Discussion topic:
When talking about HVL, you will often see low-energy beams described in terms of their HVL. Why is that? Why doesn't it apply to megavoltage beams? How do filters play a role in HVL? Give a specific example of how a HVL is used in radiation therapy. Tell us why and how it is used. Please explain.
Discussion post:
HVL or half value layer is a term often used to describe the energy of a radiation beam. However, HVL alone is not sufficient to describe all radiation beams. In short, HVL describes the amount of a given material required to reduce the intensity of a radiation beam by one half. The reason HVL is not adequate to describe all radiation beams is due to the energy and method used to produce the beam. For radioisotopes like 60CO, gamma rays given off by the cobalt source are the same discreet energy and thus there is no need to use filtration or measure the beam in HVL. In the case of linear accelerator produced beams filters are used to modify beam due to the variable spectrum of energies these beams produce. With the use of filters comes the need to classify HVLs in relation to beam energy. A filter removes lower energy photons from the beam to increase a beam’s HVL. With kilovoltage range produced x-ray beams HVL and peak voltage are used to most accurately describe to beam. In the kilovoltage beam the HVL is given in mm of AL. In the higher orthovoltage range HVL is given in mm of Cu. HVL is not used to describe megavoltage beams because a flattening filter is used to harden the beam and further filtration is not of any benefit.1
In my research of the role of HVLs and filters in radiation therapy I found an interesting article detailing new found benefits of non-filtered therapeutic megavotage beams. The study by the Journal of Applied Clinical Medical Physics found that IMRT and SRS treatment plans can actually benefit from a raw unfiltered beam. The study mentioned that previously with 3-D treatment technology it was advantageous to have homogenous flattened fields to deliver dose to the patient. However, with modulated beams and smaller fields it can be beneficial to have a heterogeneous beam. The unfiltered beam delivers more dose at the central axis which results in greater beam intensity and consequently a higher dose rate. The higher dose rate is helpful in reducing lengthy SRS treatment times. The exclusion of a flattening filter reduces fall off of dose from the central axis which can be beneficial for modulated fields because it reduces transmission and out of field dose. We are all aware of the importance and common uses of HVL and filtration in radiotherapy treatment but I found it interesting how a lack of filtration can also impact our patients for the better.
References
1.Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th Philadelphia, PA: Lippincott Williams and Wilkins. 2014.
2. Xiao Y, Kry S, Popple R, et al. Flattening filter-free accelerators: a report from the AAPM Therapy Emer ging Technology Assessment Work Group. Journal of Applied Clinical Medical Physics. 2015;16(3). http://www.jacmp.org/index.php/jacmp/article/view/5219/html_276. Accessed September 21, 2016.
When talking about HVL, you will often see low-energy beams described in terms of their HVL. Why is that? Why doesn't it apply to megavoltage beams? How do filters play a role in HVL? Give a specific example of how a HVL is used in radiation therapy. Tell us why and how it is used. Please explain.
Discussion post:
HVL or half value layer is a term often used to describe the energy of a radiation beam. However, HVL alone is not sufficient to describe all radiation beams. In short, HVL describes the amount of a given material required to reduce the intensity of a radiation beam by one half. The reason HVL is not adequate to describe all radiation beams is due to the energy and method used to produce the beam. For radioisotopes like 60CO, gamma rays given off by the cobalt source are the same discreet energy and thus there is no need to use filtration or measure the beam in HVL. In the case of linear accelerator produced beams filters are used to modify beam due to the variable spectrum of energies these beams produce. With the use of filters comes the need to classify HVLs in relation to beam energy. A filter removes lower energy photons from the beam to increase a beam’s HVL. With kilovoltage range produced x-ray beams HVL and peak voltage are used to most accurately describe to beam. In the kilovoltage beam the HVL is given in mm of AL. In the higher orthovoltage range HVL is given in mm of Cu. HVL is not used to describe megavoltage beams because a flattening filter is used to harden the beam and further filtration is not of any benefit.1
In my research of the role of HVLs and filters in radiation therapy I found an interesting article detailing new found benefits of non-filtered therapeutic megavotage beams. The study by the Journal of Applied Clinical Medical Physics found that IMRT and SRS treatment plans can actually benefit from a raw unfiltered beam. The study mentioned that previously with 3-D treatment technology it was advantageous to have homogenous flattened fields to deliver dose to the patient. However, with modulated beams and smaller fields it can be beneficial to have a heterogeneous beam. The unfiltered beam delivers more dose at the central axis which results in greater beam intensity and consequently a higher dose rate. The higher dose rate is helpful in reducing lengthy SRS treatment times. The exclusion of a flattening filter reduces fall off of dose from the central axis which can be beneficial for modulated fields because it reduces transmission and out of field dose. We are all aware of the importance and common uses of HVL and filtration in radiotherapy treatment but I found it interesting how a lack of filtration can also impact our patients for the better.
References
1.Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th Philadelphia, PA: Lippincott Williams and Wilkins. 2014.
2. Xiao Y, Kry S, Popple R, et al. Flattening filter-free accelerators: a report from the AAPM Therapy Emer ging Technology Assessment Work Group. Journal of Applied Clinical Medical Physics. 2015;16(3). http://www.jacmp.org/index.php/jacmp/article/view/5219/html_276. Accessed September 21, 2016.