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Beam Precision
Planning margins around the (imaging discernable) GTV are subsumed in a larger clinical target volume (CTV) to ensure irradiation of microscopic tumor infiltration into surrounding patient tissue. An additional margin about the CTV, called the planning treatment volume (PTV), accommodates uncertainties of patient set-up, patient motion, and predicted anatomic change over the treatment course.
Increasingly, image-guided adaptive radiotherapy uses inter-fraction imaging to confirm changes to tumor size and position, allowing modification or adaptation of the treatment plan when necessary, and allowing more precisely defined and redefined or modified PTVs.
“When patients lose weight, they will be rescanned and a new plan will be created to account for the changes in anatomy,” Dr Stathakis said.
Common Adverse Event
Skin burns are common. Radiation burns are rarely caused by aging radiotherapy equipment or equipment miscalibration.
“Causes [for burns] range from unaccounted scatter from the support devices — the couch pads, etc — and patient noncompliance with instructions on how to use creams,” Dr Stathakis surmised.
Some patients’ skin is more radiosensitive than others’, he noted. “If a linac is properly calibrated then it is not the cause of skin burns.”
References
1. Stathakis S. Principles of radiation oncology: a medical physics perspective. Presentation at: Oncology Nursing Society (ONS) Bridge; September 8017, 2020.
