[Dosimetric evaluation of conformal radiotherapy: conformity factor]

Area of Science:

• Radiation Oncology
• Medical Physics
• Cancer Treatment

Background:

• Three-dimensional conformal radiotherapy (3DCRT) aims to deliver prescribed doses to the Planning Target Volume (PTV) while sparing normal tissues.
• Current evaluation tools like DVHs, TCP, and NTCP do not fully quantify dose distribution conformity to the PTV.
• Accurate dose conformity is crucial for enhancing local control and minimizing treatment-related toxicity in cancer patients.

Purpose of the Study:

• To extend existing definitions of dose conformity to multiple dose levels using a novel conformity index (CI).
• To evaluate the clinical utility of the CI in assessing dose distribution conformity for lung and prostate cancer patients undergoing 3DCRT.
• To explore the application of CI in comparing different 3DCRT treatment planning techniques and optimizing dose prescription.

Main Methods:

• Developed a conformity index (CI) based on the relative volumes of the PTV and surrounding tissues receiving a specified dose.
• Evaluated the CI in a clinical study involving 82 lung cancer and 82 prostate cancer patients treated with 3DCRT.
• Compared CI values across different dose levels and analyzed its variation with treatment planning parameters (e.g., number of beams).

Main Results:

• The CI was found to be lower for lung cancer (0.35 at 66 Gy) compared to prostate cancer (0.57 at 70 Gy), attributed to factors like spinal cord sparing and build-up effects.
• The CI successfully differentiated between treatment plans with varying numbers of beams for both lung (2 vs. 3 beams) and prostate (4 vs. 7 beams) cancer.
• The study demonstrated that the CI can effectively quantify dose conformity and aid in the comparison of 3DCRT treatment plans.

Conclusions:

• The proposed conformity index (CI) provides a valuable quantitative measure for assessing dose distribution conformity in 3DCRT.
• CI evaluation in lung and prostate cancer patients highlights its applicability in real-world clinical scenarios.
• The variation of CI with dose offers a promising approach for optimizing dose prescription and treatment planning in 3DCRT.