Junctioning longitudinally adjacent PTVs with Helical TomoTherapy.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Helical TomoTherapy (HT) allows for precise radiation delivery.
• Treating longitudinally adjacent planning target volumes (PTVs) presents challenges in dose distribution, particularly in the junction region.

Purpose of the Study:

• To investigate parameters influencing dose distribution when treating longitudinally adjacent PTVs with HT.
• To evaluate methods for achieving optimal dose homogeneity in the junction area between adjacent PTVs.

Main Methods:

• Calculated in-phantom dose distributions for various on- and off-axis cylindrical PTVs using different HT field widths and pitches.
• Verified dose distributions with ion chamber and film measurements.
• Analyzed 3D dose distributions in the junction region of two adjacent PTVs.

Main Results:

• Equal field widths provided the best junction uniformity; larger fields enhanced intra-PTV dose homogeneity.
• Using different field widths created significant dose inhomogeneity, improved by dividing the smaller field's PTV into feathered sub-PTVs.
• The sub-PTV technique achieved dose homogeneity comparable to equal field widths, with 95% PTV coverage and dose variations from -19% to +13%.

Conclusions:

• Junctioning adjacent PTVs with HT is feasible.
• Treating adjacent PTVs of different widths is challenging but manageable by segmenting the PTV into feathered sub-PTVs to broaden the junction region and improve dose uniformity.