[OA132] An investigation of possible radiophysical causes of recurrence for non-melanoma facial skin cancer treated with superficial X-rays

Purpose A 2010 cohort of 177 patients with 195 non-melanoma facial skin cancer received superficial kV X-rays radiotherapy (RT) with 14 cases (7.2%) of recurrence. The cases were analyzed with respect to tumor location on the head and within the radiation field. In this study, we investigate whether the recurrences could potentially be caused by underdosage of irregular surface areas within a treatment field. Methods The pathology of the tumors was mostly (77%) basal cell carcinoma (BCC) and the remaining squamous cell carcinoma (SCC). The recurrence rate was 10 and 4 cases for BCC and PCC, respectively. The recurrences were located on the nose (5), scalp (3), cheek, forehead and ear (2 each). Within the radiation field, 42% of recurrences were located on flat surfaces receiving the prescribed dose (i.e. radioresistant) and the remaining on curved surfaces or at the edge. To estimate the dose coverage of the latter, we used the EGSnrc RZ (cylindrical symmetrical) Monte Carlo software. A nose geometry was approximated by stacked water cylinders with dimensions (radius  ×  depth in cm): 1 × 1+1.5 × 1 + 2 × 1 + 3 × 2 and compared with a standard full scatter geometry of 3 × 5. The dose was calculated for a 4 cm circular treatment field (1 cm margin) with a mono-energetic 40 keV point source which is the approximate effective energy of 80 kVp source (used clinically) as simulated by SpekCalc. The percentage depth dose (PDD) curve along the central axis and dose profile at the surface and therapeutic depth (85% dose level about 5 mm) for the two geometries were collected and compared. Results There was no difference in PDD for the nose and standard geometry. The radial nose dose profiles, however, showed a large underdosage of 50% (surface) to 30% (therapeutic) over the 3 most lateral mm as compared to the standard geometry. Conclusions The study showed that a large portion of the recurrences were placed on curved parts or edge of the radiation fields. Monte Carlo simulations suggest that such recurrences could potentially be caused by lateral underdosage. This could partly be corrected for by adding compensating scattering material around the treatment field.