Skin cancer is the most common malignancy worldwide. Radiotherapy (RT) with either kilovoltage x-rays and/or megavoltage electron and/or photon beams are treatment options for skin cancer, particularly for non-operative candidates due to comorbidity or cosmetical concerns or patient refusal. Our objective is to model tumor control probability (TCP) for RT of primary basal or squamous cell carcinoma (BCC/ SCC) based on published clinical data in order to optimize treatment schemes. Reports citing crude estimates of local tumor control for primary BCC and SCC of the head/face by tumor size ≤ 2 cm or > 2 cm were considered in our TCP modeling based on a TCP model: TCP = 1/(1+exp(-(BED14 - BEDprolif·T - TD50)/k), where BED is the biological effective dose calculated from the report using the linear quadratic model with a/β = 14 Gy, BEDprolif is the BED recovered per day due to proliferation, T is the treatment time in days, TD50 is the dose required to achieve 50% tumor control, & k is a fitting constant related to the slope of the dose response curve at TD50. These model parameters were estimated using the Chi-squared fitting method. Sample equivalent fractionation schemes and the effect of varying treatment time on TCP were estimated. Literature search yield eight reports useful in the development of a TCP model for BCC / SCC. The reported a median total dose of 48.2 Gy (range: 35-63.1 Gy), dose per fraction of 3.6 Gy/fx (range: 2.4-7.3 Gy/fx), [BED14 = 62.0 Gy, (range: 52.5-85.5 Gy)] & total treatment time of 23.8 days (range: 10-39.0 days). Four TCP models were generated for BCC ≤ 2cm, BCC > 2cm, SCC ≤ 2 cm, and SCC > 2 cm. For BCC, the model parameters were found to be TD50 = 0 Gy, k = 14.7 Gy-1, & BEDprolif = 0.507 Gy/day & TD50 = 48.8 Gy, k = 2.744 Gy-1, & BEDprolif = 0.091 Gy/day for tumor sizes of < 2cm & > 2 cm, respectively. Model parameters for SCC were: TD50 = 0 Gy, k = 18.986 Gy-1, & BEDprolif = 0.166 Gy/day & TD50 = 54.1 Gy, k = 2.98 Gy-1, & BEDprolif = 0.060 Gy/day for tumors < 2cm & > 2 cm, respectively. Additional calculations with a fixed BED14 = 60 Gy show that increasing the treatment time from 5 to 30 days has a 1-2% reduction in TCP for tumors ≤ 2 cm, while this increased time results in an 6-7% reduction in TCP for tumors > 2 cm. The scheme of 5 x 7.2 Gy in one week is found to be equivalent to 15 x 3 Gy in 3 weeks in term of TCP. A TCP model for primary BCC / SCC was developed based on published clinical data and may be used to design new RT regimens for advanced RT technology. Prolonged treatment times could have a significant effect on control for tumors > 2 cm in size. Shortened treatment times may permit the delivery of larger fractional doses provided the normal tissue tolerance is respected.
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