DOS 522 - Week 3 Discussion
In addition, provide examples of how irregular fields were used in radiation oncology historically.
Initial Post: Irregular Field Calculation Methods
Irregular field calculations are no longer performed in regular clinical practice at Loyola University Medical Center now that planning is computer driven and CT-based. Even simple plans that are delivered in 2D fashion, such as heterotopic ossification of the hip, have their doses calculated by drawing a field on a DRR and then letting the computer compute the doses.
In the course of researching this topic, I spoke with physicists Murat Surucu (oral communication, February 4, 2015) and Sébastien Gros (oral communication, February 4, 2015). They explained the typical cases where irregular field calculations might be used, such as the typical textbook examples of mantle field blocking of lungs and spinal cord.1
There are a few methods that might be used for performing calculations of irregular fields. A general solution to the problem is Clarkson's Method, which samples the distance from the measurement point to the geometric edge of the beam at regular angle intervals around a circle centered at the measurement point. The distance along each line is related to an equivalent circle field's Scatter Air Ratio (SAR), and each segment is assigned a weight in the overall average is based on the wedge size. This is a time consuming process, both in measurement and in calculation.
A simpler method developed by Day works fairly well for partially blocked rectangular fields. It simply breaks up the overall shape into four quadrants and each quadrant is related to an equivalent square/rectangle by spreading the area of the blocked region evenly along the edges of the rectangle, somewhat like shifting sand evening out by gravity (Anil Sethi, oral communication, January 15, 2015).
We don't use either of these methods in clinical practice anymore, but we did a bit of department archaeology today and found some films with hand-drawn fields on them. Below is what appears to be a head and neck treatment plan from 1999 with three dose prescriptions: one for a large field, one that blocks the cord and posterior neck, and a boost to a small area. (Apologies for the clipping - it was bigger than our scanner)
Dose for this plan probably could have been computed with either Day's Method (quick approximation) or Clarkson's Method (actual measurement), but films like this would have had their fields digitized with a plotter and imported into the Pinnacle system we used to use. Once in Pinnacle, the dose would have been computer-calculated rather than hand-calculated, so we have to go back farther than this to get to a time when hand-calcs were a regular occurence for anything other than second checks.
- Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.