Each facility prescribes to the PTV a little bit different. For example, some facilities want the 95% isodose line to cover 100% of the PTV. Others want the 100% isodose line to cover 95% of the PTV. Talk to your preceptor and/or radiation oncologists to find out how your facility prescribes dose to the PTV. Also, what is the maximum hot spot allowed on any plan? How is the hot spot defined at your facility (is the hot spot any area larger than 3cc or 10cc)? What if the hot spot lies in the PTV? Is the hot spot allowed to be higher than normal?
At my internship site, UW Hospital, we generally try to achieve 95% coverage of the planning target volume (PTV) by the 100% isodose line. That being said, it tends to vary slightly from case to case based on the area being treated, physician preference, patient comorbidities, and prescribed dose. The PTV is the clinical target volume (CTV) plus some margin, typically 0.5-1.5 cm depending on location and physician, added to it to account for patient motion, beam penumbra, and daily setup inconsistencies to ensure desired coverage of the treatment volume.[1] An example of when we deviate from 95% of the PTV receiving 100% of the prescribed dose is seen for breast plans at our facility. The treatment planning order (TPO) written by the physician indicate the desired coverage, constraints, and maximum dose (hot spot) they would like to achieve. The desired dose constraints for breast plans are that 95% of the PTV receives 95% of the prescribed dose and a Dmax under 110% of the prescribed dose, but acceptable dose constraints that the physician will accept are that 90% of the PTV receives 90% of the prescribed dose and a Dmax under 115%. A hot spot at our facility is considered just a point dose and is not a designated volume. Physicians will look at the area of 105% when evaluating a plan and would ideally like to see the hot spot under 110% and within the PTV, although I have seen them accept plans with hot spots as high as 120% depending on the case and outside of the PTV. An example of a case when a hot spot of over 110% would be acceptable would be an AP/PA spine for an obese patient. These types of treatments tend to run quite hot due to the large separation of the patient.
Reference
Reference
- Washington CM, Leaver D. Principles and Practice of Radiation Therapy. 3rd ed. St. Louis, MO: Mosby-Elsevier; 2010:445.