Increase In Skin Dose Research Articles

Managing Skin Reaction From Radiation or Combined Chemoradiation Therapy

Managing Skin Reaction From Radiation or Combined Chemoradiation Therapy

Increased Skin Dose With the Use of a Custom Mattress for Prone Breast Radiotherapy

The purpose of this study was to measure and compare the loss of buildup to the skin of the breast in the prone position due to 2 different positioning systems during tangential external beam irradiation. Two experiments were performed; one with a standard nylon-covered foam support and another with a novel helium-filled Mylar bag support. The choice of helium-filled Mylar was to reduce the contamination to as low as possible. The experiments were designed to allow a surface dose measurement and a depth dose profile with the pads placed in the path of the beam in front of the detector. All measurements were taken using a Capintec PS-033 thin-window parallel plate ionization chamber. The standard nylon-covered foam pad caused the surface dose to rise as it got closer to the skin. When the pad was directly touching the surface, the surface dose increased by 300% compared to the result when no pad was present. This loss of buildup to the surface was similar to that of a custom bolus material. The opposite effect occurred with the use of the helium-filled Mylar bag, namely the surface dose gradually decreased as the pad got closer to the phantom. When the Mylar pad was directly touching the phantom, the surface dose was decreased by 7% compared to when no pad was present. The use of a foam pad could potentially result in a significant higher dose to the skin, resulting in an enhanced acute skin reaction. Therefore, special care should be taken in this clinical scenario and further investigation of an air- or helium-based mylar support pad should be investigated in the context of definitive breast radiation treatment.

SU-FF-T-381: Skin Dose Determination for Helical and Serial Tomotherapy and MLC Based IMRT

Purpose: To utilize ultra‐thin thermoluminescent dosimeters(TLDs) to quantify and compare skindose for patients undergoing radiotherapy on various IMRT delivery systems. Method and Materials: Three treatment modalities and treatment planning systems were employed for comparison, including helical tomotherapy, MLC‐based IMRT and serial tomotherapy. For each modality a reference field was delivered. Surface dosemeasurements were taken with ultra‐thin TLDs (0.3 cm × 0.3 cm × 0.01 cm) at the center of each field. For comparison, treatment plans for prostate patients were developed on all modalities to deliver the same prescription. All measurements were made on a uniform cylindrical phantom using ultra‐thin TLDs placed on the phantom's surface for skindosemeasurements and an ion chamber placed at isocenter. Results: Helical tomotherapy was observed to give the highest average skindose, followed by serial tomotherapy, while the MLC — based step‐and‐shoot gave the lowest average skindose. For the prostate plans with an isocentric delivery of 2.0Gy, verified by a calibrated ion chamber, the average skindose from the helical tomotherapy delivery was about two times higher while serial tomotherapy delivery was about 1.2 times higher than the MLC‐based step‐and‐shoot method. Conclusion: The observed high skindose for the helical tomotherapy delivery can be accounted for by the absence of a flattening filter. Beams reaching the patient surface will be relatively softer compared to those where lower energy x‐rays have been filtered out by the presence of a flattening filter. However, the observed increase in skindose for the serial tomotherapy delivery can be attributed to the greater beam modulation, which leads to increase in monitor units required to deliver the same dose, and as a result greater leakage and head scatter.

SU‐FF‐T‐307: Is PTV Coverage a Valid Indicator for Plan Evaluation in Shallow Tumor Cases Where the PTV Extends Into the Build‐Up Region?

Purpose: The Planning Target Volume (PTV) is defined by ICRU report 50 as a geometrical concept, used to select appropriate beam sizes and beam arrangements. Clinically, a plan is normally acceptable if the 95% isodose surface covers the PTV. The goal of this study is to investigate the validity of using the PTV coverage for plan evaluation in shallow tumors where the PTV extends into build‐up region. Method and Materials: A conventional 3‐D conformal technique with a 10mm CTV‐PTV margin was used to produce 3‐field plans for a deep tumor (Prostate) and a shallow tumor (Head&Neck). For the H&N case, as the CTV is normally at a fixed distance below the skin, the geometric uncertainties are mainly from patient‐beam positioning errors. The CTV coverage with these errors was assessed by shifting the isocenter to 200 separate positions. The isocenter offsets were randomly selected using 3 uniform random Cartesian deviations between −10mm and 10mm, with a 3D distance constraint of 10mm. For the shallow tumor case, another plan with a 5mm larger field was made. The DVH of each plan with 200 shifted isocenters was analyzed. Results: In the H&N case, an “unacceptable plan” was obtained, which has V95=81% for the PTV. In the prostate case, we obtained an acceptable plan which has V95=94.9% for the PTV. However, both cases gave the same good CTV coverage for 200 shifted isocenter plans. For the H&N case, PTV coverage (V95) can be improved (from 81% to 83%) by increasing field sizes, but without improvement in CTV coverage, and larger fields increase skin dose. Conclusion: The current method of evaluating plan quality by means of PTV coverage is adequate for deep tumors, but is not adequate for shallow tumors. A new indicator for CTV coverage evaluation in shallow tumor cases needs to be developed.

The effect of optimization on surface dose in intensity modulated radiotherapy (IMRT)

Although IMRT has been shown clinically to increase skin doses for some patients, it has also been shown that intensity modulated delivery does not, of itself, increase skin doses. The reason for this apparent difference is that inverse planning can result in solutions that give high fluence to tangential beam segments near the skin surface, in an attempt to counter the build-up region. In cases where the clinical target volume (CTV) stops short of the skin surface, but the planning target volume (PTV) does not, there is no clinical reason to treat the skin. The CTV–PTV margin exists purely to ensure that fields are large enough to allow for geometrical uncertainties. With an objective function based on the doses to the PTV, it is possible for a plan that gives excess fluence to the skin to have a lower objective function, and hence to be preferred in an optimization. We describe a technique of plan evaluation, based on analysis of a plan by recalculating several plans in which the isocentre has been offset by a distance equal to the CTV–PTV margin. We demonstrate that changes to a plan that reduce a PTV-based objective can give a worse dose distribution to the CTV when systematic and random set-up errors are accounted for, and increase skin dose. Several possible strategies for avoiding this problem are discussed, including the use of the skin as an organ at risk, modification of the PTV to avoid the skin, and the use of ‘pretend bolus’ applied in planning but not in treatment. The latter gave the best results. The possibility of using the evaluation method itself, as the basis of an objective function for optimization, is discussed.

Influence of thermoplastic masks on the absorbed skin dose for head and neck tumor radiotherapy

The influence of thermoplastic masks used in clinical routine for patient immobilization in head and neck radiotherapy treatment on the absorbed skin dose has been investigated at Gustave-Roussy Institute. The measurements were performed in 60Co gamma-rays, 4 and 6MV X-rays and in 8 and 10MeV electron beams. Initially, the measurements were performed with thermoluminescent dosimeters (LiF) and a NACP chamber on a polystyrene phantom in order to study the influence of physical parameters (distance, field size, energy...) on first millimeters depth variation dose. The study was completed with in vivo measurements on 14 patients using various dosimeters (thermoluminescent detectors, diodes) in order to assess the increase of dose on first millimeters depth and to verify the delivered dose during treatment sessions (quality control). In treatment conditions, masks lead to an important increase of dose on the first millimeter in 60Co gamma-rays beams (dose value normalized to maximum of dose increase from 57.1% to 77.7% for 0.5 mm-water depth and from 78.5% to 88% for 1 mm-water depth); its contribution is less important in 4 and 6 MV X-rays beams (dose value normalized to maximum of dose increase from 49.5% to 63.2% for 0.5 mm-water depth and from 59% to 70.1% for 1 mm-water depth). Concerning 8 and 10 MeV electron beams, the normalized dose value increase respectively from 78.4% to 81.7% and from 82.2% to 86.1% for 0.5 mm-water depth. In vivo dosimetry enabled the quality control of delivered dose during treatment. Measured dose is in agreement within +/- 5% with the prescribed dose for 92.3% of cases. In routine, in vivo dosimetry allowed to quantify the increase of skin dose induced by thermoplastic masks for various energies of photon and electron beams as well as quality control.

Skin toxicity due to intensity-modulated radiotherapy for head-and-neck carcinoma

Purpose : To investigate the cause of acute skin toxicity observed in the treatment of head-and-neck cancer with extended-field intensity-modulated radiotherapy (EF-IMRT). Methods and Materials : EF-IMRT was used to treat head-and-neck cancer, with the gross target volume receiving 70 Gy and the clinical target volume 60 Gy. A thermoplastic mask covering the head, neck, and shoulder was used for immobilization. Dosimetric studies were conducted to investigate the possible causes of the skin reactions, such as the bolus effect of the mask, the use of multiple tangential beams with IMRT plans, and the way in which the physicians contoured the lymph nodes. The dose-volume histograms of conventional opposed-lateral fields were compared with that of the multiple tangential EF-IMRT fields. IMRT plans with neck nodes contoured up to and including the skin surface were compared with plans that contoured the neck nodes 5 mm away from the skin surface. In addition, IMRT plans defining the skin as a sensitive structure were compared with plans that did not define the skin as a sensitive structure. All plans were created using an anthropomorphic Rando phantom, and the skin doses were measured with and without the mask. In each measurement, 6 thermoluminescent dosimeters (TLDs) were placed at the lateral and medial surfaces of the neck. Results : For all four plans, the measured skin doses with the mask were consistently higher than those without the mask. The average dose increase was about 18% owing to the bolus effect of the mask. Multiple tangential fields used in IMRT plans contributed to an increase in skin dose by about 19% and 27%, with and without the mask, respectively. If the skin of the neck was contoured as a sensitive structure for dose optimization, the volume of skin that received >45 Gy was further reduced by about 20%. Five patients immobilized with head and shoulder masks were treated with EF-IMRT plans with the neck nodes carefully delineated away from the skin surface. The neck skin was identified as a sensitive structure for dose optimization. Grade 1 toxicity was observed in 3 patients, Grade 2 in 1 patient, and Grade 3 in 1 patient toward the end of treatment. Conclusion : Multiple factors contributed to the observed acute skin reaction for head-and-neck cancer patients treated with EF-IMRT. By taking into consideration the skin as a sensitive structure during inverse planning, it was possible to reduce the skin dose to a tolerable level without compromising tumor target coverage.

Variations in skin dose associated with linac bed material at 6 MV x-ray energy

Treatment with radiotherapy x-rays at 6 MV energy produces a build-up effect whereby a smaller dose is delivered to the patient's skin compared to the tumour dose. With anterior fields, no material is normally placed over the patient's skin, thus providing the maximum skin sparing possible with the beam configuration used. A posterior beam normally passes through the treatment couch top and increases the dose delivered to the patient's skin. Both the Mylar sheeting and the support ribbing material produce a significant increase in skin dose. Measurements at 6 MV have shown that the basal cell layer dose can be increased by up to 51% of maximum dose with a carbon fibre/Mylar couch and by 28% for a tennis string/Mylar couch when compared to anterior beams. These values are associated with the position of the carbon fibre or tennis string ribbing. Dermal layer doses are increased by up to 30 and 24% of maximum dose for carbon fibre and tennis string, respectively. These values include a combination of dose due to the support ribbing and the Mylar sheeting. Due to the variability in patient positioning on the couch top, these increases would be spread out over the skin surface producing an average increase per unit area at the basal layer of up to 32 and 20% of the maximum, respectively, for carbon fibre and tennis string couch tops and 21 and 12% at the dermal layer compared to dose at Dmax.

Measurement of radiotherapy x-ray skin dose on a chest wall phantom.

Sufficient skin dose needs to be delivered by a radiotherapy chest wall treatment regimen to ensure the probability of a near surface tumor recurrence is minimized. To simulate a chest wall treatment a hemicylindrical solid water phantom of 7.5 cm radius was irradiated with 6 MV x-rays using 20x20 cm2 and 10x20 cm2 fields at 100 cm source surface distance (SSD) to the base of the phantom. A surface dose profile was obtained from 0 to 180 degrees, in 10 degrees increments around the circumference of the phantom. Dosimetry results obtained from radiochromic film (effective depth of 0.17 mm) were used in the investigation, the superficial doses were found to be 28% (of Dmax) at the 0 degrees beam entry position and 58% at the 90 degrees oblique beam position. Superficial dose results were also obtained using extra thin thermoluminescent dosimeters (TLD) (effective depth 0.14 mm) of 30% at 0 degrees, 57% at 90 degrees, and a metal oxide semiconductor field effect transistor (MOSFET) detector (effective depth 0.5 mm) of 43% at 0 degrees, 62% at 90 degrees. Because the differences in measured superficial doses were significant and beyond those related to experimental error, these differences are assumed to be mostly attributable to the effective depth of measurement of each detector. We numerically simulated a bolus on/bolus off technique and found we could increase the coverage to the skin. Using an alternate "bolus on," "bolus off" regimen, the skin would receive 36.8 Gy at 0 degrees incidence and 46.4 Gy at 90 degrees incidence for a prescribed midpoint dose of 50 Gy. From this work it is evident that, as the circumference of the phantom is traversed the SSD increases and hence there is an inverse square fluence fall-off, this is more than offset by the increase in skin dose due to surface curvature to a plateau at about 90 degrees. Beyond this angle it is assumed that beam attenuation through the phantom and inverse square fall-off is causing the surface dose to reduce.

An isocentrically mounted stand for total body irradiation.

The purpose of this study was to construct a stand to support a patient for total body photon irradiation and to expedite the set-up and treatment by rotating the stand. As in other isocentric treatments, the midline dose is impacted less by source-to-skin distance variations. The method of immobilizing the patient is described. A 10 mm lucite plate is supported in front of the patient to increase skin dose. A matrix of holes in this plate serves to index the location of blocks used to shield the lungs. The dosimetry of the set-up is described, as is the production of tissue deficit compensators. The results of phantom studies and in vivo thermoluminescent dosimetry measurements are presented.

Skin damage probabilities using fixation materials in high-energy photon beams

Introduction: Patient fixation, such as thermoplastic masks, carbon-fibre support plates and polystyrene bead vacuum cradles, is used to reproduce patient positioning in radiotherapy. Consequently low-density materials may be introduced in high-energy photon beams. The aim of the this study was to measure the increase in skin dose when low-density materials are present and calculate the radiobiological consequences in terms of probabilities of early and late skin damage.Method: An experimental thin-windowed plane-parallel ion chamber was used. Skin doses were measured using various overlaying low-density fixation materials. A fixed geometry of a 10×10 cm field, a SSD=100 cm and photon energies of 4, 6 and 10 MV on Varian Clinac 2100C accelerators were used for all measurements. Radiobiological consequences of introducing these materials into the high-energy photon beams were evaluated in terms of early and late damage of the skin based on the measured surface doses and the LQ-model.Results: The experimental ion chamber gave results consistent with other studies. A relationship between skin dose and material thickness in mg/cm2 was established and used to calculate skin doses in scenarios assuming radiotherapy treatment with opposed fields.Conclusion: Conventional radiotherapy may apply mid-point doses up to 60–66 Gy in daily 2-Gy fractions opposed fields. Using thermoplastic fixation and high-energy photons as low as 4 MV do increase the dose to the skin considerably. However, using thermoplastic materials with thickness less than 100 mg/cm2 skin doses are comparable with those produced by variation in source to skin distance, field size or blocking trays within clinical treatment set-ups. The use of polystyrene cradles and carbon-fibre materials with thickness less than 100 mg/cm2 should be avoided at 4 MV at doses above 54–60 Gy.

Acute and late morbidity of using a breast positioning ring in women with large/pendulous breasts

Purpose: To assess acute and late effects of radiation therapy in women with breast cancer treated with a breast positioning ring. Materials and methods: Fifty-six patients with large and/or pendulous breasts were irradiated using a breast positioning ring. The incidence of acute morbidity was correlated with patient weight and breast ‘size’. Cosmesis was scored at ≥1 year following radiation therapy by the patients. Dose changes in the buildup region under the ring were measured using a computer-controlled scanning system. Results: Moist desquamation (MD) occurred in 60.7% (34/56) of patients treated with the breast ring. The incidence of MD was more common in patients with larger breasts ( P=0.08), the severity necessitating a treatment interruption in 5 out of 56 (9%) patients. Cosmesis at ≥1 year following radiation therapy was scored as ≥ good by all patients. The surface dose under the ring was approximately 85% of the D max dose. Conclusions: The incidence or severity of acute MD in patients treated with a breast positioning ring appears high in patients with large pendulous breasts, and might be related in part, to the increased skin dose due to the positioning ring. To date, there appears to be no significant late normal tissue effects in patients treated with the positioning ring. Additional follow up is needed to assess the long-term consequences of the ring on cosmesis.

190 TLD-measurements of the increase of surface dose due to wound dressings during percutaneous irradiation

Objective Since any prolongation of overall radiotherapy time mayaffect the outcome of treatment, skin protection promises to improve the results of percutaneous irradiation. Different non-irritant dressing materials have been tested in the treatment of radio induced skin lesions. However, the increase of skin dose caused by wound dressings may enhance skin reaction. Therefore we measured the dose due to different wound dressings. Material and methods Dose increase at the skin covered by wound dressings was measured during therapeutic irradiation. The investigated materials were a silicon-coated polyamide net (Mepitel, Molnlycke), a hydrocolloid dressing (Varihesive, Merck&Co.) and an alginate dressing (Kaltostat, Convatec). Measurements were carried out by thermoluminescent dosimetry during irradiation with electrons (5 MeV to 40 MeV) and photons (6MV and Co60). Results Dose increase depended on quality and energy of beam and on the beam arrangement. For electrons absolute values at the surface were relatively high for all materials (85–96%), but there was only a small dose increase (5–10% compared to uncovered skin). For photons dose increase depended also on the material. The lowest dose increase was measured for Mepitel, the thinnest coating. During irradiation with a single stationary field perpendicularly to the skin dose values of 32% (6MV) and 43% (Co) related to the maximum dose and a dose increase of 39% and 65% were measured. For tangential fields (e.g. irradiation of the breast, head and neck) absolute dose values were higher (up to 60% (6MV) and 75% (Co)), but dose increase was much lower (up to 20%). The other coatings caused dose increase up to 156% related to the uncovered skin. Dose values of 85% of the maximum dose (Co) are possible. Conclusions Wound dressings need not be removed during irradiation with electrons, thin dressings like Mepitel need not be removed during irradiation with photons, but thicker coatings like Varihesive or Kaltostat should be taken away during photon therapy.

Increased Skin Dose Using a Mylar Section in the Treatment Couch During Megavoltage Irradiation

Use of a thin polyester (Mylar) window in the treatment couch during megavoltage irradiation can cause a notable increase in skin dose. The authors suggest that a nylon mesh be used instead.