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?
Every plan is kind of a trial and error system as you try to get the prescribed dose to conform around the PTV but limit the amount of dose being delivered to the normal structures nearby.1 Getting 100% coverage of the 100% isodose line around the PTV while sparing the normal tissue is not always possible. At our site, the physicians will generally accept 95% of the PTV volume covered by the 100% isodose line, with the exception of prostates. For prostates, they like 98% of the PTV covered by the 100% isodose line.2
Generally, when as we are planning we try to eliminate as much of the hot spots as possible down to at least the 105% isodose line. However, they physicians will except up to a 110% hot spot with-in a plan, especially if it’s a palliative case. When we plan our Canadian fractionation breasts we do have strict limits that only 1cc of the 107% isodose line can remain in the plan.2
There are not any “rules” regarding the hot spot when it falls within the PTV volume itself. We still try to get it down to at least the 110%. I know for our prostate plans we try to even limit how much 107% and 105% are in the PTV. Also with our breast tangents we do the same, we try to limit how hot the breast tissue itself is.
Every plan is kind of a trial and error system as you try to get the prescribed dose to conform around the PTV but limit the amount of dose being delivered to the normal structures nearby.1 Getting 100% coverage of the 100% isodose line around the PTV while sparing the normal tissue is not always possible. At our site, the physicians will generally accept 95% of the PTV volume covered by the 100% isodose line, with the exception of prostates. For prostates, they like 98% of the PTV covered by the 100% isodose line.2
Generally, when as we are planning we try to eliminate as much of the hot spots as possible down to at least the 105% isodose line. However, they physicians will except up to a 110% hot spot with-in a plan, especially if it’s a palliative case. When we plan our Canadian fractionation breasts we do have strict limits that only 1cc of the 107% isodose line can remain in the plan.2
There are not any “rules” regarding the hot spot when it falls within the PTV volume itself. We still try to get it down to at least the 110%. I know for our prostate plans we try to even limit how much 107% and 105% are in the PTV. Also with our breast tangents we do the same, we try to limit how hot the breast tissue itself is.
- Lenards, N. Acquisition of Data. [SoftChalk]. La Crosse, WI: UW-L Medical Dosimetry Program; 2016.
- Conversation with Mathews, R. Certified Medical Dosimetrist. UF Health Cancer Center at Orlando Health. March 27, 2017.