Discussion topic:
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?
Discussion post:
At UF Cancer center there are multiple metrics for PTV coverage. We have several physicians and different physicians have different preferences. In some cases, such as breast treatments we follow RTOG protocols to define and cover the treatment area. We also utilize preset “scorecards” in Pinnacle that assess PTV, CTV, and critical structure coverage. The scorecard will automatically evaluate coverage and will give the value in red, yellow, or green to indicate failing, within tolerance, or passing. In the case of breast plans the scorecards are programed to show breast RTOG metrics and evaluate the plan accordingly.1 The scorecards are a great tool to indicate if a plan is likely to achieve approval. Doctors still look at each plan individually and may accept less than optimal coverage if it is necessary to cool a plan down. Additionally, a plan with all green indicators may still not receive approval if the physician doesn’t like the dose distribution. In the breast example, I cited earlier the metrics call for 95% of the PTV to receive 100% isodose line coverage.2 Some physicians are very strict on the metric and will not accept less coverage. On the other hand, today a physician approved a breast plan I did that only had 93.5% PTV coverage (100% isodose line). In this case the reduction in coverage was necessary to reduce the 107% hotspot volume. I showed a comparison plan meeting the metric at 95% with increased hotspot but the physician preferred the cooler plan.1,2
At UF Cancer center hotspots much like PTV are assessed on a plan by plan basis. Anything over 100% is considered a hotspot but the global max volume and location are always considered. For example, on a whole brain plan we seek no 110%, preferably no 107%, and as little 105% as possible. This isn’t realistic on every patient and beam isocenter placement and patient anatomy may limit what is possible. In planning, something like an AP PA L spine our physicians aren’t likely to approve a plan with a hotspot in the bowel. If the hotspot is in to PTV or posterior soft tissue the plan is more likely to be approved.
At my clinic, we always strive for the best possible plan. However, physician preference is what will dictate what the best plan is. Typically, we reduce global max volume until the 100% isodose coverage starts breaking for there a nuanced understanding of physician preference dictates how you will tackle the plan.
References
- White J, Tai A, Arthur D, et al. Breast Cancer Atlas for Radiation Therapy Planning: Consensus Definitions. RTOG.org. https://www.rtog.org/LinkClick.aspx?fileticket=vzJFhPaBipE%3d&tabid=236. Accessed March 29, 2004.
- Burch, Doug. Breast Protocol Tx planning guide. [Sharepoint procedures]. Orlando, FL UF Cancer Center Physics Residency Program; 2017.