Surface dose prediction and verification for IMRT plans using line dose profiles † Ronald E. Berg, † Michael S. Gossman and ‡ Stephen J. Klash † Erlanger Medical Center, Department of Radiation Oncology, 975 East 3 rd Street, Chattanooga, TN 37403 and ‡ SJK Physics, Radiation Oncology, 2209 Ranch Road, Sachse, TX 75048
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Surface dose prediction and verification for IMRT plans using line dose profiles † Ronald E. Berg, † Michael S. Gossman and ‡ Stephen J. Klash † Erlanger.
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Surface dose prediction and verification for IMRT plans using line dose profiles
† Ronald E. Berg, † Michael S. Gossman and ‡ Stephen J. Klash
†Erlanger Medical Center, Department of Radiation Oncology, 975 East 3rd Street,
MOSFETs are used to verify the entrance dose for conventional radiation therapy patient treatments. For sliding window IMRT treatments a combination of fluence patterns are used to provide an integral dose to the target tissue. This renders predicting measurements at the skin surface more difficult because of substantial dose gradients in each beam. The objective was to find a method of predicting MOSFET measured entrance dose and test its applicability in the clinic.
Materials
• Helios CadPlan IMRT software
• Varian 2300C/D with MLC using 6 MV and 25 MV x-rays
• Thomson-Nielson Electronics Limited standard MOSFET system
This Research
• We report a planning methodology using line-dose profile for determining surface dose, operating with Helios CadPlan IMRT software
• Dose delivery to patient was confirmed, when surface dose from plan was compared to the MOSFET reading during sliding window IMRT actual treatment
• Response under energy specific bolus is indicated for 59 patients
• Both 6MV x-rays and 25MV x-rays were used in this data• Reasonable agreement from MOSFETs validate approach
to IMRT surface dose determination.
Other Published Research• Mutic et.al. in 2000 reported under estimation to be 15 %
in the dose to the near surface (3-6mm) during IMRT using PEACOCK 1.12 software (NOMOS Corp), x-ray energy was 6MV, TLDs used including 3mm bolus in a cylindrical phantom, serial tomotherapy delivery
• Dogan et.al. in 2003 reported that the convolution superposition algorithm in FOCUS 3.2.1 inverse planning software (CMS) overestimated dose to surface of polystyrene phantom by 25% and to the first millimeter by 5%, pp-ionization chamber and TLDs comparable, 6MV x-rays used with single field IMRT comparisons
Other Published Research
• Jones et.al. 2003 reported that by applying a max/min range to PTV detector structure volumes deep in a NOMOS phantom, using CORVUS software, that IMRT doses were within 7 % of that measured with MOSFETs. TLD response also compared were typically worse, IMRT plan delivery with 6MV x-rays only.
References
• Mutic S and Low DA. Superficial doses from serial tomotherapy delivery. Med Phys. 2000;27(1):163-165.
• Dogan N, Glasgow GP. Surface and build-up region dosimetry for obliquely incident intensity modulated radiotherapy 6 MV x rays. Med Phys. 2003;30(12):3091-3096.
• Jones AO and Kleinman MT. Patient setup and verification for intensity-modulated radiation therapy (IMRT). Med Dos 2003;28(3):175-183.
Methods
• Create structure on patient surface to resemble bolus
• Calculate dose for IMRT plan
• Use tools to draw a line from isocenter through bolus for a line-dose-profile
• Determine dose from profile to the surface beneath the bolus, where the MOSFET would lie
• Treat patient with the MOSFET and bolus in place and record the MOSFET reading
• Quantify differences between MOSFET reading and plan
Bolus creation on plan
Using line-dose-profile tool
Stop point
Start point
Line-dose-profile from plan
Dose percentage is on Y axis
Distance from start point to stop point (in mm) is on X axis
Drastic decline
Extrapolation of surface dose
Helios software does not do this for you. The extrapolation shown here represents the manual operation performed on plot printouts.
Extrapolation to surface
In vivo measurement
Data (measurements 1-18)E (MV) D Planned FDD D Expected D Measured Accuracy (%)