brain targets and compared to the original physician intent.The median difference in the isodose covering the PTV-0.03 cm(3) was 0% for Methods 1 and 2 and 3.6% for Methods 1 and 3.The median difference in the isodose covering the PTV-0.01 cm(3) was 0% for Methods 1 and 2 and 2.2% for Methods 1 and 3.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Oncology, Division of Radiation Oncology, McGill University Health Center ; Department of Radiology, Radiation Oncology and Nuclear Medicine, University of Montreal ; Department of Radiation Oncology, Centre hospitalier de l'université de Montréal (CHUM) ; Department of Oncology, Division of Radiation Oncology, McGill University Health Center.

ABSTRACT

Background:  In radiosurgery, the convention has been to prescribe radiation dose to a "covering isodose volume". This is presumed to be the minimum dose received by the entire tumor. Our purpose was to review our practice, assess different means of specifying a prescription isodose, and test how reliable they were in the face of various calculation methodologies.

Methods: Different minimum doses were calculated using three calculation methods for 20 brain targets and compared to the original physician intent. Monte Carlo (MC) calculations were run down to 2% (1 sigma) statistics. Voxel size depended on the imaging field of view and the calculation engine. For pencil beam convolution ("finite size pencil beam" - FSPB) calculations with or without heterogeneity correction (Methods 1-2), the calculation grid matched the CT scan resolution. For MC calculations (Method 3), the highest available in-plane resolution was 256 x 256 pixels. The median voxel volume was thus 0.58 mm(3) (0.47 to 0.95 mm(3)) for FSPB and 2.3 mm(3) (1.87 to 3.80) for MC.

Results: The absolute minimum target dose varied substantially between the three calculation methods - up to 25% difference between Methods 1 and 3. The differences were reduced when comparing near-minimum doses with absolute minimal volumes ΔV, DPTV-ΔV. The median difference in the isodose covering the PTV-0.03 cm(3) was 0% for Methods 1 and 2 and 3.6% for Methods 1 and 3. The median difference in the isodose covering the PTV-0.01 cm(3) was 0% for Methods 1 and 2 and 2.2% for Methods 1 and 3. In our data, the smaller the volume in which the minimum dose is calculated, the more sensitive this calculation was to dose calculation parameters. The standard deviation of the difference between physician intent and the isodose covering the PTV-0.01 cm(3) was 2.9% (range from -3.3% to 9.3%).

Conclusion: In radiosurgery, absolute minimum doses are sensitive to changes in dose calculation grids and dose calculation algorithms. Based on our experience, standardizing dose prescription to the isodose volume covering the PTV-0.01 cm(3) or the PTV-0.03 cm(3) would have little impact on clinical practice and would be relatively insensitive to dose calculation parameters.

No MeSH data available.


Related in: MedlinePlus

Physician intent vs. PTV coverageWhich isodose covers the tumor using different potential minimum or near-minimum metrics is plotted relative to the real life dosimetry, as clinically approved by the physician (for example, the graph illustrates that clinical plans all excluded the last GTV voxel from the prescription isodose).
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FIG1: Physician intent vs. PTV coverageWhich isodose covers the tumor using different potential minimum or near-minimum metrics is plotted relative to the real life dosimetry, as clinically approved by the physician (for example, the graph illustrates that clinical plans all excluded the last GTV voxel from the prescription isodose).

Mentions: Using the same calculation parameters implemented clinically, the isodose volumes selected by the prescribing physicians were compared to four versions of near minimum dose: V - 0.1 cm3, V -0.03 cm3, V - 0.01 cm3, and D95. The standard deviation of the difference between physician intent and minimum dose parameter was smallest for V - 0.001 cm3 (2.9%, range from -3.3 to 9.3). Prescribing to cover V - 0.1 cm3, V - 0.03 cm3, V - 0.01 cm3, or D95 would have changed the delivered dose by a median of 6.3%, 3.4%, 2.9%, and 2.9% (Figure 1).


How Low Should You Go: Choice of Minimum Dose Prescription in Cranial Radiosurgery.

Roberge D, Leclerc-Champagne C, Doucet R, Seuntjens J - Cureus (2015)

Physician intent vs. PTV coverageWhich isodose covers the tumor using different potential minimum or near-minimum metrics is plotted relative to the real life dosimetry, as clinically approved by the physician (for example, the graph illustrates that clinical plans all excluded the last GTV voxel from the prescription isodose).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4494511&req=5

FIG1: Physician intent vs. PTV coverageWhich isodose covers the tumor using different potential minimum or near-minimum metrics is plotted relative to the real life dosimetry, as clinically approved by the physician (for example, the graph illustrates that clinical plans all excluded the last GTV voxel from the prescription isodose).
Mentions: Using the same calculation parameters implemented clinically, the isodose volumes selected by the prescribing physicians were compared to four versions of near minimum dose: V - 0.1 cm3, V -0.03 cm3, V - 0.01 cm3, and D95. The standard deviation of the difference between physician intent and minimum dose parameter was smallest for V - 0.001 cm3 (2.9%, range from -3.3 to 9.3). Prescribing to cover V - 0.1 cm3, V - 0.03 cm3, V - 0.01 cm3, or D95 would have changed the delivered dose by a median of 6.3%, 3.4%, 2.9%, and 2.9% (Figure 1).

Bottom Line: Different minimum doses were calculated using three calculation methods for 20 brain targets and compared to the original physician intent.The median difference in the isodose covering the PTV-0.03 cm(3) was 0% for Methods 1 and 2 and 3.6% for Methods 1 and 3.The median difference in the isodose covering the PTV-0.01 cm(3) was 0% for Methods 1 and 2 and 2.2% for Methods 1 and 3.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Oncology, Division of Radiation Oncology, McGill University Health Center ; Department of Radiology, Radiation Oncology and Nuclear Medicine, University of Montreal ; Department of Radiation Oncology, Centre hospitalier de l'université de Montréal (CHUM) ; Department of Oncology, Division of Radiation Oncology, McGill University Health Center.

ABSTRACT

Background:  In radiosurgery, the convention has been to prescribe radiation dose to a "covering isodose volume". This is presumed to be the minimum dose received by the entire tumor. Our purpose was to review our practice, assess different means of specifying a prescription isodose, and test how reliable they were in the face of various calculation methodologies.

Methods: Different minimum doses were calculated using three calculation methods for 20 brain targets and compared to the original physician intent. Monte Carlo (MC) calculations were run down to 2% (1 sigma) statistics. Voxel size depended on the imaging field of view and the calculation engine. For pencil beam convolution ("finite size pencil beam" - FSPB) calculations with or without heterogeneity correction (Methods 1-2), the calculation grid matched the CT scan resolution. For MC calculations (Method 3), the highest available in-plane resolution was 256 x 256 pixels. The median voxel volume was thus 0.58 mm(3) (0.47 to 0.95 mm(3)) for FSPB and 2.3 mm(3) (1.87 to 3.80) for MC.

Results: The absolute minimum target dose varied substantially between the three calculation methods - up to 25% difference between Methods 1 and 3. The differences were reduced when comparing near-minimum doses with absolute minimal volumes ΔV, DPTV-ΔV. The median difference in the isodose covering the PTV-0.03 cm(3) was 0% for Methods 1 and 2 and 3.6% for Methods 1 and 3. The median difference in the isodose covering the PTV-0.01 cm(3) was 0% for Methods 1 and 2 and 2.2% for Methods 1 and 3. In our data, the smaller the volume in which the minimum dose is calculated, the more sensitive this calculation was to dose calculation parameters. The standard deviation of the difference between physician intent and the isodose covering the PTV-0.01 cm(3) was 2.9% (range from -3.3% to 9.3%).

Conclusion: In radiosurgery, absolute minimum doses are sensitive to changes in dose calculation grids and dose calculation algorithms. Based on our experience, standardizing dose prescription to the isodose volume covering the PTV-0.01 cm(3) or the PTV-0.03 cm(3) would have little impact on clinical practice and would be relatively insensitive to dose calculation parameters.

No MeSH data available.


Related in: MedlinePlus