Tumor Thickness Predicts Long-Term Complete Response of Facial Basal Cell Carcinomas in Asian Skin Types IV/V Treated with Methyl Aminolaevulinate Photodynamic Therapy

Abstract

Background: Basal cell carcinoma (BCC) responds well to topical photodynamic therapy (PDT), with high clearance rates of 72–100%, although the therapy showed limited effectiveness for lesions >2 mm thick. Tumor thickness is thought to be associated with therapeutic response. Objective: The purpose of this study was to investigate the efficacy, safety, and response depth of methyl aminolaevulinate (MAL) PDT for BCC. Methods: After application of MAL emulsion, each lesion was irradiated with 633-nm red light (total dose: 339 J/cm²). Complete response (CR) rates were assessed by histological examination at 6, 12, and 24 months. Results: Forty-seven patients (95 lesions) with skin type IV/V were enrolled. Overall CR rate at 24 months was 75.8%. Superficial BCC was more responsive than other subtypes. Tumor thickness beyond subtype was significantly associated with CR rate. Three response depths are proposed: absolute CR (<1.3 mm), relative response (1.3 –1.8 mm) and no response (>1.8 mm). Although the recurrence rate (24%) is higher than with conventional surgical excision, 90.3% of patients were satisfied with the cosmetic outcome. Conclusions: MAL-PDT offers a noninvasive effective treatment; however, it is not the first option for most BCCs, except inoperable cases. The tumor thickness, independent of subtype, is predictive of PDT response.

Introduction

B asal cell carcinoma (BCC) is the most common skin cancer in Caucasians, Hispanics, Chinese Asians, and Japanese.^1

–4 Since the 1960s, the incidence of BCC has annually increased at 5–8% in Caucasian and Asian populations.^5
–7 Clinically, BCC is a malignant epidermal skin tumor that grows slowly and invades locally in a three-dimensional contiguous fashion, as an irregular growth of subclinical fingerlike outgrowths.⁸ The standard treatment is surgical excision with intra-operative margin control, with laser, cryotherapy, and curettage therapies being used as alternatives. The prognosis is good for most surgically excised BCCs, but such treatment can lead to poor cosmetic outcomes when local flaps or skin grafts are required for reconstruction. Cosmetically sensitive areas such as the face and neck, which are the most common sites because of exposure to UV light,^2,3 present particular challenges for treatment. For some anatomical regions, such as the eyelids, direct closure of these lesions after excision can result in the distortion of important landmarks, which leads to complications including ectropion and epiphora.

Topical photodynamic therapy (PDT) may prevent these potential complications that are associated with invasive therapies. This drug-based approach was first described by Kennedy et al.⁹ 5-aminolaevulinic acid (ALA) is preferentially taken up and converted into protoporphyrin IX (Pp IX) by neoplastic cells. When Pp IX is activated by a 630-nm laser, it selectively destroys tumor cells while sparing healthy tissue and cells. Methyl aminolaevulinate (MAL), a novel photosensitizer precursor, showed deeper tumor penetration and less adverse effect than 5-ALA, which was a result of the enhanced lipophilicity.^10,11 This innovative therapy with MAL or ALA has been approved in Europe, and reported in China as a treatment for BCC,^12,13 actinic keratosis,¹⁴ and Bowen's disease.¹⁰ Some studies on MAL-PDT showed high clearance rates and excellent cosmetic outcomes.¹⁵ However, the effectiveness appeared limited for lesions >2–3 mm thick.¹⁶ One ALA-PDT study showed that tumors that were <1.0 mm thick responded well in 53 cases of BCC, suggesting that tumor thickness appeared to closely predict therapeutic response.¹⁷ Because an accurate prognosis is critical for physicians performing PDT, it is necessary to determine the maximum depth of tumor that responds in topical PDT. One pharmacokinetics study showed that fluorescence could reach a depth of 0.98 ± 0.04 mm after occlusive application of MAL in superficial BCC.¹⁸ However, to date, no clinical trials have addressed the tumor thickness that may accurately predict therapeutic response to MAL-PDT in BCCs.

We investigated the relationship among tumor thickness, histological subtypes, cosmetic outcomes, and long-term complete responses (CR) in facial BCCs treated with MAL-PDT. Based on 95 lesions of BCC, the results showed that tumor thickness, which is independent of histological subtype, played a key role in therapeutic response and presented an obvious negative correlation.

Materials and Methods

This was a prospective non-comparative observational study. Ethics Committee of the Fourth Military Medical University approved this study, and all subjects gave informed consent.

Patients and lesions

Between September 2004 and October 2008, patients were consecutively recruited from the dermatology clinic of Xijing Hospital in Xi'an, China. Patients with excisional biopsy proven facial BCC gave prior written informed consent. BCC was confirmed clinically by two physicians and histologically by 4-mm punch biopsy taken from the thickest part of lesions. The tissue was fixed in 10% formaldehyde, embedded in paraffin and stained by hematoxylin and eosin (HE). This histology examination was repeated at 6, 12, and 24 months. The thickest tumor depth was calculated from stratum corneum to the lowest boundary of the tumor tissue. Slides were made by serial paraffin sections, and a 100-um scale was added as reference on pathological photomicroscopic images (Nikon 90i, Tokyo, Japan). In addition to atypical changes, subtypes such as superficial, nodular, pigmented, fibroepitheliomal, sclerosing, and “unclassified”, classified with reference to Raymond Barnhill's Textbook of Dermatopathology (Third Edition), were reported. Lesions that presented clinical remission but histological remnants of tumor cells were regarded as “treatment failure” (i.e., only partial disappearance or recurrence). Only CR lesions confirmed by three clinical and histological examinations at 6, 12, and 24 months were regarded as “treatment success” (complete disappearance of tumor).

Patients in this study were at least 18 years of age. Exclusion criteria were the following: Gorlin's syndrome, general poor health with expected poor compliance, pregnancy, breast-feeding, or other treatments for the BCC lesions within the previous 6 months.

Treatment procedure

Crusts were removed by curette to facilitate access by the photosensitizer. Topical MAL 20% (wt/wt) (Biosynth, AG, Staad, Switzerland) in an oil-in-water emulsion (Tanabe, Tokyo, Japan) was applied to the lesion and to a 10-mm margin of disease-free skin, which were sealed with thin plastic wrap and covered with non-adhesive light-shielding dressings for ∼6 h. After removal of excess cream using saline buffer, the lesion with at least 20-mm of normal skin border was irradiated with a 633-nm red light (126 mW/cm²) from a light-emitting diode (LED) lamp (Omnilux, Altrincham, Cheshire, U.K.). Each lesion was irradiated for 15 min with a total light dose of 113 J/cm². To mitigate any hyperthermic effects or burning sensation, the treatment was divided into two 7.5-min sessions separated by 30 min. The light source was calibrated before illumination using a MACAM SR-9910 spectro-radiometer (Macam Photometrics, Livingston, U.K.). The abovementioned procedures were conducted three times, 1 week apart for each time, for a cumulative dose of 339 J/cm².

Post-treatment evaluation and follow-up

Six months after final treatment, the clinical response was evaluated as complete or incomplete, by clinical inspection operated by the same investigator. “Complete” was defined as total disappearance of the lesion. Lesions with incomplete response received an alternative treatment of surgical excision, cryotherapy, or CO₂ laser therapy. The investigator-assessed cosmetic outcomes were evaluated by a blinded observer. The patient outcome was graded as excellent (no hypertrophic scarring, atrophy, or induration; no redness; no difference in pigmentation compared with neighboring skin); good (no hypertrophic scarring, atrophy, or induration; but slight redness or pigmentation); fair (slight-to-moderate hypertrophic scarring, atrophy, or induration; moderate redness or pigmentation) or poor (extensive hypertrophic scarring, atrophy, and induration; or serious redness or obvious pigmentation). Each patient and/or one family member completed an additional questionnaire for self-evaluation that used these four criteria. The evaluations were repeated at months 12 and 24.

Statistical analysis

Statistically, data were reported as numbers and/or percentages. Student's t-test and Pearson χ² test were used for comparing mean values and CR rates, respectively. Binary logistical regression analysis was used for determining tumor response depth. A p value of <0.05 or 0.01 was considered statistically significant. Statistical analysis was performed using the SPSS 10.0 software application.

Results

Therapeutic response

Forty-seven patients with previously untreated biopsy-proven BCCs were enrolled. All had type IV/V skin. The mean age was 62.7 years, and 51% were men. Relevant clinical details, such as lesion number, location, size, histology subtype, and tumor thickness, are summarized in Table 1. In the 95 treated lesions, CR rates were 89.5 %(85/95), 77.9% (74/95), and 75.8 %(72/95) at month 6, 12, and 24, respectively. No significant differences in CR rates were seen between months 6 and 12 (x ² = 4.66, p > 0.05), but they were seen between months12 and 24 (x ² = 0.11, p < 0.05). The 23 lesions with incomplete response were offered alternative therapies. There were no serious adverse events or deaths.

Table 1.

Clinical and Histological Characteristics of the Patients and Lesions

No. of patients	47
Male/female	20/27
Mean age (range)	62.2 years (42–81 years)
No. of lesions	95
Lesion location, No. (%)
H-mid zone	52 (54.8%)
Scalp	10 (10.5%)
Non-H-mid zone	33 (34.7%)
Mean size of lesions (range)	1.22 cm (0.4–4.7 cm)
Histological subtypes, No. of lesions
Superficial BCC	39
Nodular BCC	26
Pigmented BCC	6
Sclerosing BCC	4
Fibroepitheliomal BCC	3
Unclassified BCC	19
Mean tumor thickness (range)	1.6 mm (0.2–3.25 mm)

Tumor recurrence

At month 24 after therapy, 23 of 95 lesions had recurred, and then received alternative therapies. At month 12 after therapy, 21 of 23 lesions had recurred. Two lesions had recurred at month 12 after therapy. The overall recurrence rate was 24.2% (23/95). Recurrent lesions consisted of two superficial, eight nodular, four pigmented, five sclerosing and three fibroepitheliomal BCCs, and one unclassified BCC. The majority (73.9%; 17/23) of the recurrent lesions were in the H-mid-face zone, and their recurrence rate was significantly higher than for those on the scalp (32.7% vs. 14%, x ² = 4.054, p < 0.05). Recurrence seemed to have no correlation with lesion size. In addition, 8 of the 23 lesions without CR received MAL-PDT again toward good cosmetic outcome, especially lesions in peculiar regions that might require surgical excision and/or local flaps. Unfortunately, all lesions recurred, despite a total dose increase to 452 J/cm², which indicated that the low dose was not a key factor.

Cosmetic outcome

Most (65/72; 90.3%) patients with CR were satisfied with the cosmetic outcome at month 24, assessed by clinician or patient (Tables 2 and 3). Side effects in the lesions scored as fair or poor consisted of atrophy (n = 1), induration (n = 1), pigmentation (n = 3), and redness (n = 2). No other side effects, such as hypertrophic scarring or depigmentation was observed, as is shown in Fig. 1. Twelve of the 23 (52.2%) recurrent lesions were rated as having good or excellent cosmetic outcomes by the investigator. There was no difference in cosmetic satisfaction rates as evaluated by the investigator versus the patient. Cosmetic outcomes were not significantly associated with location, size, or histology subtype (Table 2).

FIG. 1.

A lesion on the H-mid zone (a) before MAL-PDT therapy and (c) 6 months after. Histological examination (H&E, original magnification × 20) showed tumor disappearance, reconstituted epidermis with flattening of rete ridges, and slight fibrosis in the dermis (d) after the treatments, (b) compared with the untreated.

Table 2.

Lesion CR and Cosmetic Outcome Over Time by Tumor Location and Size

	CR rate			Cosmetic outcome ^a (excellent/ good/ fair/ poor)
	6 months	12 months	24 months	6 months	12 months	24 months
Lesion location
H-mid zone	82.7% (43/52)	69.2% (36/52)	67.3% (35/52)	15/ 14/ 6/ 2	16/ 14/ 5/ 1	18/ 13/ 3/ 1
Scalp	100% (10/10)	90.0% (9/10)	90.0% (9/10)	8/ 2/ 0/ 0	8/ 1/ 0/ 0	8/ 1/ 0/ 0
Non-H-mid zone	96.8% (32/33)	87.9% (29/33)	84.8% (28/33)	19/ 10/ 2/ 1	19/ 8/ 3/ 0	20/ 5/ 3/ 0
Mean lesion size^b (cm)
<1	91.4% (32/35)	80% (28/35)	77.1% (27/35)	15/ 11/ 4/ 2	14/ 10/ 3/ 1	15/ 10/ 2/ 0
1–2	90.3% (28/31)	80.6% (25/31)	80.6% (25/31)	16/ 8/ 4/ 0	17/ 4/ 4/ 0	19/ 3/ 3/ 0
>2	86.2% (25/29)	72.4% (21/29)	69.0% (20/29)	14/ 8/ 2/ 1	12/ 6/ 2/ 1	13/ 4/ 2/ 1
Total	89.5% (85/95)	77.9% (74/95)	75.8% (72/95)	42/ 26/ 8/ 3	43/ 23/ 8/ 1	46/ 19/ 6/ 1

Rated by investigator.

Largest diameter.

Table 3.

Lesion CR Rate Over Time by Tumor Thickness and Histological Subtypes

	CR rate
Histological subtype	6 months	12 months	24 months	Tumor thickness (mean ± SD, mm)
Superficial	97.4% (38/39)	94.9% (37/39)	94.9% (37/39)	0.63 ± 0.35
Nodular	82.1% (23/28)	78.6% (22/28)	71.4% (20/28)	1.49 ± 0.50
Pigmented	88.8% (8/9)	55.6% (5/9)	55.6% (5/9)	1.92 ± 0.60
Sclerosing	60% (3/5)	0% (0/5)	0% (0/5)	2.10 ± 1.58
Fibroepitheliomal	85.7% (6/7)	57.1% (4/7)	57.1% (4/7)	2.02 ± 0.43
Unclassified	100% (7/7)	85.7% (6/7)	85.7% (6/7)	1.70 ± 0.29

Therapeutic response and tumor location

CR rate was significantly associated with tumor location. At month 12 (Table 2), the lesions in the H-mid zone were more responsive to PDT than those outside of the H-mid-zone (69.2% vs. 87.9%, x ² = 3.91, p < 0.05;).^11,19 The differences observed in the H-mid zone versus the scalp (69.2% vs. 90.0%, x ² = 1.8179, p > 0.05) were not significant, because of the small sample sizes, but the CR rate for the H-mid zone tended to be lower than that for the scalp. Similar results were seen at month 24 (data not shown). In addition, significant correlation was not found between lesion size and tumor response at each follow-up. In contrast, superficial lesions were more responsive than nodular lesions (x ² = 6.68, p < 0.05; x ² = 4.12, p < 0.05; x ² = 7.05, p < 0.05, respectively at 6, 12, and 24 months). At month 24, the superficial lesions showed that the CR rate (94.9%) was higher than for the pigmented lesions (55.6%) and the fibroepitheliomal lesions (57.1%) (x ² = 10.33, p < 0.01; x ² = 8.72, p < 0.01, respectively) (Table 3). The sclerosing lesions showed the poorest response. Because of the small sample sizes, there were no significant differences between nodular and other subtypes.

Therapeutic response and tumor thickness

To investigate the relationship between the tumor subtype or thickness and the treatment response, statistical analyses were performed. We found that the subtype with best response tended to have the least tumor thickness (Table 3). At month 24, the superficial BCCs with the best response showed the thinnest mean tumor thickness among these subtypes. Significant differences in tumor depth were observed between each tumor subtype (t = 8.332, p < 0.01). Similar results were seen for nodular versus pigmented (t = 2.209, p < 0.05). Other comparisons showed no significant difference, because of small sample sizes. In contrast, there were significant differences in CR rates between the previously recommended CR depth groups of <2.0 and >2.0 mm (Fig. 2).¹⁶ Notably, no significant difference was seen among the five subtypes in each depth group. This suggested that tumor thickness, independent of tumor type, played a key role in therapeutic response.

FIG. 2.

Tumor depth was more closely associated with PDT response than with tumor subtype. Mean CR rate is significantly higher in BCC lesions <2.0 mm thick than in those >2.0 mm thick (x ² = 5.48, p < 0.01). No significant differences are observed among the five subtypes at either <2.0-mm thickness or >2.0-mm thickness (x ² = 0.32, p > 0.01).

To determine the CR depth, we evaluated the relationship between CR rate and tumor thickness by logistical regression analysis. As shown in Table 4, the tumor thickness was significantly related to CR rate at month 24 (F = 38.06, p < 0.01). Two groupings were used to determine the CR depth: artificial category grouping and serial grouping without artificial intervention (Table 4). The former obtained a CR depth of <1.2 mm. By binary logistic regression analysis using the latter grouping model, we found that 95% of the CR lesions distributed between 1.3 and 0.31 mm in thickness (respectively cumulative frequencies of 97.5% and 2.5% of normal frequency distribution area). Of the “no response” lesions, 95% distributed between 1.8 and 3.1 mm. Therefore, CR depth is defined as between 1.3 to 0.3 mm, and “no-response” depth is defined as between 1.8 and 3.1 mm. The new descriptive parameters of absolute CR depth (<1.3 mm), relative response depth (1.3 to1.8 mm), and “no-response” depth (>1.8 mm) are proposed, and closely predict the therapeutic response. These depths correspond to the papillary dermis plus the upper reticular dermis, the middle reticular dermis, and the lower reticular dermis plus subcutaneous layer, respectively. The thickness of tumor lesion was not significantly associated with the cosmetic outcome (Table 4).

Table 4.

Relationship Between Tumor Thickness ^a and CR Rate by the Cosmetic Outcome ^b at 24 Months

Tumor thickness (mm)	CR rate	p Value	Cosmetic outcome	p Value
Thickness grouping ^c		<0.01 ^d		NS ^d
< 0.4	100% (5/5)		100% (5/5)
0.4–0.6	100% (9/9)		100% (9/9)
0.6–0.8	100% (13/13)		92.3% (12/13)
0.8–1.0	100% (15/15)		93.3% (14/15)
1.0–1.2	100% (12/12)		83.3% (10/12)
1.2–1.4	88.9% (8/9)		87.5% (7/8)
1.4–1.6	62.5% (5/8)		100% (5/5)
1.6–1.8	50% (3/7)		66.7% (2/3)
1.8–2.0	42.9% (2/7)		50% (1/2)
2.0–2.2	0% (0/4)		-
2.2–2.4	0% (0/3)		-
>2.4	0% (0/3)		-
Category grouping		<0.01 ^e		NS ^e
<1.2	100% (54/54)		92.6% (41/54)
1.2–2.2	51.4% (18/35)		83.3% (15/18)
>2.2	0% (0/6)		-

Calculated from stratum corneum to low boundary of tumor.

Rated by patients with complete response.

Mean margin of error is 0.017 mm at 95% confidence interval.

Binary logistical regression, the equation of probability event = 1/(1+e^{−1.141+0.878×}), (e = 2.718281828).

Pearson χ² test.

NS, not significant.

Discussion

This study suggests that MAL-PDT is a moderately effective treatment option for facial BCCs, which has an excellent cosmetic outcome, and that tumor thickness is a predictor of PDT response. The long-term CR rate is 75.9%, which seemed to be slightly lower than previously reported for singular (79%) or combined MAL-PDT (81%).^15,19 This slight difference possibly correlates to the different skin type. Asian skin (type IV/V) is more resistant to light penetration than is Caucasian skin (type II/III).²⁰ To date, no multi-center and/or multi-ethnic study has been conducted; therefore, directly comparison cannot completely explain the difference in treatment effectiveness.

PDT response was significant correlated with BCC distribution and subtype with the same total dose, and therapeutic response was poorer in the H-mid-zone than in the non H-mid zone and scalp (67.3% vs. 84.8% and 90%), which is consistent with previous studies.^{19, 21} The H-mid zone is the most common area of BCC lesions, and it consists of embryonic fusion planes (regions of mesenchymal migration and fusion of the five primordial facial processes during the 5th to 10th weeks of human development).^22,23 Atypical cells penetrating to unexpected depths along these planes might result in the high recurrence rate. In previous PDT studies using ALA, the weighted average clearance rates for superficial BCCs were higher than for nodular BCCs (87% vs. 53%).²⁴ Similar results were seen in this study (94.9% vs. 71.4% at month 24, Table 3). In contrast, our CR rates are higher than the weighted rates achieved by ALA-PDT, and similar to the remission rates in one MAL-PDT study (92% and 87% for superficial and nodular BCCs, respectively),¹¹ and the high lipophilicity of MAL may be one of the major reasons. Peng et al. reported that MAL penetrated to a 2.2-mm depth in BCC, contrasting with more limited penetration of ALA. Interestingly, these data not only support the idea that superficial BCCs respond better to PDT than nodular BCCs do, but also suggest that the penetration depth of MAL is an important determining factor of PDT response.¹⁸

With respect to tumor clearance, our present study found that tumor thickness was a more important factor than tumor subtype. Actually, histological subtyping depends upon infiltrating depth of atypical cells.²⁵ As is shown in Table 3, for superficial BCC, the CR rate increased as tumor thickness decreased. Nevertheless, other histological characteristics that affected tumor clearance were not completely precluded. Definition of tumor depths was representative and scientific because of logistic regression analysis with small bias in this study. Statistical analysis from the artificial grouping inevitably introduced a bias, as is shown in this study. This bias was observed from the difference between the defined depths in this study (1.2 mm, 2.2 mm) and that of Morton's (1 mm, 2 mm).¹⁷ Moreover, it is possible that a sampling error was derived from the 4-mm punch biopsy that is recommended for large lesions or in cosmetically sensitive areas by the National Comprehensive Cancer Network guidelines. Excisional or shave biopsies were used for accurate evaluation on lesion margins.²⁶ Although the punch biopsy has a big limitation on representative sampling due to only the 4-mm lesion taken, the multi-site biopsies may to some extent minimize the sampling bias.

Here we propose a classification of “absolute CR depth” (<1.3 mm), “relative response depth” (1.3–1.8 mm), and “no response depth” (>1.8 mm). In the failure of PDT for lesions of >1.8 mm thick, the primary limiting factor is not light source but MAL penetration depth. Theoretically, 630-nm light may penetrate up to 6 mm into skin, which is supported by one case in which 7-mm-thick extramammary Paget's disease (EMPD) was successfully treated with the PDT by using intralesional injection of ALA followed by illumination of 633-nm LED light.²⁷ Peng et al. and other investigators reported that MAL penetration depth ranged from 0.7 to 2.2 mm in tumor tissue or normal epidermis, suggesting that tumor response depth possibly depends upon MAL penetration depth.^24,29 Therefore, depth of protoporphyrin IX fluorescence should be further analyzed in BCCs biopsies 4–6 h after application of photosensitizer. An in vitro study is needed for further understanding of the importance of penetration depth of MAL in PDT response in the future. Certainly, the recurred tumors with >1.8 mm thick could be treated by intralesional application of MAL. Henta et al. reported that a vulval EMPD case with tumor depth of 7 mm was successfully treated with the combination PDT using topical application and intralesional injection of ALA,²⁷ which also had been used in dysplastic Barrett's esophagus,³⁰ cervical intraepithelial neoplasia,³¹ carcinoma in situ of the urinary bladder,³² and Bowen's disease.³³

Conclusion

MAL-PDT is a noninvasive, safe, moderately effective, and well-tolerated option for treating cosmetically vulnerable lesions in facial BCC. The tumor thickness should provide a prediction for physicians performing PDT. Although it is not recommended as the first option for most patients, the PDT should be suitable for the patients who have poor surgical condition or high requirements on cosmesis and quality of life.

Footnotes

Acknowledgment

We are indebted to professors Chengxin Li, Bing Liu, and Yufeng Liu; technicians Li Ruo, Ru Xue, and Yanping Cao; doctors Lei wang and Xianlong Qi, in the Department of Dermatology, Xijing Hospital; and doctors Heather Ann Long and Hongjiang Qiao in the Department of Dermatology, Hokkaido University Graduate School of Medicine, Sapporo, Japan. This study was supported by Hospital High & Creative Technique Fund (XJGX0629M26), Xijing hospital, Xi'an, China.

Author Disclosure Statement

No conflicting financial interests exist. The sponsor had no role in the design and conduct of the study.

The first two authors contributed equally to this paper and should be considered as first authors.

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