Society 'The Souther Chapter 09' held on 6th â 8th November 2009 at the Zon Regency Hotel, Johor. Bahru, Johor ... PTV (Purple Line). Figure 3 shows that the ...
MINIMIZING SKIN TOXICITY IN INTENSITY MODULATED RADIOTHERAPY OF HEAD AND NECK CANCER PATIENTS Radiotherapy Unit, Pantai Cancer Institute, Pantai Hospital Kuala Lumpur The following submission paper was presented in the Annual Scientific Congress Malaysian Oncological th th Society ‘The Souther Chapter 09’ held on 6 – 8 November 2009 at the Zon Regency Hotel, Johor Bahru, Johor
Introduction Intensity Modulated Radiation Therapy (IMRT) is the state-of-the-art of radiation therapy technique has been established in Pantai Hospital Kuala Lumpur since August 2008. More than 60 patients been treated using the IMRT technique in PHKL. Intensity modulated radiation therapy (IMRT) is a type of conformal radiation, which shapes radiation beams to closely approximate the shape of the tumor. The intensity (the dose) of the radiation in IMRT can be changed during treatment to spare more normal tissue and an increased dose of radiation can be delivered to the tumor. Although advances in radiotherapy such as Intensity Modulated Radiotherapy (IMRT) have led to improved outcomes in head and neck cancers, toxicity following IMRT remains substantial; especially acute skin reaction, as shown in figure 1. The aims of this paper are to demonstrate the factors contributing to this acute skin toxicity and discuss the possible solutions to this problem. There are at least four factors contributing to the skin toxicity, included planning target volume (PTV) contouring, the inverse treatment planning (ITP), the bolus effect of immobilization mask and the use multiple tangential beams.
Figure 1. Acute Skin Toxicity
Factor 1, for IMRT planning, the oncologist will outline the clinical target volume (CTV) that stops well short of the skin surface this implies that there is no disease in the skin (Figure 2a). Then, a separate PTV will add to the CTV to compensate for the treatment set‐up uncertainties (Figure 2b).
Figure 2a. CTV (Green Line)
Figure 2b. PTV (Purple Line)
Figure 3 shows that the dose at the surface is much higher if there is no gap between the PTV and patient skin. The maximum surface dose of phantom B is 4099.4cGy whereas the surface dose at phantom A is 2084.7 cGy. Phantom APhantom B
Figure 3. Surface point dose measurement in solid phantom Factor 2, in ITP used in IMRT, if the PTV was drawn near the skin, the computer optimizer will increase the intensity (Monitor Unit) near the skin surface to compensate for the lost dose in the buildup region, resulting the increase of skin dose. Figure 4 shows that segment 1 and segment 6 have higher monitor unit because both segments are nearer to patient skin.
Figure 4. Multileaf Collimator segment monitor unit Factor 3, the bolus effect of immobilization mask can increase skin dose about 15% when compared with and without head and shoulder immobilization mask. Table 1 shows that the patient skin dose investigation by Dr Ping Xia. As you can see from figure 5, because there is over folding to the BDS, which has increased the thickness of the BDS, this patient had developed redness on that small area.
Table 1. Skin dose investigation with TLD
Figure 5. The bolus effect of immobilization mask Factor 4, the IMRT plan often uses multiple tangential fields (fields do not perpendicular to the skin surface), which reduce the skin sparing effect. Figure 6 which shows that the surface dose is higher if using more tangential beams. Maximum Surface Dose: 748
Figure 6. Multiple tangential beams versus four direct beams Several possible strategies for avoiding the skin toxicity had been discussed, including the use of the patient skin as organ at risk, modification of the PTV to avoid the skin and the use of immobilization mask instead of patient skin in planning. The latter gave the best results.
Methods Nine extended fields IMRT plans were generated for our patients with nasopharyngeal cancer (Gross Tumor Volume receiving 70Gy and the clinical target volume 60Gy) with the use of patient immobilization mask for planning. An immobilization covering the head, neck and shoulder was used for immobilization, showed in figure 7. Patient’s skin reactions were then evaluated after completed 7 weeks of radiotherapy treatment.
Figure 7. Immobilization mask
Results
Figure 8. Skin Reaction
Conclusion By including patient’s BDS during inverse planning, the optimizer will not work so hard to cover the superficial tumor volume so that spares the skin better. It is possible to reduce the skin dose without compromising tumor coverage
Reference: 1. www.aapm.org/meetings/03SS/Presentations/Xia.pdf 2. Lee N, Chuang C, Quivey JM, et al. Skin toxicity due to intensity‐modulated radiotherapy for headand‐ neck carcinoma. Int J Radiat Oncol Biol Phys. 2002; 53 (3): 630‐637. 3. N. Dogan and G.P. Glasgow. Investigation of surface and build‐up region dosimetry for oblique Incidence IMRT Beams. Med Phys. 30, 1405‐1406, 2003. 4. Thomas S J and Andrew C F Hoole. The effect of optimization on surface dose in intensity modulated radiotherapy. Phys.Med.Biol.2004; 49: 4919‐4928
