doi:10.1016/j.meddos.2008.08.004 CHARACTERISTICS OF MOVEMENT-INDUCED DOSE REDUCTION IN TARGET VOLUME: A COMPARISON BETWEEN PHOTON AND PROTON BEAM TREATMENT MYONGGEUN YOON,PH.D., DONGHO SHIN,PH.D., JUNGWON KWAK,PH.D., SOAH PARK,PH.D., YOUNG KYUNG LIM,PH.D., DONGWOOK KIM,PH.D., SUNG YONG PARK,PH.D., SE BYEONG LEE,PH.D., KYUNG HWAN SHIN, M.D., TAE HYUN KIM, M.D., and KWAN HO CHO, M.D. Proton Therapy Center, National Cancer Center, Goyang, Korea (Received 17 April 2008; accepted 21 August 2008) Abstract—We compared the main characteristics of movement-induced dose reduction during photon and proton beam treatment, based on an analysis of dose-volume histograms. To simulate target movement, a target contour was delineated in a scanned phantom and displaced by 3 to 20 mm. Although the dose reductions to the target in the 2 treatment systems were similar for transverse (perpendicular to beam direction) target motion, they were completely different for longitudinal (parallel to beam direction) target motion. While both modalities showed a relationship between the degree of target shift and the reduction in dose coverage, dose reduction showed a strong directional dependence in proton beam treatment. Clinical simulation of target movement for a prostate cancer patient showed that, although coverage and conformity indices for a 6-mm lateral movement of the prostate were reduced by 9% and 16%, respectively, for proton beam treatment, they were reduced by only 1% and 7%, respectively, for photon treatment. This difference was greater for a 15-mm target movement in the lateral direction, which lowered the coverage and conformity indices by 34% and 54%, respectively, for proton beam treatment, but changed little during photon treatment. In addition, we found that the equivalent uniform dose (EUD) and homogeneity index show similar characteristics during target movement. These results suggest that movement-induced dose reduction differs significantly between photon and proton beam treatment. Attention should be paid to the target margin in proton beam treatment due to the distinct characteristics of heavy ion beams. © 2009 American Association of Medical Dosimetrists. Key Words: Proton beam treatment, Target movement, Equivalent uniform dose, Dose-volume histogram. INTRODUCTION Proton therapy has been shown to be superior to conven- tional radiation therapy with photons and electrons for treating cancers. To understand the advantages of proton- based treatment compared with photon radiotherapy, it is necessary to review the physical characteristics of both modalities. 1,2 Conventional x-rays are composed of pho- tons of the same electromagnetic waves as visible light, but of much higher energy. Proton beams are different, because they are composed of positively charged parti- cles with an atomic mass of one. Unlike photons in x-rays, protons lose only a small amount of energy in tissue until they approach their maximum penetration depth, at which all the residual energy is lost over a short distance, resulting in a sharp increase in the absorbed dose. This portion of the particle track, where energy is lost rapidly over a short distance, is known as the Bragg peak. During proton therapy, several beams of closely spaced energies are superimposed, creating a region of uniform proton dose over the depth of the target, called the spread-out Bragg peak (SOBP), because the Bragg peak is too narrow to treat most tumors. In contrast, x-ray beams deposit their maximum dose in subcutaneous tis- sues, with the dose decreasing exponentially as tissue depth increases until exiting the body. This important difference in energy loss in tissue results in superior dose distributions with protons compared with photons. 3-5 Proton-based radiotherapy has a lower entrance dose to normal tissues, and there is no exit dose to normal tissues beyond the tumor. While intensity modulated radiother- apy (IMRT) can produce the same level of dose confor- mity in the tumor, it may cause more radiation-induced secondary cancers than proton therapy due to the higher integral dose deposition in normal tissue. One important drawback of proton therapy is that the dose coverage is more sensitive to the movement of the target than occurs in conventional radiotherapy. That is, proton beam treatment requires more accurate patient positioning to take full advantage of its superior dose conformity. 6,7 This sensitivity to target movement is basically due to differences between photon and proton beam characteristics. Figure 1 shows schematic pictures of target movement in phantom (Fig. 1A), revealing increased depth of the target from the skin and corre- Reprint requests to: Sung Yong Park, Ph.D., Proton Therapy Center, National Cancer Center, 809 Madu 1-dong, Ilsandong-gu, Goyang, 411-769, Korea. E-mail: cool_park@ncc.re.kr Medical Dosimetry, Vol. 34, No. 3, pp. 191-201, 2009 Copyright © 2009 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/09/$–see front matter 191