What can a conservatively set PTV - CTV margin lead to?

A conservatively set PTV (Planning Target Volume) to CTV (Clinical Target Volume) margin can lead to under-treatment and recurrence because it may not adequately account for uncertainties in tumor position and movement during treatment. The purpose of adding a margin to the CTV is to ensure that the radiation dose is effectively delivered to the target area, taking into consideration factors such as patient movement, variations in anatomy, and limitations in imaging.

If the margin is too tight and does not encompass the full extent of the tumor or its potential movement, there is a high risk that the entire volume of tumor tissue will not receive the necessary radiation dose. This under-treatment can result in cancer cells surviving, leading to treatment failure and potential recurrence of the disease. Therefore, while conservatively setting the margin might seem like a safe approach, it can compromise the overall effectiveness of the treatment plan.