Evaluation of surface and build‐up region dose for intensity‐modulated radiation therapy in head and neck cancer

Abstract

Despite much development, there remains dosimetric uncertainty in the surface and build-up regions in intensity-modulated radiation therapy treatment plans for head and neck cancers. Experiments were performed to determine the dosimetric discrepancies in the surface and build-up region between the treatment planning system (TPS) prediction and experimental measurement using radiochromic film. A head and neck compression film phantom was constructed from two semicylindrical solid water slabs. Treatment plans were generated using two commercial TPSs (PINNACLE3 and CORVUS) for two cases, one with a shallow (approximately 0.5 cm depth) target and another with a deep (approximately 6 cm depth) target. The plans were evaluated for a 54 Gy prescribed dose. For each case, two pieces of radiochromic film were used for dose measurement. A small piece of film strip was placed on the surface and another was inserted within the phantom. Overall, both TPSs showed good agreement with the measurement. For the shallow target case, the dose differences were within +/- 300 cGy (5.6% with respect to the prescribed dose) for PINNACLE3 and +/- 240 cGy (4.4%) for CORVUS in 90% of the region of interest. For the deep target case, the dose differences were +/- 350 (6.5%) for PINNACLE3 and +/- 260 cGy (4.8%) for CORVUS in 90% of the region of interest. However, it was found that there were significant discrepancies from the surface to about 0.2 cm in depth for both the shallow and deep target cases. It was concluded that both TPSs overestimated the surface dose for both shallow and deep target cases. The amount of overestimation ranges from 400 to 1000 cGy (approximately 7.4% to 18.5% with respect to the prescribed dose, 5400 cGy).

Experimental evaluation of the accuracy of skin dose calculation for a commercial treatment planning system

“…Fraas et al (4) (American Association of Physicists in Medicine Task Group 53) reported the collective expectations of the Radiation Therapy Committee Task Group members for pass–fail criteria in the buildup region during the commissioning of a treatment planning system (TPS) as 20% of the central ray normalization dose for regular open fields, increasing to 40% for source‐to‐surface distance (SSD) variations, and 50% for wedged fields. Using radiochromic film, Chung et al (5) reported that two TPSs [${\text{Pinnacle}}^3$ (Philips Medical Systems, Andover, MA) and CORVUS (North American Scientific, Chatsworth, CA)] overestimated surface dose by $7.4\%-18.5\%$. Using thermoluminescent dosimeters and parallel‐plate ionization chambers, Mutic and Low (6) found that a tomotherapy TPS underestimated doses to the surface and first few millimeters below the surface by approximately 15%.…”

Comparing dose in the build‐up region between compensator‐ and MLC‐based IMRT

“…These parameters are not modeled well in commercial treatment planning systems (TPS). As a result, those TPS are known to be inaccurate in calculating dose in the build‐up region, as reported by Chung et al (4) These authors found that the two commercial TPS overestimated surface dose by 7.4% to 18.5%.…”

A simple technique to improve calculated skin dose accuracy in a commercial treatment planning system

“…A number of investigations have focused on the accuracy of skin dose or entrance dose (or surface dose as sometimes referred) calculations in commercial TPS 11, 12, 13, 14, 15, 16, 17. Court et al11 studied pencil beam convolution (PBC) algorithms in the Varian (Varian Medical Systems, Palo Alto, CA, USA) Eclipse system by comparing measured entrance dose to the Eclipse predicted dose.…”

“…All of these studies are based on phantom measurements and not by real CT based patient treatment planning. Results vary widely partly because of different TPS systems utilized, as well as the uncertainties in measuring entrance or skin dose accurately, especially in buildup regions 13, 15…”