Session Item

Saturday
November 28
08:45 - 10:00
Physics Stream 1
Where automation can or cannot help the medical physicist
1120
Symposium
Physics
09:09 - 09:33
Which external beam modality is optimal?
SP-0012

Abstract

Which external beam modality is optimal?
Authors: Barnes|, Toni(1)*[toni.barnes@sunnybrook.ca];
(1)Odette Cancer Centre, Radiation Oncology, Toronto, Canada;
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Abstract Text
Abstract text

Superficial skin cancer can be treated with a number of external beam modalities. The optimal technique is guided by depth of the target volume and proximity to organs at risk (OAR). The patient population is often elderly, and the ability to travel to the cancer center for repeat visits, as well as compliance with treatment setup and delivery must be taken into consideration when deciding on the dose fractionation regime and treatment modality. In this presentation, different methods for the treatment of superficial skin cancer with external beam radiation will be reviewed.  Advantages and limitations of these methods will be discussed, along with comparison to brachytherapy techniques.  Kilovoltage X-rays, including superficial and orthovoltage therapy, have the advantage of simple beam collimation, narrow penumbra, and relative ease of treatment setup; disadvantage includes the relatively slow dose drop off with higher energies and the high absorption of bone (f-factor) with lower energies. When using electron therapy the physical properties of the beam needs to be properly understood when prescribing treatment; bolus is required to ensure full surface dose, and an adequate margin around the target is needed to account for the wide penumbra, and as lower isodoses lines bow out at depth. The nuances of shielding are often more complicated with electron than with kilovoltage X-rays. Megavoltage photons are required for targets at depth, and IMRT or VMAT can be used to tailor the dose to cover large complex target volumes while minimizing dose to nearby OAR. IMRT and VMAT are useful techniques when treating large superficial targets over cured surfaces, such as the scalp. With stereotactic radiation, dose escalation to target is possible as using narrow margins, and may provide higher rates of tumor control and symptom palliation. However, rigid immobilization and longer treatment times are required and may not be well tolerated by all patients. Target delineation can be guided by incorporating ultrasound and MRI into the treatment planning process. Clinical cases will be used as examples, and recommendations given to help guide clinical practice.